Wednesday, December 31, 2008

a Happy New Year...

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

Tuesday, December 23, 2008

Wattage and headroom on stage

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

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.

Monday, December 22, 2008

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

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.

Friday, December 12, 2008

How do valves work?

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.


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*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.