Power Scaling in Morgan amps?

[shocki]

I have a Badger 18 which uses London Power scaling which works really nicely. I tried a Morgan Marshally kind of amp the other day and liked the scaling even better. Especially at lower volumes. Does anybody know how he does it?

[Alan0354]

What is the London power scaling?

[shocki]

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[Jazz P Bass]

Tube Amp FAQ, Tube Amp Info, Tube Amp Lore

[Alan0354]

Are those power scaling just control the screen grid voltage using a FET source follower? Anyone has schematics? Is it this one:

http://luisamark.com/mark/files/Powe...018%20watt.gif

What is the difference between this ande Morgan?

[Merlinb]

Are those power scaling just control the screen grid voltage using a FET source follower? Anyone has schematics?

Power Scaling usually controls the supply voltage to the whole amp, not just the screens. The schematic you linked to is just one example (not an official "london Power" circuit). Kevin O'Connor has published many of the schematics in his Ultimate Tone series of books. You'd probably be best asking questions relating to it on the Power Scaling forum:
Powerscaling.com • Index page

Morgan amps may be using an official London Power circuit, or they may be using their own proprietary scheme- possible just for the screen grids. Marshall also do screen scaling.

[Alan0354]

So this is just an old idea of variac using transistor source or emitter follower to control the B+. No wonder, I was wondering why having such a big transistor and need heat sink.

Why is this not more popular if it is so good?

[Steve Conner]

I've seen tracking regulator circuits used to make the bias track the screen voltage in fixed bias amps. Cathode bias makes life easier, though.

I remember one Marshall amp that used a pot and a few diodes to simulate grid current clipping on the power tubes. The pot adjusted the clipping level, from 0V right down to the negative bias voltage. Since power tubes generally clip by running into grid current anyway, the effect was similar to power scaling, but with no high voltage power electronics required.

If your amp had a SMPS, variable output voltage could be implemented quite easily.

[Alan0354]

I've seen tracking regulator circuits used to make the bias track the screen voltage in fixed bias amps. Cathode bias makes life easier, though.

I remember one Marshall amp that used a pot and a few diodes to simulate grid current clipping on the power tubes. The pot adjusted the clipping level, from 0V right down to the negative bias voltage. Since power tubes generally clip by running into grid current anyway, the effect was similar to power scaling, but with no high voltage power electronics required.

So this is just an old idea of variac using transistor source or emitter follower to control the B+. No wonder, I was wondering why having such a big transistor and need heat sink..

Yeh, I am still battling with the transformer for the SMPS!!! I have been studying the "Handbook of Transformer design & Application" by Flanagan. Other than they totally forgot to put in the 1/n^2 in the equations, actually it is a well written book.....at least to chapter 4.

Why the power scaling not as common? It works, I did one with variac and extra filament tranny and HV for preamp long time ago. I would still go with the variac just because I don't like to deal with heat sink and high Drain/Collector voltage as the insulator has to be very thin to conduct heat. Reliability should be much better with the good old tried and true variac.

[Steve Conner]

Why is power scaling not common? I think it's all about cost-quality tradeoffs. There seem to be two kinds of amp buyers: people who want cheap amps and people who want expensive vintage-correct ones. Power scaling increases the price and breaks the vintage-correctness, so it doesn't fit either niche.

Matamp used to sell an amp head with a variac built right into it.

[Alan0354]

Fender, Marshall and Peavey don't have it. They cover 70 to 80% of the Market.

[Steve Conner]

This may be controversial, but I also think a properly implemented master volume can give you much of the benefits of power scaling with none of the cost.

Another issue with the HT regulator circuits is how to protect them against tube shorts. It can be done, but the protection components add even more cost and complexity. The circuits I've seen tend to just ignore the issue.

[Alan0354]

This may be controversial, but I also think a properly implemented master volume can give you much of the benefits of power scaling with none of the cost.

Another issue with the HT regulator circuits is how to protect them against tube shorts. It can be done, but the protection components add even more cost and complexity. The circuits I've seen tend to just ignore the issue.

That's why still variac for me. Chances are you blow the fuse before the variac.

Yes, I think from tinkering with the circuit in the last half a year and reading people that know what they are talking here, circuit is a major major part of the sound, not any particular component and design. With a good preamp distortion design, you can go really far. I never thought I would say this before. I am playing with all possibility, I really don't not see a clear winner compare with preamp distortion( master volume), scaling AND even the THD Hotplate at least down to -12dBm ( I think it's like 6.25W or something).

[d95err]

Why is power scaling not common? I think it's all about cost-quality tradeoffs.

I don't think cost is the issue. For a mass-produced amp, the cost of Powerscaling would just be a few cent. The difficulty is in how to market the Powerscaling feature. Your average customer would have no idea of what the difference between Powerscaling and a regular master volume is. Also, your average user is usually more interested in lots of channels and features, which is easier to do with a regular master volume amp (although Kevin O'Connors more advanced circuits can be made switchable).

I think the bigger brands will get around to Powerscaling eventually though. In fact, Marshall is already using a form of Powerscaling on the new Slash and Yngwie signature models.

[Alan0354]

It is not going to be a few cents. It's a pcb and the heat sink that cost money. And then the protection circuit. Like Steve said, you blow the whole thing if the tube shorted. Variac is not cheap plus the separate power supplies. It's not the electronic components, it's the hardware, circuit boards, knob and pots.

AND I agree with Steve. I am experimenting with all options, I sure do not see a clear winner. A good preamp distortion is better than most people expect. Don't use Mesa as a bench mark ( I can't help it!!!).

The new Marshalls sound pretty crappy to me. I have the JCM900, I never know how bad it sound until it blow 3 months ago and I was forced to pull out my Bassman 100 and started designing my own. The old Marshalls sounded very good for hard rock, but still, they are not refine like Fender stuffs. They are kind of a one trick animal. I am just surprised so many Marshalls seen on tv. Is that marketing or real?!!! I particularly dislike the LED diode clipping distortion.

Marshall reliability is legendary!!!! They blow and they blow. I had a Plexi and the JCM900, blow after blow after blow. I am not even sure I want to fix the 900 or gut it and use the chassis and the PT. I like the enclosure, small and sturdy. Maybe buy a Jensen MOD and Bandmaster OT and start designing my own. My Bassman has been used as a breadboard, extra holes for tubes were drilled to the point it is starting to look like Swiss Cheese!!!!