Why don't we see this more often as opposed to, say, use of attenuators?
I'm thinking specifically of the London Power Powescaling modules, which I understand have some form of tone control on them as well.
I'm unqualified to comment on the technical details but they've been recommended to me by a number of people.
My limited understanding is they reduce the plate voltage on the power tubes achieving a kind of variac effect (although not the voltage reduction to the whole amp including input and output transformers), I.e. the tubes will break up at lower volumes, and because voltage is reduced, tube life is extended.
So are there any considerations I'm not aware of. It seems like a great idea.
Comments
None of them sound exactly like each other, so power scaling doesn't make them redundant.
Because half power (if it's just turning off one pair of valves), low power (operating a push-pull amp single-ended, eg Mesa), and pentode/triode are far simpler and cheaper to implement, and the first two sound much better.
But your autocorrect typo for triode is true .
I know some people do like power scaling, and in theory it should do what it says, but for me it's the worst of all the options in practice - expensive, invasive, restricted to one amp and the worst-sounding.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Whenever I've tried an amp with both, it always sounds better with the power scaling set to maximum and the master volume turned down, not the other way round or any other combination.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
By good master volume, he means one with a useful taper.
You can of course voltage-scale the preamp valves as well as the power amp valves, I don’t know why this isn’t done more often, as it seems to me a more logical approach to scale all the voltages.
For example, diming the master volume into an attenuator rarely sounds natural to me, and neither does a preamp gain on full into a clean power amp.
For me, the magic happens when you have both of them working quite hard, but neither flat out.
I would assume the same with power scaling, but my experience of them is limited to one I really didn't like, because it didn't react very naturally to guitar volume changes, or at least not as well as a loud amp.
Continuously variable, down to about 0.1 Watt.
No. It depends on who installs it, but when I do it, I prefer to leave the preamp unscaled, i.e. running at full system voltage. The Drive control (usually, but not always, a PPIMV) restricts the size of the signal fed to the output stage in order not to 'over' overdrive it. Use of this control to restore the level of drive is important.
Single knob voltage control systems scale the whole amp and, IMO, don't sound very good below a certain point. Some builders restrict the range of the VVR system in order to sidestep this.
Like an attenuator, I think Power Scaling works well with use of a master volume (in this case, the Drive control) and in my opinion, it sounds better than an attenuator, when correctly installed and correctly set up. (Which isn't hard). IMO, the effect of a pushed power supply isn't essential if what you're after is a little output valve compression to add into the mix. Its light, doesn't require an external box to carry around, not expensive compared with a decent attenuator, doesn't cause any extra stress on the output valves or shorten their lives, and in fact may lengthen it.
It's one option, take it or leave it. I think it works well, but there are other options.
By the way, I demo'd Power Scaling for Pete Thompson, design consultant at Celestion with a view to building a test rig for their speakers. I even went as far as coming up with a proposal for the test rig, which would have included multiple preamps driving multiple Power Scaled power amps, all switchable and with the tester able to footswitch between all the common types of amp architectures. He was very impressed with it's dynamic response, feel and sound at reduced volumes, which was the whole point of it as he was interested in the interaction between power amp and speaker, whilst not exceeding safe sound levels. Unfortunately, the funding was never approved by the guys in China, despite being submitted twice and Pete pushing very hard for it.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein