MV, Powerscaling in 50W, 100W Amps

[shocki]

I know powerscaling works great in 18W-30W amps. I have not seen this alot in bigger amps though. Did anybody try this in Plexi Marshalls for example? Most of the Marshalls use PPIMV it seems.

[soundguruman]

A variac is used to drop the line voltage to 115AC..a power attenuator is used to lower the output 6db...
The line voltage should maintain 6 volts on the heaters. Any lower and it tends to shorten the tube life.
I have used diode stacks to lower the voltages. Of course when you lower voltages, you gotta readjust bias circuits...
and anyhow, a lot of players are changing pentodes over to semi / quasi triode output operation, for lower volume, better saturation, and they like the way it sounds.
You can see boogie has been making a lot of pentode / triode outputs like that....
But again, I think it shortens the life of the tube to run it more class a triode. It does sound better, probably.

[mfreqmaster]

I have a home built JCM800 2204 clone where I've done the 50W-25W triode switch mod. Sound is a little different. I actually like the full pentode sound of the EL-34s, seems to have a fuller sound, but the triode sound isn't bad. It's also loud as hell. It doesn't feel like half power though, maybe that's just psychoacoustics in action.

[soundguruman]

I have a home built JCM800 2204 clone where I've done the 50W-25W triode switch mod. Sound is a little different. I actually like the full pentode sound of the EL-34s, seems to have a fuller sound, but the triode sound isn't bad. It's also loud as hell. It doesn't feel like half power though, maybe that's just psychoacoustics in action.

The thing is, when you knock a 100W head down to 1/2 power....
The difference in volume is almost non-detectable. (you will see...........you will see!)
That's where your power attenuator comes in sooooo handy.
That's just the way it works out. What you want is to reduce it to like, 35 watts.

[GainFreak]

According to a lot of people it works in 50 and 100W amps as well.

[Tage]

I think the reason power scaling, that is scaling beck the amplifiers B+, works in smaller amps, read sub 30watt, is because they are mostly cathode biased. Thus when the amps B+ or power amp B+ to be specific is scaled the power tube Bias adjusts to that new voltage, sorta self biasing.

SGM mentioned a variac, well that scales everything and so it would work, but it also scales down the preamp voltages as well, not to mention the heaters. I don't think the OP was referring to Variac power scaling, nevermind an attenuator.

When you get into most amps that are 50W and up you now have fixed bias, the fixed bias would need to be scaled along with the B+. this adds complxity to the system and so takes it beyond the relm of most DIY and most smaller scale amp manufacturers. But if you look at soe of the big brands they have 10W heads that will scale down to 1W. Just head over to London Power and order the applicable TUT volumes with info on the subject and you will get a much much much more in depth examination of Power scaling, or get Merlin B's Power amp Design book, lots in there too.

Hope this helps clear some of the mud without adding to much more.

[jpfamps]

I think the main issues hear are (assuming you are reducing power by reducing the HT) firstly, in a larger amp you need to dissipate more power in the devices controlling the HT, which may mean heat sinking, fan etc.

Secondly, I would think most people would consider a lower powered amp to be better starting point for power reduction; it would seem perverse to build a 100W amp and then run it at 1W or so.

[Gingertube]

I Powerscale a Quad 6V6 Amp, there is absolutely no reason why you can't power scale an amp of ANY size. Cathode bias is easy, just provide a B+ regulator, fixed bias is a little trickier in that you need a tracking bias supply regulator.
In both cases having scaled down the output section, it can be more easily overdriven by the preamp and so a "Drive Compensation" control is required to be used inconjunction with the powerscale control to determine the relative levels of pre and power amp overdrive. This can be Part automated by also scaling the diff splitter supply BUt is usually done using either the existing master volume control or an added X-Line or Dual Gang Master after the PI.

How much extra power you need to dissipate in the control device of teh B+ reg depends upon whether you feed in the filtered DC or if you feed in the half wave recified waveform straight off the rectifierand use the pass device (MOSFET) as a straight switch, either works well. KOC has Kits for both options.

Here is the simple PSK1, Note I have put the switching MOSFET on that piece of Aluminium Channel for adequate heatsinking (not yet wire up in this picture),

Cheers,
Ian

Edit:
The transformer is a 115V Ac to 12V AC reverse connected across a 6V3 Heater winding on the main power tranny. When rectifying and filtering of the "original" 115V AC primary (which is now a 115/2 V AC secondary) we get about a -75V DC Raw Bias Supply. That goes to a tracking regulator so that teh Bias Supply Tracks the B+.
The couple of components sticking up from the board are mods I did to incorporate a SAG/COMPRESSION control in the B+ reg to emulate a tube rectifier SAG.

[shocki]

Gingertube. Is this some come kind of ready made circuit board?

[GainFreak]

When you get into most amps that are 50W and up you now have fixed bias, the fixed bias would need to be scaled along with the B+. this adds complxity to the system and so takes it beyond the relm of most DIY and most smaller scale amp manufacturers.

There's nothing complex about that. You just nee to scale down B+ and bias supply simultaneously using a stereo pot and you'll need a heatsink.
Also as already mentioned you'll need to install a "drive compensation" pot in case you don't scale down the preamp B+ (I wouldn't).

[Diablo]

I think this is what you're looking for.

Page Title

[soundmasterg]

Power scaling is something that Kevin O'Connor came up with years ago and a bunch of places have copied it. His earlier PSK style stuff is basically what VVR and lots of others are. He has a newer style of power scaling that works in a bit better way such as his SB or SF kits. He talks about all of this in his books TUT4 and TUT6 especially and has kits and things on his website. VVR only work sup to 50 watts whereas power scaling from O' Connor works up to hundreds of watts. Check it out... Tube Amp Kits, Tube Amp Books, Tube Amplifiers by London Power

Greg

[Gingertube]

Yes,
Several variations now available from here:
Power Scaling Kits - Tube Amp Kits, Tube Amp Books from London Power
Cheers,
Ian

[FredB]

I put LP Power Scaling with sag control into a cathode biased 50W 2 channel amp that I built. It works great.

The power scaling and sag circuitry is on the left side of the upper middle board.

[GainFreak]

Powerscaling by Marshall found in their AFD100 amp:

AFD100 Main PCB.pdf

It's digitally controlled self bias circuit for each tube. Only screen voltage is variable (which was suggested by Steve in another thread) in 21 steps.