Master Volume on Champ-Style Circuit

[The New Steve H]

Is there any reason I can't install a PPIMV-style pot between the preamp plate and the power tube grid on an amp with a 5F1-type circuit?

[Colossal]

Well, no phase inverter in a Champ, but I know what you mean...post preamp master, basically varying the grid leak on the power tube. Nope, no reason you can't add one. It's been done before; check out the Trinity Tramp for example (kind of a hot rodded Champ variant). It may or may not float your boat sonically, but sure you can do it.

[Firestorm]

Just replace the 220K grid load with a 220K-250K pot and you're there. Wiper goes to the grid stopper (if you have one).

[roberto]

You can also change the 220k to a 500k log pot if you want a little bit more gainy amp.

[The New Steve H]

I'm still fiddling with the 6BM8 Bassman I built, but the one feature that has me sold is the PPIMV. I ordered parts to put one on my 5F6A.

[John_P_WI]

You can also change the 220k to a 500k log pot if you want a little bit more gainy amp.

Depending on which tube you use and circuit, be careful to observe the maximum grid to ground resistance values. Not an issue when the master is turned down, but may be an issue at maximum volume when the resistance of the master pot and output tube grid-to-ground is 500 k ohm.

[roberto]

With a 6V6 champ it isn't a problem, with a EL84 you can go up to 1M. With EL34 and 6L6 you have to stay aroung a 250k pot, better with a 2M2 from wiper to ground.

[katopan]

Just be aware it'll be inside the feedback loop so as you turn the new master down the feedback will become less effective. That may or may not be a positive thing depending on what you like.

[The New Steve H]

The amp I want do put this in isn't QUITE a Champ. It's the Herzog attempt I made last year. It has no negative feedback loop.

Still, I'm trying to wrap my head around the difference it would make. The volume would be reduced, so the total feedback would be reduced...I guess that with the preamp cranked up and the output tubes at lower volume, the end effect is less negative feedback, just as you say.

The amp I just built has a 5F6A-style presence control instead of a plain old 22K resistor. The presence seems to work very well in spite of the PPIMV. I wonder if that's because there isn't a whole lot of gain in the PI.

[rp]

Personally I find MVs on 5w tube amps pointless, but that's just me. There are simply are no good tube bedroom amps, if that's your goal - they are all still too loud for righteous distortion, choke them way down with an MV and they sound SS. Just buy a (used) small SS amp if you want low volume versatile sounds, SS is great for that and more dynamic to me than a throttled little tube amp.