Power Scaling.

tekbow

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.

sw67

I have it on my MJW - good solution. Using the gain / boost / power scaling / volume i can get pretty much the same sound at home and gig levels. Search MJW roadstar on youtube - mine is the blue roadstar combo

tekbow

@sw67 , yeah I saw MJW used it, and I've seen it on a couple of other hand built amps like Rodenberg, but less so on high end production amps. I think either the Marshall Yngwie or AFD had it, some of the H&K amps use it.

Is the MJW system continuous or fixed values? Kruse amps have it but it reduces in fixed increments.

If it's as good as it seems to be and gets reasonably authentic sounds, I just don't see why companies bother with half power and pentode/tripe switching any more.

p90fool

It's one solution amongst many, like attenuators and conventional master volumes. 

None of them sound exactly like each other, so power scaling doesn't make them redundant. 

sw67

My amp has a London power scaling module ( i think ) - just like a volume knob so zero to off and anything in between

tekbow

@p90fool, well, the idea isn't so much as how they sound relative to each other, it's more which sounds closest to a cranked amp, excepting speaker interaction obviously.

So an attenuator will be the real deal, but you'll cook your tubes quicker plus attenuators *can* kill high end. A master volume isn't really running the tubes at breakup levels unless you're actually running the amp loud, a half power switch is still really loud from 100 down to 50w assuming it's straight switching out a pair of tubes. If you already have a 50w amp, there's nowhere to go unless you start using Pentode triodes switching, which I don't think sounds that great, or you can use something like TAD tonebone converters with EL84's that convert your amp to cathode bias, but because it's EL84's will get darker the more you push them.

Whereas with the power scaling, you're actually reducing the voltage on the tubes (as is my understanding, I may be wrong) leading to a variac type effect as made famous by EVH where you're clipping the output tubes sooner because of said lower voltage.

It begs the question as to whether this is the most "authentic" as compared to just cranking the amp, but from @sw67 experience, it would seem so.

Btw SW67, does yours have the little tone control I see on some of the London Power kits? It may have sone weird name like frequency compensation or summat.

ICBM

tekbow said:

Why don't we see this more often as opposed to, say, use of attenuators?

Because it requires extensive modification of the amp (if not built with it), can't be used with more than one amp (unlike an attenuator), doesn't give the full dynamic response of the power supply being pushed hard as well as the power stage (unlike an attenuator), and doesn't sound as good as an attenuator or a good master volume (in my opinion!).

tekbow said:

If it's as good as it seems to be and gets reasonably authentic sounds, I just don't see why companies bother with half power and pentode/tripe switching any more.

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.

tekbow

@ICBM

Ok, good answers. What do you mean by Good Master Volume, because what I'm under the impression a Master volume as generally implemented is, and what you call a good Master volume is (you would know and I defer to you) may be 2 completely different things.

And yes, the autocorrect on my latest phone constantly fucks with me. It's favourites are "cone" instead of come and "sane" instead of same.

ICBM

tekbow said:

Ok, good answers. What do you mean by Good Master Volume, because what I'm under the impression a Master volume as generally implemented is, and what you call a good Master volume is (you would know and I defer to you) may be 2 completely different things.

A good master volume is simply one that works well, and gives a good volume reduction without a large tone change, at least until very low volume. Many amps have them, some don't. They won't ever give the sound of the power section being pushed because without that, there's no additional load on the power supply - but the same is true of power scaling.

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.

olafgarten

tekbow said:

@ICBM

Ok, good answers. What do you mean by Good Master Volume, because what I'm under the impression a Master volume as generally implemented is, and what you call a good Master volume is (you would know and I defer to you) may be 2 completely different things.

And yes, the autocorrect on my latest phone constantly fucks with me. It's favourites are "cone" instead of come and "sane" instead of same.

By good master volume, he means one with a useful taper. 

thermionic

I’ve built two amps with voltage scaling as part of the design, and I find it very useful. There’s an obvious restriction with my 5W single-ended amp in that if you have the preamp gain and master volume maxed and the voltage scaling very low the preamp valve is on full voltage and at its max gain, feeding into a power amp valve that’s running on about a tenth of its normal voltage. This really overloads the power valve and the resulting distortion isn’t pretty. You can overcome this easily by using a combination of preamp gain, master volume, and voltage scaling. Interestingly, my 18-watt amp doesn’t seem to do this, possibly because the phase inverter effectively buffers the signal, I dunno...

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.

tekbow

@thermionic, I'm presuming, but not certain, that the drive control pot in the London Power kits scales the voltage in the preamp.

It certainly does something to the preamp.

p90fool

In my experience I tend to prefer the sound of everything being pushed "a bit". 

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.

martinw

tekbow said:.

Is the MJW system continuous or fixed values? Kruse amps have it but it reduces in fixed increments.

Continuously variable, down to about 0.1 Watt.

tekbow said:

@thermionic, I'm presuming, but not certain, that the drive control pot in the London Power kits scales the voltage in the preamp.

It certainly does something to the preamp.

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.

Lebarque

The variable attenuation control (VAC) on my Lazy J doesn't sound as good as my Swart Night Light attenuator. The Tone King Iron Man sounds good too. Anyone know how that works?

crunchman

I haven't tried all the attenutuators on the market, but in my experience, power scaling sounds better than the attenuators I've had experience of.

It's also easier, as it's all built into the amp, so you don't have to faff around with extra cables to plug an attenuator in.

ICBM

Lebarque said:

The variable attenuation control (VAC) on my Lazy J doesn't sound as good as my Swart Night Light attenuator. The Tone King Iron Man sounds good too. Anyone know how that works?

It's an attenuator, which claims to be reactive - although with most of these, there is still a significant resistive element.

tekbow

@martinw , thanks for that, much appreciated.

Just how invasive is it to install in an amp that it wasn't built in to? Something like a Soldano HR50+ with 2 channels, separate channel masters and shared EQ? Would that require 2 drive controls or one?

And if I may ask one more question, what's the purpose of the RBX bias control?, why wouldn't the on-board biasing be adequate?

The way you're describing it's use is how I envisioned it, I.e. find a level of power amp effect I'm happy with (the point where the amp "opens up" perhaps) then use the scaling to bring that down to a manageable volume level.

sweepy

I love the principle but I’ve never found one that actually sounds any good  

normula1

I power scaled the output section of my Laney Lionheart but didn't like the result as much as a simple l-pad attenuator mounted in the back. I have fitted the same unit to the stereo power amp I use where it's really effective at reducing the power down just enough to give a little bit of compression. I don't tend to run it much less than 40% though as I'm not trying to get the output valves to distort.

jpfamps

ICBM said:

Lebarque said:

The variable attenuation control (VAC) on my Lazy J doesn't sound as good as my Swart Night Light attenuator. The Tone King Iron Man sounds good too. Anyone know how that works?

It's an attenuator, which claims to be reactive - although with most of these, there is still a significant resistive element.

There has to be a significant resistive element other wise you wouldn't dissipate much power in the attenuator.....