Steve's Amps Blog

2010-09-13T07:31:00.005+01:00

I decided not to move the blog after all. It attracted malware over on the main site and I had to clean it all up, very tiresome. So here it stays.

Plenty of older posts below right - click on the years, months etc and they will all open up.

...and after

2010-05-02T20:29:00.002+01:00

...as promised, some shots of the Selmer T&B Mk II after I returned it to some kind of electronic sanity. Sorry they're a bit shaky, it's been a lo-o-ong day.

Oh dear a bit of a lashup

2010-04-29T18:19:00.005+01:00

My goodness what a dreadful mess met my astonished eyes inside this Selmer Treble'n Bass MkII. Click on the images to see the full horror of it. Funny thing is, it actually worked, kind of. Anyone want to own up?

... and yes I will post pictures when I've sorted it out!

But take us the Voxes, the little Voxes, that spoil the vines

2010-04-22T08:22:00.005+01:00

A couple of thing occurred to me after thinking about EBEEP's comment on my Voxiness post. Just two bits of random stuff.

First, about the AC15/AC10. There is a weak spot on these old amps. The EZ81 rectifier valve runs a 6.3v filament, giving cheapskate manufacturers the chance to run it off the same filament supply as all the other valves, rather than having an extra 5v supply as required by most rectifiers. However there is a good reason why rectifier valves should have separate filament supplies - they do have a tendency to short the high-voltage supply to the filaments, and if they're not on a separate supply this can cause damage throughout the amp (I have seen two examples of this, lovely old power transformers burned out, ugh). So if you have one of these old amps, get a really good unused EZ81 and fit it. Most of the early 60s AC10/AC15s I see still have the original Mullard, but even Mullards aren't immortal. Best replacement is of course... an NOS Mullard.

And in an unconnected development... The outstanding good point of an AC10/AC15 is that it gives you Vox power amp overdrive at lower volume levels. A neat and kind-of-fairly non-invasive mod for an old AC30 is to separate the four EL84 outputs into two pairs, each on its own cathode resistor, and make one pair liftable from earth, giving you a switchable AC15 output stage in your AC30. Bigger output transformer and bigger speakers than an AC15, sure, but in a stage situation it is for many people just what they need to get their optimum sound without drowning out the band, and it does sound good (...or get an attenuator. The Weber MASS is the best I've heard).

2010-01-20T13:56:00.003Z

- thought I might twitter a bit too, maybe I'll post more often if 140

characters is all I have to think up.

PS sorry to be slow in responding recently.

Stuff happened.

2009-09-24T09:32:00.000+01:00

Don't you just love these guys?

Head Office Branch
Plot 84, Ajose Adeogun Street
Victoria Island, Lagos, Nigeria.

RE:YOUR PAYMENT NOTIFICATION
From:Dr Kingsley Paul.
Remittance Manager.
Zenith Bank Of Nigeria.

ATTN: Esteemed Beneficiary,
This is to officially inform you that we have verified your contact file presently in our database, as regards your African Union (AU)solidarity fund that was awarded to you upon been one of the lucky Beneficiary in its last draw on the 15TH of September 2009.
This award is funded by the African Union( AU) and the United Nations(UN) so as to strengthen tires between Africa and the rest of the world with particular reference to Nigeria.

We have discovered you have not received your payment due to your lack of co-operation and not fulfilling the obligations giving to you in respect to your contract payment.
Secondly, you are hereby advised to stop dealing with some non-officials in the bank as this is an illegal act and will have to stop if you so wish to receive your payment immediately. After the board meeting held at our headquarters, we have resolved in finding a solution to this problem, and as you may know, we have arranged your payment through our SWIFT CARD PAYMENT CENTRE in Europe, America,Africa and Asia Pacific, which is the instruction given by our president, ALAHAJI USMAN AMIR YARADUA (GCFR) Federal Republic of Nigeria.

The ATM Card Value is $7.000.000.00 (Seven Million Dollars). Our bank( Zenith Bank Plc) will send you an ATM CARD which you will use to withdraw your money totaling to a tune of $7.000.000.00 (Seven Million Dollars) in an ATM MACHINE in any part of the world, but the maximum is ($15,000) per transaction. So, if you like to receive your fund this way, reply to this office immediately for the issuing of your (ATM) CARD with the below Information.

(1) Your Full Name:
(2) Address where you want the payment center to send your ATM CARD:
(3) Phone And Fax Number:
(4) Bank Name:
(5) Occupation:
(6) Age:

Alternatively, you can come down to the header address to claim your fund with the original notification mail that will be sent to you shortly.
We shall be expecting to receive your information you have to stop any further communication with anybody or office apart from this office of the presidency.
On this regards, do not hesitate to contact me for more details and direction, and also please do update me with any new development.
Thanks for your co-operation.

Note: Because of impostors, we hereby issue you with our code of conduct, which is (406) so you have to indicate this code when contacting or emailing this CARD CENTER.

Regards,
Dr. Kingsley Paul
Remittance Manager
Zenith Bank Plc.

Marshallness, or an amp's guide on how not to be tweedy

2009-07-20T23:40:00.015+01:00

Number 2 - the Marshall sound

The Marshall overdrive sound is instantly recognisable. If ‘crunch’ is the word, then this is the crunch of broken glass underfoot. Anything further removed from the Isley Brothers’ ‘Summer Breeze’ kind of overdrive – or the Carlos Santana type of overdrive – is impossible to imagine. Marshallness is toppy, aggressive, harsh… and the instantly recognisable core sound of rock.

Marshall overdrive was certainly available from the very first amps Ken Bran put together in the back of Jim Marshall’s drum shop in 1962 (though he may not have meant you to turn them up to 10 to obtain it). And it hasn’t changed that much. From the JTM45, through the plexis and JMPs, the Master Volumes, the JCM800 and 900s through to the DSLs and TSLs, that mouthful-of-ground-glass sound remains essentially the same. Marshall have been pretty good at synthesising it in their solid state amps too (if you see a 5005 ‘Lead 12’ at the car boot, buy it – Billy Gibbons recorded some classic ZZ Top numbers on this 12-watt solid state Marshall, and it sounds, well yes, just like a Marshall).

So if the Marshall sound was there from the start, we should be able to see it being shaped in those early JTM45 circuits, and follow its essential elements down the years.

You may know that the essentials of the classic Marshall circuit are very close to those of the 1959 ‘tweed’ Fender Bassman. The 1950s Tweed Fenders tend to go into overdrive sooner than the 60s Fender classic 'blackface' models, and in that respect to resemble Marshalls. However, tweed overdrive is softer and looser than ballsy Marshall crunch. An understanding of the reasons for the differences beween tweed Fender overdrive, and the overdrive sound of the basically very similar Marshall amps, will help us to understand the components of the Marshall sound.

The Marshall/tweed Fender Bassman circuit is more complex and sophisticated than that of the basic AC30 circuit I wrote about last time. The basic AC30 preamp has just one gain stage and a volume control. The Marshall/Bassman circuit, however, goes like this:

input gain stage > volume control/attenuation > gain stage > EQ driver > three-knob EQ

As you can see there are two extra stages in there before the EQ, which on these amps is the full treble-middle-bass array (the presence control is elsewhere, in the negative feedback circuit). The volume control comes before these stages and is perhaps better described as a gain control, as it also controls the degree to which those 'extra' stages are overdriven. The reason for the first of the two, the extra gain stage, is that a three-knob passive EQ saps loads of gain out of a preamp. The second, the driver stage that also comes before the EQ, is a ‘cathode follower’. It has no intrinsic gain, but serves to make the signal low-impedance and high-current, the more efficiently to drive the EQ. It isn't actually that necessary though, it's an electronic luxury. Fender didn't bother using it in the blackface preamps they went on to design (and they were ok!).

By the way, I have a sneaky feeling that a cathode-follower stage in itself changes the tone for the better somehow. The Vibro-King has a cathode follower as its first stage; it drives the complicated valve reverb in that amp, but an experience I once had when building a Vibro-King preamp circuit into another amp suggested to me that the cathode-follower stage actually brings something good to the sound – perhaps in the way it drives the next stage. One fine day when I'm not too busy failing to fix your amp I will try an experiment…

Anyhow, back to the story. Remember that we haven’t yet said what makes the difference between the Fender tweed Bassman and the Marshall; we’re still looking at the shared nature of their very similar circuits.

The EQ is the last stage in the Bassman/Marshall preamp. It comes after preamp overdrive has already happened. Modern overdrive preamp designs all do it this way around, but many earlier amps made the mistake of putting the tone-shaping controls earlier in the preamp. My beloved Rivera-era Fender Concert falls into this category – and that is the reason why its overdrive only sounds good at certain settings of the gain controls. The right way is to create the overdrive, then have an EQ stage to shape it. Back in 1959, as in so many other things, Leo Fender got it right.

You do in fact need to do something about the signal EQ before you overdrive anything, too. And there Marshall diverges crucially from Fender... I will come to this...

Ok let’s look at the differences between the Marshall circuit and its American predecessor. The first one is that Fender used a much lower gain preamp valve – a 12AU7 – in the first stage. Marshall plugged in an 12AX7/ECC83 – the high-gain type that most guitar preamps use in most positions. This gave much more gain pushing right through the rest of the preamp, and it means that the preamp goes into overdrive sooner. Whether this was deliberate or not is a moot point – quite possibly it wasn’t, because just after the volume control Marshall also introduced a larger value series resistor, which cut down the signal a little (though not removing anywhere near as much gain as that high-gain ECC83 valve had introduced). Much more crucially in producing the Marshall sound, though, on the ‘lead’ preamps they parallelled this resistor with a capacitor designed to let higher frequencies through. Here is where the pre-overdrive EQ shaping that I was talking about, happens. In a little parallel box that brakes the signal with a resistor, and shapes it with a cap that lets higher frequencies through.

So this resistor/capacitor filter that follows the volume control in the classic Marshall circuit has the effect of cutting back the bass and boosting the treble. I would guess that this was probably a response to guitarist’s demands that the amp help them ‘cut through’ and be heard. Well you know what guitarists are like (...oh I forgot, you are one).

Anyhow what it did in my view was to shape the Marshall overdrive sound. Toppy, harsh, aggressive. What Marshalls do via this filter and the extra gain in the first stage is to drive lots of high-end frequencies through the first three stages, then let the player shape the sound with the TMB knobs and the presence control. Not that I imagine they intended to do that - in 1962 nobody was designing amps to create distortion. As I wrote, I expect they were going for a treble boost so guitarists could rise above the general racket with a solo - same thing Vox were after when they put a treble boost in front of the simple AC30 preamp. And with the same probably unintended result - not searing clean solos but screaming treble-y overdrive when the amp was turned up higher than it was ever designed to be.

You know how bass controls on Marshalls are. Kind of pointless. Doesn’t really matter much what you do with them. That’s because the filter I’m talking about cuts back the bass so much that there’s little left for the bass control to do (it comes last in the chain, remember) . As I said, this may well have been the result of the designer trying to help guitarists ‘cut through’, but what it also did was to create an overdrive sound that people liked to hear. Overdriven bass does not sound good, it’s muddy and, well, farty. Overdriven middles with the treble rolled off gives you ‘woman tone’, and brings that jasmine into bloom. Open up the treble and drive it and you’re listening to the sound of rock, the Marshall sound.

A little later, in the early 70s, Marshall cottoned on to this essential misuse of their amplifiers and, after listening to their customers, introduced a master volume control to tame output levels whilst allowing some overdrive, and also tacked on an extra gain stage at the front end of the preamp. This too was followed by a resistor/cap filter, rolling off the muddy bass before overdrive, and shaping what was still the same Marshall sound, with the same essential components. At the heart of the DSL/TSL preamps is the same basic circuit.

So the Fender/Marshall differences that make all the difference are (a) a high-gain valve in the Marshall stage one, and (b) a filter after the volume control that cuts the bass and boosts the treble. Now all the Marshall was waiting for was for Eric Clapton to come along and turn it up to 10.

But there’s a couple more crucial differences. For one, the Marshall has a bit more negative feedback than its ol’ pappy the tweed Fender. This actually cuts down the gain a little (but the Marshall still has plenty to spare thanks to its ECC83 in stage one). It also produces a sound that is little more controlled, a little less ‘natural’ maybe. This adds the final touch, keeping the Marshall’s inherent wildness in check just enough. Turn a Bassman up full and you will often get some slightly odd effects on loud notes. With an ECC83 in stage one a Bassman becomes almost untameable, and these ugly overtones, which are caused by too much gain causing blocking distortion and/or internal oscillation, become intrusive. I would suggest that Marshall found this to be the case after putting in that ECC83, and introduced a touch more negative feedback, and a slightly bigger series resistor after the volume control, to tame things a wee bit. Then they added the cut-through-the-band cap to give guitarists' fragile egos a bit of a support. And they were nearly there.

One last difference. The Fender has four 10" alnico speakers in an open back cabinet. Sweet and airy. Marshall really wanted a 2 x 12" cab for reasons of economy, and they were used to making closed back units because they had started out making bass cabs in response to bass players’ understandable grumbles about how the likes of 'bass' amps with 4 x 10" open back cabs (can you think of one?) were not helping them compete with guitarists. But speakers in those days couldn’t handle much more than 15 watts, and Marshall kept burning their little voice coils out in 2 x 12"s when they turned up these new 40-plus watt amps they'd been building. And so, in response to some very simple maths, the closed-back 4 x 12" cab was born, right back at the start of Marshall history, the little box was stacked on top of the big box, it sounded like rock music, it looked like rock music, and the circle was closed.

Voxiness: what makes the AC30 sound that way?

2009-02-22T19:51:00.009Z

- An occasional series which will attempt to account for the character of the great classic guitar amps. First, the Vox AC30.

The AC30 has a characterful clean sound, not at all hi-fi and quite 'coloured'. Turned up loud it has a fat smooth overdrive. Wonderful and unique, but how does it do what it does?

The basic non-top-boost AC30, played through one of its non-vib/trem channels, is as simple a guitar amplifier as you can imagine. There is a gain stage (just the one), a volume control, and a power amp with a tone cut control. That's it.

The absence of a tone stack, indeed of any tone controls at all in the preamp, means that the AC30 preamp's one little gain stage is actually capable of pushing the power amp into overdrive. Tone stacks - especially the three-knob sort - suck out loads of gain, and usually have at least one extra gain stage to drive them. With none of that in the way, in the basic AC30's signal chain there are a minimal number of components and stages between your guitar and the power amp. Good for gain, and really good for clarity and 'chime' and that lovely airy thing you get from such glorious simplicity in a valve amp.

The top boost section on the 'brilliant' channel (if you have treble and bass controls, then you have a top boost version), adds a gain stage and a further 'cathode follower' stage dropping the impedance and bringing up the current, before introducing its two tone controls. However it adds them after the basic AC30's gain stage and volume control are allowed to overdrive these extra stages, making the preamp portion of the overdrive sound subject to the tone-shaping power of the bass and treble controls. Which is good - you want your tone stack after your distortion so you can shape the crunch the way you like it. The overall result on the top-boosted Brilliant channel is a still-simple preamp, though now with three stages and some more stuff in the signal chain, which puts out distinctly more gain than the basic preamp, and which can itself be overdriven internally.

Is that all there is to the AC30? Nope, there's the power amp, which has several unique tone-shaping factors. But first, some philosophising...

I think it helps to understand the nature of old equipment if you first try to work out why its manufacturers did what they did in designing it, rather than looking at the fascinating but misleading business of how it was actually used by the wayward genuises of popular music in creating works of musical art. So what was in Dick Denny's mind when he sat down in a smoke-filled room at Jennings Musical Instruments and sucked his pencil with a new amp in mind? Well one thing we can be sure of is that he was not thinking about the Brian May squeal, the Rory Gallagher howl, chimey Beatley strumming, or any of that. Above all he definitely was not thinking about overdrive, as he sat down to plan an amp that was to give some of the best overdrive sounds the world has ever grooved to. One thing we can be absolutely sure of is that he was thinking about the market as it was in the late 50s/early 60s, and doing so with some responsive accuracy, or his designs would not have sold.

If you've ever taken the back off an old AC30 you will have seen quite a bit of complexity inside there - and thus you might now be wondering how I can I describe it as an utterly, beautifully simple amplifier. The explanation is even simpler - the majority of the components in there are not employed in amplifying your guitar, but in creating the vibrato and tremelo options you probably never use. Tremelo - the regular variation in volume familiar from many early Sixties amp designs, especially blackface Fenders - involves an oscillator, and some way of applying the wobble it makes to the signal - 'modulating' the basic sound. Vibrato, which the AC30 also offers, is a much rarer beast and involves a regular variation in the pitch of the output. The same oscillator can be used for both vibrato and tremelo but a different and more complex modulation system is required for the variation in pitch that vibrato requires. That's what most of the guts of the AC30 is doing - oscillating and modulating - and nowadays hardly anyone uses the options those circuits provide, especially the most complex one, vibrato.

The Jennings company's first product had been a valve synthesiser called the Univox. It was either this, or the very similar Clavioline, that was used to produce the space-age melody line on 'Telstar'. I happen to have one of these - a quite complex piece of valve technology. If you think of where the company was coming from when they began to expand their range it's easier to understand the AC30's design.

This was a synth manufacturer moving into the growing guitar market. Rock'n roll bands were forming all around them. There was a market for their piano-add-on space-age sound-generator, but a much bigger market was taking off, and it featured guitars - and those innovative bass guitars. These bands needed amplifying so the guitars could keep up with the drums - and Vox offered them a one-box solution. An amplifier with a section for bass, a section for a bright-sounding guitar, and an effects section to build on their brand's reputation for spooky sputnicky sounds - each section with two inputs, and each with a volume knob to get the mix right. The AC30 was conceived as a band-in-a-box amp, but this was a synthesiser firm, and to build on that reputation the AC30 circuit was also designed as a guitar sound synthesiser, making the then-radical new vibrato and tremelo effects available to enhance the sound.

With all these things to think of, each of those channels needed to be as simple as possible, as every complexity designed into in the preamp channels was going to be multiplied by three.

So the simple guitar amplifier that sounds so wonderful in its simplicity is kept so simple in order to make room for three channels, and for some state-of-the-art sound-processing circuitry.

My reading of the development of this circuit is that the prime goal in designing the Vox amplification circuits was to keep the preamp down to one gain stage, which is only really possible with no tone control at all. Just a gain stage and a volume control to turn up nice and high.

Which brings us to the stage after the preamp, where Denny put his one tone control - the phase inverter, or PI, which splits the signal waveform into top and bottom halves for the push-pull power amp to drive into the two-part output transformer. He used a long-tailed pair PI, which has some inherent gain and good fidelity. It is possible to use a single triode for the PI (eg in the 'concertina splitter' that I discussed a month or two ago), but despite its economy this doesn't give any gain. The AC30 needed that extra gain - but it is more economical to provide it in the PI, which all three input sections use, rather than trebling the effort by putting extra gain into each of those sections.

The PI splits the signal into two inverse halves. If you mix those halves together they will cancel each other out, the result being silence. Denny put a high-frequency filter across the two halves, and made it adjustable, so that if you turned it up you cancelled out the tops. Which is why the Vox's 'cut' control works the 'wrong way round' for a treble control - the more you turn it up, the more tops it cuts out. It's a very effective filter for the sharp highs of this amp's brilliant channel - and it costs nothing in preamp gain. It is there because you can't put tone controls in single gain stage preamps. Oh, and it just happens to provide exactly the top cut you need to smooth out an AC30's overdriven sound, though nothing could have been further from its designer's mind.

Now to the AC30 power amp. It uses EL84 valves, which were a UK design and make. Very important, that, in those days of import controls. To get the amp up to the 30 watt (ok, 27) that Vox felt (quite rightly) was necessary to keep up with rock'n roll drummers, you needed four of these slim, light, cheap British valves. And you needed to run them hot. Denny knew that you could drive these little beasts above their normal plate dissipation for extra watts, and he did so. Perhaps he had noticed that Leo Fender was doing the same to squeeze extra volume out of USA valves - more voltage, more current, more watts. He biased his quartet of El84s with a single resistor on the cathodes, and bypassed it with a fat capacitor to maximise gain. That cheap, simple method of biasing - 'cathode bias' - also provided a sweet compression that Fenders didn't have when you opened them up - but once again, that was a completely unintentional byproduct of a market-driven design.

One more thing about this power amp. It has no negative feedback. Negative feedback is just what it sounds like - you take a part of the signal from late in the amp's signal chain, often from the speaker output itself, and you feed it back into an earlier part of the amp where the wave is running a negative version of the signal you are using. It's a bit like the two halves of the signal in the PI, so to avoid cancelling the signal out you cut down the fed back part to a fraction of the signal at the point where you're feeding it back in. It cuts down the gain a bit, but it also flattens down the response, making it more linear and hi-fi. It helps control that demon of the high-gain amplifier, internal oscillation. Leo Fender always used it, probably because his amps without it were prone to oscillation and it helped avoid tiresome fiddling at the quality control stage. But the relatively low-gain design of Denny's AC30, with its spacious chassis and good layout, avoided oscillation problems anyway, so Denny could dispense with negative feedback. Probably he had his one-stage preamps in mind, and avoided everything that might stop him wringing every last ounce of gain out of his amp.

Negative feedback doesn't sound bad in itself - but when I sit nine of out ten guitarists down by the bench with a guitar in their hands and lift the negative feedback out of their amps, they want me to leave it out. The sound without it is a bit gainier, and a bit less 'tight', a bit more 'natural' and a bit less hi-fi. And that, my friends, is the final piece in the complex sonic jigsaw that makes the AC30 circuit the unique musical instrument that we all know it to be. Especially cranked.

Oh yes and alnico speakers. Jennings fitted a good pair of current-manufacture speakers to their amp, which seem to have been chosen for their high efficiency - giving more volume. The famous Celestion blue alnicos. As it happens, they also compress the sound quite nicely as they approach their relatively low power ceiling, putting the finishing touch to the the overdriven AC30 sound - a sound it was not designed to produce!

Do you have an AC30 with ceramic-magnet speakers, and are you thinking of buying it an expensive pair of Celestion Blues for its birthday? Here is a wee hint to end with - don't change both speakers, just change one. The mix gives you the best of both worlds. Try it and see, and save yourself £120 or whatever it is these days.

Pulling out valves, running without a speaker, and other output impedance matching issues

2009-01-25T12:10:00.014Z

Greetings and welcome to all my patient readers. I received the following questions from one of you (thanks Steve, by the way!):

I have read your FAQ with great interest, learning a lot and answering questions I have had for years. It has reminded me though of two points that I really want to understand and which I cannot find straight answers to. Please could you help?

The first is why I can't simply take two of the 4 EL34 valves out of my 100 watt amp to get it to run at lower power? The valves seem to be in parallel pairs and so surely taking out one from each pair would just cut the power getting through to the output transformer?? I'd love to be able to do this but Marshall warn against doing this but without saying why (just that it will damage the remaining valves/transformer but not why)

The second query is why switching a valve amp on with no load is so bad for it. OK I have lived for years just accepting I shouldn't do it but the kid inside me wants to know why.

In my experience the kid in you is way more likely to run amps with no speakers plugged in than is the intelligent questioning adult. If I plotted proportion of jobs featuring burned output transformers against the average age of clients I think we would have an indicative curve.

Both of these are output impedance matching issues, so I have to explain that first before moving on to specifics.

Impedance is resistance to AC. It's measured in ohms, the same as resistance, but you can't read it on an ohmmeter.*

Your output valves will work best if they are looking into (or driving, or loaded by, pick your metaphor) a certain output (or 'plate') impedance range. This will be a few thousand ohms per valve; each valve type has different expectations, and the type of circuit changes impedance expectations too. Your output transformer is an impedance converter (amongst other things) - it multiplies your speaker impedance into something your output valves want to see.

A typical output transformer will turn an 8 ohm speaker load into, say, 8000 ohms for the valves - in a typical push-pull amp with paired output valves, 4000 ohms per side.

Valve output impedance is a ball-park affair. A given set of valves might be happy looking into anything from (say) 3000 to 8000 ohms. So plugging 16 ohm speakers into an 8 ohm load is probably not too scary.

Right now let's deal with pulling two valves out. This doubles the output impedance that the amp requires from its speaker (equal impedances in parallel, like the two valves one one side of a push-pull quartet, produce half the impedance of each component). So to match the doubled impedance requirement from the valves, we should double the speaker impedance. Or if possible turn the selector down one click. So... with two output valves pulled out playing into 8 ohm speakers we should have the selector on 4 ohm - or arrange to have a 16 ohm speaker loading. But if you don't do that it will probably still be ok, output impedances being a ball-park business.

Why do Marshall say don't do it? Because for all they know you are 17 (Sorry 17 yr olds but statistics are statistics).

Why do output valves need a certain output impedance range? Because that impedance will also set a voltage/current relationship that they can handle. V=IR, or voltage = current x resistance, as every 17 yr old once knew and has now forgotten anyway.

OK now for running valve amps without a speaker. With no speaker plugged in there is, in the typically-wired amp, no load at all, which is an infinite impedance. Put an infinite value for R into V=IR, remembering that the bias and the potential in the amp will try to hold I constant, and have a think about it. What happens is that the valves make attempts to chuck out enough voltage to drive an infinite impedance. They can't, of course, but they are high-voltage devices and they have a damn good try. The flash voltages they generate can be high enough to cause arcing inside your output transformer- and outside it too, spectacularly. Arcs can occur inside the valves too, and on the valve bases. Internal feedback from these events probably makes things even worse. Typically one or more valves go out, and there are fused turns in the output transformer, causing either no output or reduced, distorted output.

There you are. Any questions (or indeed corrections, I'm a repairman not a scientist remember) - use the comment button.

*Indeed you can't measure impedance on an ohmmeter, but if you want to know the impedance of a speaker or combination of speakers, an ohmmeter across the terminals or the jack will read approximately 70% of the impedance in ohms. So an 8 ohm speaker will read 5 or 6 ohms.

The form 696 petition

2009-01-03T00:41:00.004Z

Below is an article worth reading, just in case you hadn't yet heard of this issue.

To sign the current petition to the Goverment protesting against this latest wearisome sign of the times, click the link: http://petitions.number10.gov.uk/Scrapthe696/

from The Independent, Friday, 21 November 2008:

How Form 696 could pull the plug on the capital's music scene

Warning sounded over new piece of bureaucracy that forces licensees to reveal a mass of information about performers

Teenage kicks will be harder to get if publicans and managers of other small venues are forced to comply with a new piece of bureaucracy called Form 696, a former punk rock star has warned.

The form demands that licensees give police a mass of detail, including the names, aliases, private addresses and phone numbers of all musicians and other performers appearing at their venue, and the ethnic background of the likely audience. Failure to comply could mean the loss of a licence or even a fine and imprisonment.

The police say they need the information demanded on Form 696, which runs to eight pages, so they can pinpoint which acts and venues attract troublemakers, and make sure venues are safe. But Feargal Sharkey, who rose to fame during the punk era as the vocalist on the single "Teenage Kicks" by the Undertones, is so angry about what he sees as a threat to live music that he is consulting lawyers about how to stop it.

As the boss of UK Music, which campaigns for musicians, he will be applying next week for a judicial review into whether a local authority has the right to make it a condition of a publican's licence that they have to fill in Form 696. The scheme was introduced by the Metropolitan Police after incidents at live music concerts in 2006, some involving guns. In theory, it applies to any licensed premises where there is live entertainment, but Detective Chief Superintendent Richard Martin, head of the Met's pubs and vice squad, said that in reality it will apply only to performances likely to draw large crowds.

It applies in 21 London boroughs, but professionals in the music business fear that if it becomes accepted, it will be copied in other cities. Martin Rawlings, director of the Pub and Beer Association, said: "I know of licensees faced with this saying they are just not going to put live music on. Form 696 is being used only in London so far, but there are similar things going on around the country, where the police are asking publicans to sign various protocols. It has gone too far, frankly."

Mr Sharkey has also complained to the Equality and Human Rights Commission that the police appear to be focusing on the music enjoyed by black and Asian teenagers. One of the questions on the form requires the licensee to specify the type of music that will be performed, giving as possible examples "Bashment, R'n'B, Garage". Another question asks, "Is there a particular ethnic group attending?"

a Happy New Year...

2008-12-31T19:05:00.002Z

...to all my readers (I know you're out there because Statcounter tells me so).

Wattage and headroom on stage

2008-12-23T23:30:00.013Z

A customer called and asked if I could improve the headroom on his Deluxe Reverb reissue. He liked the Fender clean sound and had bought the amp to provide it, but he found that on stage he couldn't get a sound that wasn't distorted.

...and if you already know the reason why, no need to read on.

In conversation with another caller, it emerged that he had bought his 100 watt Marshall head to get 'more gain', and was disappointed to find that on stage it actually gave him a less distorted sound than his old 50 watter.

Both these people had a similar misconception about wattage. Let me explain.

Amps have a preamp and a power amp. The tone is shaped in the preamp. The watts are made in the power amp, and as everyone knows they give you the loudness. Preamp - tone; power amp - volume. Or anyway that's the intention, though with guitar amps there's a complication, which is that if you turn the thing right up, the power amp starts to make tonal contributions too.

Preamps do not need to be different in amps of different wattages. Preamps do the same job in any amp - they shape tone via volume and tone controls and other tonal components, give you control over your sound, and they churn out a signal of a few volts AC, big enough to drive a power amp. You could take the output of a 100 watt Marshall TSL preamp and feed it into a Fender Champ power amp, no problem. And vice versa. In high-gain amps with master volume controls the preamp makes distortion sounds too.

At a certain level of preamp output (as set by the volume or master volume control), the power amp will start to distort. This is usually a nice kind of distortion. It is however only available at a certain volume level. You can't put a volume control on power amp signals, they would fry your pots. Hence attenuators.

OK so here we are on stage with our noisy drummer, doing a sound check. We turn up the preamp volume control until it is driving the power amp hard enough to keep up with Animal. If we have a power amp capable of 100 watts then it is unlikely we will overdrive it. If we have one capable of 12 watts then it is very likely we will overdrive it. Thus at stage volume levels the Deluxe Reverb (18 watts at most) power amp will be overdriven, and the 100 watt Marshall power amp won't.

So the Fender Deluxe Reverb will not give you the option of a clean sound on stage because it will inevitably be overdriving its power amp, whilst the 100 watt Marshall will not give you the option of power amp overdrive until everyone is weeping blood.

A 50 watt Marshall in a loud band on stage may be beginning to offer power amp overdrive; a 100 watt Marshall won't. So if you want a great overdrive sound live, go for a lower wattage amp. If you want a good clean sound you will need relatively more watts.

Here then is a brief guide to what levels of wattage will give you what kinds of result on stage in terms of headroom and power amp overdrive.

Amps below 10 watts, usually 'single-ended' ie one power valve only, eg Fender Champ. Not enough volume for stage work in any but the quietest band, but lots of lovely power amp overdrive. Single-ended amps are renowned for good power amp overdrive. Not totally sure I agree with this orthodoxy actually, as I like the sound of an overdriven push-pull power amp, though an overdriven Champ does sound lovely. However you can mic up an amp like this, see below.

12-20 watt amps, usually 2 x EL84, 2 x 6V6. Examples would be the Orange Tiny Terror, the classic Fender Deluxe and Princeton, the Marshall 18 watt, the Mesa Studio and Subway models, the Vox AC15. These amps will give enough volume to keep up with a subtle drummer. They will be on the edge of overdrive if the drummer gets energetic, or if there are other lead instruments. They will not give you a good clean sound on stage unless the band knows how to play quietly, or unless you mic up.

30-60 watt amps, usually 2 x EL34, 2 x 6L6, or 4 x EL84. Including 50 watt Marshalls, many Fenders including the modern Deluxe and Deville series, Vox AC30s and the majority of gigging amps. These will give you enough volume to keep up with a drummer and a few other electric instruments. You will almost certainly be able to keep the sound clean if you want to, and also in a loud band you may well be able to push some nice distortion out of the power amp. 50 watts is LOUD however.

80-120 watt amps, usually 4 x EL34 or 4 x 6L6. 100 watt Marshalls, Fender Twins, etc. These amps have lots of volume on tap. The important difference between a 50 watt and a 100 watt amp is that the 100 watt amp absolutely guarantees that you will be able to keep your sound clean even at earsplitting volume. As you experience the volume levels a 50 watt and a 100 watt amp are not that different in fact - the difference will be that at a given setting on the volume control the 100 watt amp will be cleaner, and the higher up the volume range you go the more noticeable this will be.

You will have realised by now that choice of wattage in an amp is also choice of live tone, and that the higher the wattage the more reliably available are the clean tones. On stage, more watts simply allows you to play cleaner should you wish to. Lower wattages make power amp distortion more likely. A 100 watt Marshall can be clean on stage with a loud band, but not so a Deluxe Reverb.

There is a certain inflexibility written into all this. The way around that is to use a small amp, set it to the tone you want, and mic it through the PA to get the volume you want. Or get someone like me to fit a line-out, if it doesn't have one; we can put a line out on any amp you like.

And if you then decided that you wanted some nice big empty Marshall boxes to hide your mic'd-up Champ behind... well you would not be the first.

Preamp valve types

2008-12-23T20:27:00.003Z

A little guide to the common preamp valve types:

ECC83 or 12AX7. High gain valves, used in most guitar amps.
ECC81 or 12AT7. Lower voltage gain high-current types, used for certain purposes in some amps.
ECC82 or 12AU7. Low gain high-current valves rarely used in guitar amps but good for dropping gain for harp.

These three types have the same pinout and can be swapped in for one another in most situations, though the higher current drawn by 12AU7 and 12AT7 types can cause minor issues. Each valve contains two triodes, ie two potential gain stages, making them very useful for designers.

The '12' designation means they have 12 volt filaments/heaters. But these are split into two 6 volt halves, so they can also work on the usual 6.3volt filament supplies most amps have. Really handy valves.

EF86. Used in the Vox AC15 and very early AC30s. Also used in modern Dr Z and VHT amps. Lovely sound for guitar, just one gain stage but with extremely high gain. Strong tendency to microphony and noise. Not in any way to be swapped for the above three types - very different valve type even though it has the same number of pins. These are made by the modern manufacturers but they are apparently a challenge to manufacture and the modern ones are much more prone to microphony etc than NOS ones. Which can easily cost you £40 or more.

Steve's credit crunch fix-it-y'self tips

2008-12-22T01:07:00.007Z

Just a few things you can do without taking the chassis out, if only to keep an amp going through a gig.

1. The effects loop cutout issue. If your amp is intermittently or permanently silent and it has an effects loop, get a spare guitar cable to use as a patch lead and plug it into the send and return jacks. The lead bridges the loop. One of the jacks, usually the return, has a cutout on it on most loops, and the contacts start to fail. If the amp comes back with the cable plugged in, it's a loop problem. Now you might need a new jack fitted anyway, but you could try some switch cleaner squirted into the jack hole. Even a bit of WD40 in an emergency. It is fine to play the amp with the patch lead in.

2. Preamp valves. These are the little valves. They can cause various problems, usually pops, crackles and hisses, but they can cause cutouts too. Pull out the small preamp-type valve nearest the big power valves; 10 to 1 this is the phase invertor (PI), which is in fact the first power amp stage, linking preamp to power amp (all the signal goes through it). If all goes quiet, you have a preamp problem. Stick the PI valve back in, get a spare valve and replace each preamp valve with it in turn. If the problem stops, leave the valve there and you've cured it. They can be tricky to get back in - use your eyes and if it don't fit, don't force it as the old song says. They can get hot but not usually skin-damagingly so. It is ok to take them in and out with the amp on. Don't stick your finger in the socket, obviously.

3. Power valves. These are the bigger hotter valves. Often they short and blow fuses. If you have a fuse the same value (and type - slo-blo fuses have a T next to the value usually), you can replace it and plug in some new valves. If the plates don't glow red you can play the amp ok. Sure it ought to be biased but if it plays ok and sounds ok, and the valves don't get red-hot and melt, that's ok.

Sometimes a power valve gets noisy. When you pull out the PI the noise continues - and you know you have a power amp problem. You can swap in a new valve to try to get rid of the noise. If there's not too much hum with the PI out you can assume you have a good balance. Watch the heat on these valves, they get well hot enough to burn skin. And the sockets have high voltage on them, often 500 volts or more.

It is well worth remembering that unbalanced power valves (which can happen if one is failing or failed) cause power amp hum, a hum which persists even with the PI valve out.

4. Microphonic valves. If your amp has a tendency to squeal like a little piggy or indeed a distressed warthog, or if you can hear it through the speakers when you knock on the case, you may have a microphonic valve. Tap each valve to find it, and replace the one that picks up too much sound from the tapping. Remember though that early stage preamp valves in higher gain amps will always be a little microphonic, as they are being amplified so much through subsequent stages. The first stage preamp valve is almost always the one nearest the inputs.

Caveat: Valve problems might have also caused internal damage, but you will know by the amp's poor performance if you have such an issue.

How do valves work?

2008-12-12T16:47:00.000Z

The relatively simple business of how valves/tubes work isn't that well treated on the net, so I'll have a go. I'm not so hot on physics so I will be very happy to be corrected on any of this.

The basic principle is that electricity can flow in a vacuum or a near-vacuum. That's what's happening inside a flourescent tube: the electricity - a stream of electrons - flows from one end to another and lights up some gas as it goes. Two poles, one at each end, are connected to the mains, and the current flows through the tube from one end to another.

Basically there are three of these wireable-uppable poles inside a valve of the type used for amplification. One (the anode or plate) is connected to a source of electricity, and a second one is earthed (the cathode). The electricity wants to flow (a bit like lightning) from the plate to earth* and it would do so if it wasn't for the third item in there, which sits between the plate and the cathode and is used to limit the current flow. This is the grid.

The grid needs to have some negative voltage on it to limit the current flowing through the valve. That's why valves are called valves - it's like a water pipe with a tap on it - the grid is the tap and it limits the flow going from one end of the pipe to the other.

The grid can only limit the current flow if there is negative voltage on it. The more negative volts, the more the current is limited. More negative volts on the grid is like turning the tap down.

This negative voltage is the 'bias' voltage. It sets a standard current flow. Without this bias voltage the current would flow too much, overheating the plates (they then glow red hot) and destroying the valve.

OK now here is how amplification happens. We also put the electrical waves that comprise the sound 'signal' on the grid. So the negative grid voltage is constantly varying with the sound of the guitar. That means that the current flow through the valve is also being varied exactly in tune with the variation on the grid. This means that the high positive voltage on the plate is varying too. The volts build up on the plate when the grid voltage limits the current, and lessen when the grid allows more current to flow.

This can happen many thousands of times a second - current flow is lightning-fast and the effect of the grid variation is equally fast. No problems with the variations that can make sound waves - electrical current flow is way faster than that.

So there is a small voltage wave on the grid - and a much bigger voltage wave on the plate. That's amplification.

_____________________________________________
*In fact electrons flow the other way, 'upwards' from earth. But by convention the direction goes the other way, because all the circuit conventions were well established before the true direction of electron flow was discovered. So the cathode is heated to make the electrons flow from it. And the cathode isn't always directly earthed... but you don't need to know any of this to get the basic drift.

Guitar meets concertina

2008-11-25T22:36:00.000Z

What do these amps have in common?

Fender Princeton and Princeton Reverb, blackface and silverface
Fender Super Champ
Peavey Classic series
Peavey Delta Blues
many classic Orange models

Seems to me that they have a sweet clean sound which develops into a good power amp crunch when you turn them up.

The other thing they have in common is an unusual phase inverter design. The phase inverter is the circuit stage that comes just before the power valves. Its function is to split the signal waveform into two halves for the paired power valves to handle in 'push-pull' - because this is a very efficient way of using valves.

The majority of amps use what is called a 'long-tailed pair' design of phase inverter, typically using the two halves of a standard preamp valve (ECC83, 82 and 81 valves have two separate triodes inside). This design has the advantage of good balance and some inherent gain.

The above amps, however, use a design which uses only one triode - one half of an ECC83/81. The split halves of the waveform are tapped from the 'top' and 'bottom' (plate and cathode) of the triode. It is called a cathodyne phase invertor, or more memorably a 'concertina splitter'. This design has no inherent gain. It is well-balanced in the signals it puts out under normal conditions, but the impedances of each half are different and when you push it a bit this produces imbalance. And that imbalance seems to sound good. It produces the kind of harmonic distortion that gives that nice up-an-octave singing sustain.

If you get the chance, give one of the above amp types a try - turn it up and see if you agree with me.

Can anyone recall any other amps with concertina splitters?

The Beefheart Rules

2008-11-12T10:30:00.000Z

Forwarded to me by my old friend Ian of www.ianstenhouse.com.

Captain Beefheart's Ten Commandments For Guitarists:

1. LISTEN TO THE BIRDS... That's where all the music comes from. Birds know everything about how it should sound and where that sound should come from. And watch hummingbirds. They fly really fast, but a lot of times they aren't going anywhere.

2. YOUR GUITAR IS NOT REALLY A GUITAR... Your guitar is a divining rod. Use it to find spirits in the other world and bring them over. A guitar is also a fishing rod. If you're good, you'll land a big one.

3. PRACTICE IN FRONT OF A BUSH... Wait until the moon is out, then go outside, eat a multi-grained bread and play your guitar to a bush. If the bush doesn't shake, eat another piece of bread.

4. WALK WITH THE DEVIL... Old delta blues players referred to amplifiers as the "devil box." And they were right. You have to be an equal opportunity employer in terms of who you're bringing over from the other side. Electricity attracts demons and devils. Other instruments attract other spirits. An acoustic guitar attracts Casper. A mandolin attracts Wendy. But an electric guitar attracts Beelzebub.

5. IF YOU'RE GUILTY OF THINKING, YOU'RE OUT... If your brain is part of the process, you're missing it. You should play like a drowning man, struggling to reach shore. If you can trap that feeling, then you have something that is fur bearing.

6. NEVER POINT YOUR GUITAR AT ANYONE... Your instrument has more power than lightning. Just hit a big chord, then run outside to hear it. But make sure you are not standing in an open field.

7. ALWAYS CARRY YOUR CHURCH KEY... You must carry your key and use it when called upon. That's your part of the bargain. Like One String Sam. He was a Detroit street musician in the fifties who played a homemade instrument. His song "I Need A Hundred Dollars" is warm pie. Another church key holder is Hubert Sumlin, Howlin' Wolf's guitar player. He just stands there like the Statue of Liberty making you want to look up her dress to see how he's doing it.

8. DON'T WIPE THE SWEAT OFF YOUR INSTRUMENT... You need that stink on there. Then you have to get that stink onto your music.

9. KEEP YOUR GUITAR IN A DARK PLACE... When you're not playing your guitar, cover it and keep it in a dark place. If you don't play your guitar for more than a day, be sure to put a saucer of water in with it.

10. YOU GOTTA HAVE A HOOD FOR YOUR ENGINE... Wear a hat when you play and keep that hat on. A hat is a pressure cooker. If you have a roof on your house the hot air can't escape. Even a lima bean has to have a wet paper towel around it to make it grow.

Looka my new blog site

2008-11-07T13:38:00.000Z

I moved the blog over here so people could add comments and ask questions. Also to give myself space for even more extended rambling when I oughter be a-soldering.

Click the list on the right to read and comment on old blog posts.

Amp porn III: original Vox AC15

2008-11-07T13:34:00.000Z

Don't see too many of these & I'm not aware of many pics on the web either. '62?

Steve's real deals

2008-11-07T13:32:00.000Z

They're out there. Decent well-build reliable great sounding secondhand valve amps for less than £500. I think of them as the Hot Rod Deluxe alternative, and a great alternative they are. You could do a lot worse than get one of these as your workhorse. 'Handwired' amps from the 70s and early 80s can be had quite cheaply and are infintely repairable and very robust. Once you get one sorted out it will run ansd run and put up with far more drops off the back of the van than any modern PCB amp.

The best second-hand bargain I know is the Rivera-period (80s-ish) Fender Concert - the one in Blackface trim usually labelled 'Concert II'. This has a beautiful Fender clean sound and, footswitch-ably, an excellent smooth 'American-style' overdrive. They're point-to-point circuit board wired - possibly the last ever Fender amp to be built this way - and include good things like a valve effects loop, and come in combo and in head form. Cool, cool amps, so much better than a Hot Rod they're in a different universe, and cheaper too. And they'll last forever. Quite a few of them around in UK 240v versions.

The 80s Super Champ is another wonderful little amp from Paul Rivera's golden period, but then you knew that.

Silverface Princetons and Princeton II amps are lovely but you'd need to mic one up in most bands. Starting to get pricey.

I've been hesitating about mentioning this next one. Maybe I should just keep schtum, take out a second mortgage and buy every one that comes up. But because it's you, I'll tell you. You know how much people will pay for a JCM800? Well, just before they made them, in the late 70s, Marshall were putting out the last of the JMP master volume amps. Lovely beasts, absolutely classic Marshall rock amps that really do make 'That Sound'. There are quite a few 50w two-speaker combos about, and some heads too. Basically they are JCM800s. It still seems to be possible to pick up a master volume Marshall combo from the late 70s for about £400. Some twerp must have dissed them on the internet or something. Anyhow, go and buy one, do. However, be careful not to buy an Artiste instead (fairly pointless amp from the same period that looks a bit similar).

Going back in time the Carlsbro 50 Top (not the reissue, don't know about that) is a great-sounding simple reliable well built amp. Let me give yours a little tweak and we'll get some fantastic plexi-type tones out of it.

If like many sensible people you've decided you want a clean loud amp with a good valve sound and you'll use pedals to get the rest, the Selmer Treble'n Bass will not let you down and is superbly built. Early Fenderish-styled ones are a bit dear, but the one with a full-depth aluminium faceplate and the knobs spread all over it is still quite cheap. Throoughly recommended. Nice compact size too.

Want to pay a bit more for a new amp but find that the reviews in Guitar Porn Monthly tend to repeat one particular brand name again and again, and you went into *****'s and tried one and it sounded like shit? Well yours is not a unique experience. My favourite new amp has a really lovely clean sound and also a nice crunchy overdrive. It is...the VHT Pittbull. Silly name I know, makes it sound like it'll chew your head off but actually it's a nice, civilised, very versatile amp. They've been trying for years to make an amp that does a great Fender clean and a nice Marshall crunch; this is the nearest they've got.

There you go, don't say I never tell you nuffink.

Amp porn II: inside a Bad Cat

2008-11-07T13:18:00.000Z

Bad Cats really are hand-built point-to-point. Lovely work.

See, no pussy jokes. Some of us have self-control.

Tranny amps - built like tanks, eh?

2008-11-07T13:14:00.000Z

Solid state amps used to be so simple. Nice old HHs, Acoustics etc - lovely. Then they started working on getting more and more watts out of smaller and smaller boxes.

I do sigh just a little bit when a bass player comes in bearing some behemoth amp capable of delivering a kilowatt or more (ie it would heat up a one-bar electric fire very nicely). Of course it has fused most of its output devices, and no doubt much else besides. Maybe he bunged it on a padded chair in a pub and blocked up the huge fan which is meant to dissipate all that heat. Pop. Or maybe he shorted the speaker leads - same result. Or maybe it just burned itself up for the hell of it. Well it can usually be fixed, but it does take time and it does cost money guys. Cooling is really crucial for these amps, and if there are twin power amps and only one speaker lead, don't forget to put it in 'bridge' mode - both amps on 5 is a lot cooler than one on 10.

Power supply faults (causing weird combinations of symptoms but usually little or no sound) and op-amp failures (distortion, loss of volume, loss of sound altogether, noise) complete the list of familiar westbound route for solid state amps.

All these things can usually be fixed. However (here follows a bit of a grumble, so ignore it if you like as like many grumbles it is no doubt a symptom of Weak Character) - some recent transistor amps tend to be made of specialised ICs - like computer chips - which are often either impossible to obtain or very expensive. Very recent amps have Surface Mount Technology or SMT, which makes their circuit boards both reminiscent of Solihull from 30,000 feet and also tricky to service. If they go wrong you're meant to approach the manufacturer for a replacement board, which of course gives the faceless, be-suited ones the opportunity to discontinue the board and force you to buy a new amp... which is possibly good for the third world economies that manufacture the things in vast automated plants but is not good for the pocket of the struggling musician or indeed that of his ol' pal Honest Steve the amp repairman.

And from now onwards amps will have to be built with lead-free solder, which melts at a higher temperature than good old tin/lead, does not stick to metal so well, and sets to a nasty matt grey. This is so they amps can be recycled safely. You guys are always just chucking your amps in the bin willy nilly aren't you? All those '68 Marshalls in landfill sites are endangering the environment, it's got to stop.

It's all a bit of a yawn. All of which goes to show how behind the times I will be, until I get a hot air SMT repair station. Then I'll be able to fix your mobile while you're here. Why does that idea seem so hateful to me? Weak Character again.

Harp through a guitar amp

2008-11-07T07:17:00.001Z

Harp players hit two problems when they try to amplify themselves through a guitar-type amp. One is howling feedback through the microphone, and the other is the lack of volume that happens when they turn down low enough to cut the feedback.

The big problem with using guitar amps for harp is that they have too much gain. Harp mics put out far more juice than guitar pickups and they don't need the gain, it just makes them scream.

Guitar amps also have a built-in mid scoop. This was no doubt the result of Leo Fender, whilst chucking his first amps together in a hurry to get down the pub and spend the proceeds, not really trying very hard to get a flat eq, but it has become the standard guitar amp sound and for most people guitars don't sound right now without the mids taken out (jazz guitarists being, as usual, the main exception). The trouble with that for harp is that the harp really does need the mids to sound right (being mainly made up of mids).

These things I can deal with. We can cut the gain by means of circuit changes and/or valve swaps, and we can level out the eq. Good candidates for this sort of conversion are the Fender Blues Junior and, for huge amount of volume, Blues Deville. Most non-high-gain Fender amps of whatever vintage can be converted to a loud, sweet harp amp, usually for less than £100.
But what if it's five minutes before gig time and you still can't get a decent sound, because you're forced to use a guitar amp for harp. What to do?

1. Turn the treble right down and the bass right up, use a long mic lead and cross your fingers.

2. Berate yourself for not reading this earlier.

3. Pull all the output valves but one out of the guitarist's amp. Watch your fingers now.

4. Give me a call after the gig. Preferably the next day, I do sleep at nights.

I built myself a couple of harp amps. Here's the big one:

- more or less '59 bassman circuit with flatter EQ and lower-gain preamp valves (12AU7, 12AT7, 12AT7). Power amp is 4 x 6V6. You can have 2 x 6V6 for more grind, or mix 6V6 and 6L6 for loads of headroom. Speakers are 4 x 8", two Weber alnicos and two Eminence ceramics. Built the cabinet too - solid pine dovetailed. But enough about me...

The Fender Hot Rod Deluxe / Deville is an uncouth little bastard.

2008-11-07T07:15:00.000Z

Oh Fender Hot Rods. They do indeed have too much raspy gain for most people's taste. These little hooligans of amps have DAMAGED THE HEARING OF MANY AN INNOCENT HARP PLAYER.

By the way have you ever noticed how loud these amps are on 2? But when you turn them up more they don't actually get that much louder? This is because the master volume pot on Hot Rods is linear rather than logarithmic, as they are on less delinquent amps. Some cynical people say this is so when you plug into one in the shop you say, "whoa, if it's that loud on two it must be like the four trumpets of the apocalypse on ten". And you like that, so you buy it. But then the singer starts bloody moaning, and you can't actually control the volume much, it's either on at two or off at one-and-a-half, so you say 'look it's only on two' which is a bit disingenuous isn't it?

This too can be put right - the volume control, not the singer (though to bring both channels into balance so you can channel-switch can be a bit complicated.)

Another standard Hot Rod problem is oscillation - anything from eldritch howling to motorboat phutphutting. The filter caps go. Not too hard to fix generally.

The famous fault though is an intermittent crackling and sometimes unbidden channel-swapping, caused by the fact the Fender put two very hot ceramic resistors on the circuit board to drop the voltage to the switching circuit, and they over heat and melt their own solder. High melting point solder is a good idea on these, or in extreme cases metal-cased resistors mounted direct to the chassis on flying leads from the board.

The speakers on these amps are a bit beamy. A death-ray of high frequency sound goes between the singer's legs and hits the front row in the teeth, but you can't hear a note you're playing. Get a Weber beam-blocker from www.tedweber.com - or just put a strip of gaffer right across the speaker hole. The highs all come from the centre of the cone, this spreads them around a bit.

If the cap fits…

2008-11-07T07:14:00.000Z

Musicians love to talk about their gear the same way (some) women like to talk about kittens and knitting.
In the old days, this was fairly manageable. The discussion usually comprised two old musos in the pub…

"Valve amps are better than transistor amps."
"No they're not."
"OK. Fancy another pint?"

Now, with the Internet, the debate has got out of hand. It's like a global crusade combined with the Spanish Inquisition. Everyone's got a view, but rarely agrees with anyone else, except to gang up and flame some poor 16 year-old for having the 'wrong' opinion.

Capacitors cause a worrying amount of debate in this area, perhaps because the signal appears to 'pass through' them on its journey through the amp. It doesn't really, but caps can affect tone because they charge at different rates, etc.

Actually decent audio caps at the kind of voltage ratings you need in a valve amp are not that widely available. I like to use Sprague Orange Drops, which I import wholesale from the USA... better than the usual little poly caps, and yes I can hear the difference and so will you, especially in high gain amps with plenty of stages for the signal to pass through.

I do think there is a slight sound difference with carbon composition resistors too (though admittedly I do feel a bit of a tosser saying this), and if I'm building an amp I use them wherever they're near the signal path and carry significant voltage (they can't make a difference if they don't, as far as I can see).

But they are hard to get and cost up to 100 times more than the quieter, utterly reliable little film types that every commercial amp uses, so I don't sweat it. Anyhow I'm probably imagining the sound difference; we're all a bit susceptible to vintage amp voodoo and I'm no different. As yet I've remained immune to paper-in-oil caps, but that's no doubt because I've managed to avoid finding out where to get them.

Amp porn I: Inside a Plexi Marshall

2008-11-07T07:06:00.000Z

Just what it says on the tin, the guts of a plexi. Not many chassis pics of these on the web, so here you go. This is a Superbass. I forget the date, it's a year or two since I had it in. '69?

The Golden Age of Valves - Now?

2008-11-07T07:05:00.001Z

These days the manufacturers in Eastern Europe are producing valves with most of the qualities of the old Western European manufacture, y'know, black plates, mica spacers, D-getters and all the rest of the b****cks.

Well, OK, maybe there are a few really high-end things you don't get, that you used to get - super-pure materials, ultra-hard vacuums - but sound for guitar is not going to be affected by that stuff, let's leave it to the hi-fi guys to worry about whether it's worth a 2000% mark-up. That's if they've any change left from the £500 they put aside for speaker cables (don't tell them about 2-core mains flex, that's our little secret).

Even previously hard-to get valves aren't so any more. 6v6s used to be a problem, for instance, but now JJ and Sovtek (badged Electro-Harmonix) both make a good one.

(BTW, never, ever buy a stubby little 6v6-labelled effort covered with black paint inside all the way up the glass. These are possibly the worst new-manufacture valves ever sold and aren't really 6v6s at all.)

Recommended buy? I really like the new JJ 6v6. Smooth yet crunchy, like a toffee crisp. My harp amp has four of them.

If you type '6L6 review' into Google into Google you'll find that there are some reviews on forums, which may be unbiased but may also be from (a) nutcases (b) people who are trying to justify having recently spent three figures on two valves. There are also some reviews by people in the trade, which won't be nutty but, being written by valve retailers, may naturally enough be affected by a wish to retail valves - especially expensive ones or ones you branded yourself. Both types of review are full of useful information, but caveat emptor and anyway because these reviews focus on the subtle little differences between brands rather than the overwhelming similarities amongst types of valve they will tend to draw lines that aren't really significantly there.

So are there any differences between brands that actually matter?

Well yes.

In my view there's a much bigger difference between a poor current-production valve and a good one than between a good current production and a NOS valve.

There's a big mystique thing about NOS valves as well as the supply-and-demand factor - witness the big prices now being asked for winged-C Svetlana 6L6s, valves that only very recently were current-production and cheap but now are expensive - though as they are now becoming available through UK distributors again we may see some more sensible prices on proper Svets soon.

The nearest you're going to get to a valve review

2008-11-07T07:01:00.000Z

Good cheap current-production valves I know about (and NOS valves I've experienced thanks largely to customers) include these:

EL34: The current JJ EL34s I sometimes use are good in terms of clean sound. They can be driven into distortion satisfactorily in non-MV Marshalls etc though they have quite a lot of headroom.

The newer JJ E34L has even more headroom if you want it loud and clean. Both valves are available in blue glass, which has a similar effect on tone to purple pilot light jewels.

The Sovtek 6CA7 (same as EL34) is cheap and well-liked - I very much like these - a bit smoother and fatter than the usual run of EL34 types.

I've heard the Mullard reissues - they don't sound like bad valves to me but I didn't try them exhaustively. There's a tendency to rubbish reissues just because of the sucker factor - but we all know they're not really Mullards don't we? Everything you buy is branded one way or another, and isn't there a strange paradoxical kind of honesty - or anyway a pleasing post-modern irony - about putting new wine in old bottles in so egregious a way? So long as they don't charge too much more for it.

I have tried real NOS Mullards in a pre-MV Marshall: clean, they have a good balance of highs and lows, lacking the harsh highs of some 90s production valves, e.g. older Teslas. Overdriven at high volume levels they hit you in the chest with overwhelming power - but still stay quite smooth-sounding. Probably it's that overdrive sound that makes the difference for me - I haven't yet heard a modern production EL34 that isn't a little bit jagged-sounding overdriven in a Marshall. But maybe you like that.

6L6: The JJs are good all round sonically and have the 6L6 sweetness in the top end that I personally look for, but tend to cost 50% more than other types, so I'm tending to use other types.

Any Russian 5881 (equivalent to 6L6) will be reliable and not sound bad, though maybe without some of that sweetness? Hard to be sure; they do sound good.

BTW avoid anything labelled 6L6B (there is a current production valve you might see cheap on Ebay) for Fender and most other guitar amps. The B suffix means they can't handle big plate voltages. 350v max on these. They can't be biased as hot either. C suffix is the one you need for high-volt guitar amps. G suffix just means made of glass, as opposed to metal - incidentally I've heard one of the old metal ones and it sounded really good, but turned out to be microphonic. Sigh. The WXTs are same rating as C-suffix, ie good for guitar.

I'm not especially a party to the Winged-C Svet magic - I've heard them but to be honest I couldn't differentiate them from the JJs. Maybe it's me.

Soon I'll be testing Sovtek's 6L6 WXT+ (NB - just got a shipment of these in because it seems New Sensor can't sell their Svet clones in the UK any more... more later on just how they sound but I stuck some in a Vibrolux to try them out and they sounded fine).

I have played RCA blackplates in various early-60s Fenders. They sound more controlled than current-production valves, perhaps because of a good balance between frequency levels. Very sweet chimey highs, tight bass.Could I tell the difference from a JJ or a Svetlana in a blindfold test? Not sure. I'd be listening for that top end sweetness I keep going on about.

I have one coke-bottle RCA I got unused, still in its brown wartime box and padding, in a garage sale. I put it in a single-ended amp my son and I built for him to use. (That's the kind of father I am - gullible). It sounds lovely, very lively and 'real' with smooth rounded overdrive.

What many NOS valves offer is breakup at much lower volume levels, that's really the most logical reason for buying them, if that's what you want.

6V6: The JJs are great - smooth and clear and sweet with a lovely rounded almost-civilised distortion sound. They are tall though, so check there's room - and they're not that cheap.

The EH 6V6 is stubbier like the old ones were, cheaper and still sounds fine.

NOS American 6V6s are still quite reasonable - about £20. I heard some GEs in a customer's Super Champ, and they sounded gorgeous - that Fender sound that is full of tops but could never be painful to listen to. Pure chime. And what a bargain compared to NOS 6L6s.

EL84: The JJs are excellent and cheap, so I use them often. They are pleasantly chimey and overdrive quite easily if you're into turning your Vox-type amp up. I've heard the Sovteks and they are not bad valves, though I've seen a few fail (maybe this is because so many manufacturers use them). Am I imagining it or do they break up a bit later than the JJs - more headroom?

I don't know much about NOS EL84s but they get a lot of praise.

ECC83/12AX7: The JJ ECC83S sounds good, is low in microphonics because of the small plate (probably) and low in hum because of the spiral filament, and is good and cheap, so for guitar amps I don't look further, though I often see various EH Sovteks and they are good too...

The Sovtek LPS valves sound very good indeed but in high-gain amps can be troublesome first stage valves because of microphonics/noise, as LPS stands for large plate structure.

I do have a recurrent experience of testing older-type Sovtek 12AX7s and finding that their gain levels have sunk below the acceptable, so if you have an amp with old Sovtek preamp valves you might spice it up interestingly with a new set.

Old used Mullards, Brimars etc usually in my experience have lost their egde gain-wise too, I'm sorry to tell you.

Chinese ECC83 valves have a distinctive shiny metal structure which partly wraps around the greyish plates; a lot of companies relabel these. The older ones are usually in a poor state in the amps I see, and so I doubt their longevity.

ECC81/12AT7: There are some very nice Phillips/Jan valves around at goodish prices (eg ten quid each from me) - they sound very fat and solid to me as first stage valves in old Fenders (if you're not after a lot of gain) or as splitters - I do like to retain ECC81/12AT7 types where they are specified as splitters because their excellent current gain makes for a nice fat meaty sound - very like the cathode follower circuits driving tone stacks in my favourite preamps.

The JJ ECC81 is fine, and I don't recall any negatives about the Sovtek/EH 12AT7s either.

ECC82: One little note about preamp valves. The ECC82 is not midway in gain between an 81 and an 83 as you can read in some places on the dear old unreliable web, but is actually a very low gain valve, way less than an 81. It has little or no usefulness in guitar amps other than as a trem modulator in AC30s etc (can be very handy as a gain-dropper for harp though). Not always easy to get. I have some very good Chinese ones, different internal structure than the Chinese ECC83, and I get an excellent harp sound from them.

A balanced view

2008-11-07T06:53:00.000Z

Balanced pairs of valves. Do you need them? the simple answer is, no you don't.

What does a balanced power amp section in a valve amp offer the guitarist? Less hum.

What do unbalanced ouput valves give you? A bit of even-order distortion - yes, they can sound better! And a bit more hum. Too far out of balance and you start to lose the handy humbucking function of the push-pull pair.

Your output transformer isn't balanced - it's only worth doing that for hi-fi - so why bother with balanced pairs of valves.

Sometimes hum really matters, and in very powerful valve amps imbalance can lead to instability. SVTs are a good example - with six power valves in push-pull the differences can really add up, and if you have a later one with the little red and green bias lights it will be hard to set up without balanced sets of valves.

Silverface Fenders often have a hum balance control which allows you to balance the bias voltage between the two push-pull 'sides'. Works a treat, you can stick in any old 6L6-es and dial the hum right out. Set the overall bias with the resistor that goes to earth on the pot body from the wiper (centre of the three tags), if you need to. Some people remove this really handy little tool when 'blackfacing' a silverface amp and convert it to a bias adjust pot, thus making willing victims of themselves for the eager sellers of matched valves. But I like it, he added redundantly.

More biased than before

2008-11-07T06:48:00.001Z

Everyone talks about it. But do we really know what we're talking about? I'll try to be clear about it.

- Biasing is done to adjust the current that flows through the valve.

- Bias is a negative voltage that acts as a brake on the main current carrying the amplified signal through the valve (in a manner of speaking).

- The more bias voltage you apply, the less current flows through the valve. Too much current destroys the valve as it can't dissipate all that heat. Too little and it doesn't work right.

Well, that's pretty dull. So why's everyone talking about it?

Apart from this proper function/valve survival issue, bias becomes a matter of taste.

High current gives you power/volume, clarity, good chimey top end. Less current gives you earlier breakup, with less volume, less clarity and top end.

Usually I bias for power and clarity - if you want less volume and a browner, distorted sound, well - some amps suit this approach better than others, but generally low currents, like low voltages, in power valves don't really deliver the distortion sound you're probably looking for.

Some amps have a self-biasing arrangement (which is the secret of those new amps you can get where you can swap output valves) - but even they are worth checking out when you put in a new valve.

One more other thing... adjustable bias and fixed bias are not opposites. In fact most amps with adjustable bias are fixed bias. The common alternative to fixed bias is cathode bias, which isn't so easily adjustable.

OK? Glad that's clear.

A side issue: some Peavey valve amps seem to have a tendency to leave the factory biased very 'cold' - that is, set up so that the current through the power valves is very low indeed.

This is particularly true of the 5150s, but I've seen it on many and various Peaveys, the exception being the EL84 amps which, like most EL84 amps including the AC30, are biased for incendiary levels of heat.

But some Peavey 6L6 amps and others in the 50-100 watt range are often biased way cold. I think they do this in pursuit of distortion. The distortion created is called 'cross-over' distortion,and in most amps it doesn't sound that good. So if you like the stock 5150 sound, good luck to you and stick with it but if you feel that your Peavey sounds a bit thin and blue-bottley this may be the reason and you might want to go with a hotter bias. Unfortunately most Peaveys don't have adjustable bias and do have PCBs, so it's not just a matter of twiddling with a trimmer, but it can be worth setting the bias properly, or installing an adjustable bias system (not very hard to do).

Speakers Cornered

2008-11-06T07:35:00.000Z

There is an immense ball of confusion out there about speakers, as I'm sure you've noticed. Alnico, Ceramic, paper cones, Hemp cones, wattage, decibels - all the disagreements are a sure sign to me that there's really not much difference anyway. What's the big deal about old speakers - as opposed to old designs?

(Waits for stones to start flying)

IMHO, old speakers at first get nice and round and mellow, they tend to sound less forward, with looser bass; then the top end goes altogether as the cone gets flabby. Is that good? Maybe there's some point that's wonderful somewhere before vintage turns to just plain old, but everything changes...

Deep breath. People like alnicos but I can't really hear the difference separately from all the other differences between speakers.

And lots of manufacturers identify the process of 'earlier break-up' - but I'm not sure I want to push my speakers that hard, can't be good for them.

For me the best thing about valve amps is what they add to the top end - a lovely mix of harmonics that makes a clean guitar really chime and sing, and makes distortion musical. I like a speaker that doesn't get in the way of that. Colouration? For me, less is more. My favourite guitar amp speaker isn't a dedicated guitar speaker but a high-quality all-purpose driver - the Eminence Delta Pro 12. This is a cast frame speaker very much in the mould of the old Electrovoice speakers of the 70s and 80s. These will let you listen to your amp all right; they don't add much colouration to the sound and won't distort or 'overdrive' as they are rated 400 watts. What I like best about them is the tight powerful bass their solid construction gives; everything else sounds a bit flabby by comparison, you really feel you can 'dig in'. A Twin with a pair of these in is a tool for world domination. Cost about £80 apiece though.

Pity Ted Weber is in the States (see my links page). His alnicos are not stupidly priced and sound good. The Signature series are only about £25. But you have to think about shipping and taxes. If you're out there and you're ever moving house from Kokomo, Indiana to Godalming, Surrey let me know before you pack the container, there's a few things I might ask you to slip into your ottoman.

Oh yeah, I like Celestion ceramics. They colour your sound (...er, but you just said!) ...ok, but in kind of a good way. Have a look at a response curve for one. Jagged-looking peaky stuff at the top end. That's what gives them that toppy / crunchy sound that goes so well with Marshalls (if you don't value your high-frequency hearing too much). I think it's OK, the Celestion sound, and it can decorate a dull-sounding amp in a valvey way so long as you don't turn it up too much and start cutting people's heads off, but it is like so many things in this life of ours, a question of taste. Tip: if you're thinking of buying an expensive Celestion ceramic try the cheapest one instead. Smaller magnet, slightly less efficiency and so slightly less volume (GOOD) but the inimitable lo-fi Celestion mouthful-of-ground-glass sound is still there. Their cheapo 8-inch guitar speaker, I forget what they call it, costs about twelve quid or something and sounds excellent in a baby combo, forget little alnico nonsense things for one-hundred-pounds-jesus-christ-almighty and try one.

Jensen ceramics are quite cheap and I think they sound good. But more meat than the alnicos, nice tops. But - always these buts - it is worth shelling out for a Jensen P8R for your silverface Fender Champ.