VVR on a JTM45 style amp

[KX36]

I'm planning to build an amp very similar to the JTM45 (at least for the phase inverter and KT66 poweramp), but it will have a VVR/Power scaling circuit. I was wondering what people's opinion was on whether this amp would sound better (when the power's turned down) with just the poweramp on the control or if I should include the phase inverter too. I'm not doing the rest of the preamp.

I'm not trying to change the tone, or to get a "brown sound" Variac type tone, just to be able to get good JTM45 overdriven tones at lower volumes for recording/practice while still being able to turn up the power for bigger gigs.

I think the answer would have to do with whether that classic tone comes more from PI or PA clipping, and I'm sure it's also interwoven a lot with the pre-PI vs post-PI master volume arguments, but I welcome the opinion of anyone who has experience with VVR on similar amps.

Cheers,
Matt

[martin manning]

The PI is part of the power amp. If you reduce the voltage on the output stage alone and let the PI signal swing stay as it is you will get a lot of clipping in the power stage. If you are after reduced power with the same tone, that won't be it. The same issue might arise if you include the PI, as the signal from the preamp will be able to cause the PI to clip harder as its supply voltrage is reduced. To prevent that, you'd have to turn the gain down on the preamp, and that will change the tone. Dialing the whole B+ string down keeps all of these things more or less in balance.

There have been many good reports on using the Lar-Mar type PPIMV to dial these things down in a transparent way. Simple and inexpensive, it's no different from the original circuit when turned up all the way.

[KX36]

Whichever way I go, there will be a volume control before the first stage with the lowered voltage. I.e. pre PI MV if PI is included, post PI MV if only the KT66s are included. Although the question is not as simple as the standard pre vs post PI MV debate. PPIMV alone without VVR will only retain the tone without the volume if the tone you like comes more from the PI clipping than the PA. If that's the case, that does lean one way for my original question.

[John_P_WI]

IMHO you will get the most consistent tone through the entire range IF you scale the power amp and use a PPIMV or other MV as a drive compensation. Remember as you dial down the power tubes their signal swing becomes smaller, therefore the signal driving them needs to become smaller. Much of the dirt of the old Marshalls is from the PI, the PPIMV will retain this, only messing with your feedback around the power amp as it is dialed down.