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Derating the power of a valve amp?

George J

George J

Herefordshire member
Derating the power of a valve amp?

Is this possible? Would it result in longer output valve service-life? Would it make the actual quality of sound worse, or even possibly better? Would it reduce the risks other than to the valves?

The amp I have in mind is my Quad II Forty, which is nominally [if not actually according to some reports] a 40 watt amp.

More than obviously this will only be considered after the current times pass and we return to normal, assuming I still have a job and so on.

The Q II forty has two KT 88s on the output side, and I find it a rather fine amp on my ESL, but if I could get longer service out of it, that would be good, and certainly the ESL needs less power than the amp can deliver in stock form. I never play loud, so if there were real service life gains, hopefully without reduced quality, I would happily go ahead one day. I would happily live with 20 watts ...

Keep safe and best wishes from George
 
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Hi George,

You could either give Quad a ring or see if you could get a pair of Quad II Classics and sell the Quad Forty.

Cheers

John
 
George J said:
Derating the power of a valve amp?

Is this possible? Would it result in longer output valve service-life? Would it make the actual quality of sound worse, or even possibly better? Would it reduce the risks other than to the valves?

The amp I have in mind is my Quad II Forty, which is nominally [if not actually according to some reports] a 40 watt amp.

More than obviously this will only be considered after the current times pass and we return to normal, assuming I still have a job and so on.

The Q II forty has two KT 88s on the output side, and I find it a rather fine amp on my ESL, but if I could get longer service out of it, that would be good, and certainly the ESL needs less power than the amp can deliver in stock form. I never play loud, so if there were real service life gains, hopefully without reduced quality, I would happily go ahead one day. I would happily live with 20 watts ...

Keep safe and best wishes from George
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Not sure why you are considering de rating the Quad II 40 as 40 watts (probably nearer to 30 watts) is quite low power for a pair of KT88s?
Back in the 1970s/80s it was quite common to get 100 watts from a pair of "proper" GEC KT88s, so 30-40 watts should be very easy on a pair of modern KT88s if the amp is properly designed and made. Andy Grove designed the circuit for this amp so it should be good.
 
You could re-work it to triode strapped push an pull , it would require new bias point depending on OT impedance
 
slawekt said:
You could re-work it to triode strapped push an pull , it would require new bias point depending on OT impedance
Click to expand...

That shouldn't be necessary as that voltage and current draw on that circuit should be well within the valves (KT88s) working spec, which I think is what the OP is alluding too, probably based on web reports of unreliability.
 
G T Audio said:
That shouldn't be necessary as that voltage and current draw on that circuit should be well within the valves (KT88s) working spec, which I think is what the OP is alluding too, probably based on web reports of unreliability.
Click to expand...
I'm not very familiar with Quad circuit but if he is after dropping output power and increasing tubes lifetime I'd go that way .
 
There are numerous failure modes for valves and you cannot tell which one is going to kill any particular valve.
Some you can influence within any one design, some you cannot. For instance, if a valve is destined to fail due to heater failure/changes, you can do nothing to alter that.
 
The Quad II 40 UK model is the same as the European model and 230Vac nominal. Just take the stock Quad amplifier and tune the mains supply voltage using a bucking transformer down to 230V or even 220V. UK mains tends to be on the high side of 240V, which makes the valves run much harder.
A 12V 25VA transformer could drop the supply by about 12V, about right for the UK.
 
David just beat me to it, a bucking transformer is the answer, very easy to implement. Even a 24 Volt tx would be good.

Running 16 Ohm speakers won't help as it's class A anyway.
This also applies to triode strapping, the valve dissipation will be the same so no saving.
 
I'd leave well alone... As others have said, it's not that simple. The bucking transformer method could also be a bad idea as if reducing voltage enough to make any real change to dissipation the heater voltage will be reduced and too low a heater voltage can be as bad as too high. If the amp uses voltage regulation, for heaters or elsewhere, reducing mains could put the regulators into the dropout zone...

Even modern KT88's should be very reliable at only 40W output.
 
Thanks for the replies. Very helpful.

My conclusion, based on them, is that it might be possible, but certainly not worth the trouble.

I don't play loud, so the extra power is not strictly necessary ... but it is not hurting either.

In response to the idea of a different speaker. That is definitely not on. I have had ESLs for ten years now. The one I have now was factory serviced at Quad about four years ago, and is very fine. I asked Quad what amp would be best assuming I could not afford an original [period] Q II. Rob Flain said that the II Forty would be ideal, and sonically he was absolutely right.

This Forum does give good answers, and most helpful on a simple question like this.

Thanks and best wishes from George
 
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I don't necessarily recommend doing this but individual cathode resistors instead of shared would probably improve reliability slightly.
 
I would not reduce the supply too far, certainly not below 220V. This would have been nominal mains in most of Europe, so the amp should work fine with it.
I don't think the Quad had regulated supplies
 
Reducing the plate voltage is likely to dull the sound. Arkless is right that using a bucking transformer is likely to reduce the heater voltage and cause technical problems. As-is you should be able to get a couple of years out of the power valves, before they need changing, which is an easy task to do.

Andrew
 
I admit triode strapping wasn't very good idea as Miller capacitance goes up by fair bit making it bit difficult to drive and as pointed dissipation stays at same level but Patrick Turners' re-work is worth checking out if anyone wants to improve their II forty .
 
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