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

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

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?"