Marshall JMP50 PPIMV master volume

[Chuck H]

What you propose with the PI to the wipers should work fine. I've never done this myself but I suggested it to a poster that wanted to use this type of master volume for an amp with cathode bias and a bias wiggle tremolo. He wired the PPIMV like below to keep it from interfering with the tremolo circuit. He reported that it worked just fine. It's going to load the PI outputs quite a bit at low master settings and that might affect the low end, symmetry and the clipped wave form a little when driving the PI hard. But it shouldn't cause the distortion you describe. I think it's possible you had something else wired differently.

[Helmholtz]

I reversed the leads on the dual pot. I put the leads from the phase inverter caps on the pot wipers, and put the leads that go to the output tube grid stoppers on the first lugs of the pot.

This way you are shorting the PI outputs at low MV setting, which makes the PI distort. Wired correctly as voltage divider the loads to the PI remain constant.

[Diablo]

This way you are shorting the PI outputs at low MV setting, which makes the PI distort. Wired correctly as voltage divider the loads to the PI remain constant.

Thanks much for the explanation. I couldn’t see the difference with the pot acting as a voltage divider.

[catalin gramada]

It works pretty well like in post #3. Shielded wires are not necessary but twist wires well. At minimum settings the effect of presence pot will be none.

[Helmholtz]

I put the leads from the phase inverter caps on the pot wipers, and put the leads that go to the output tube grid stoppers on the first lugs of the pot.

There is another effect involved with this kind of MV wiring.
As with turning down the MV the load resistance to the coupling caps strongly decreases, the lower corner frequency shifts from maybe 25Hz to e.g. 120Hz. So bass content reduces at lower settings. May be desirable - or not.

[catalin gramada]

Those MV is not "transparent" and think no one will be. But is great useful tool even for bedroom levels. Put it into max position and restore the amp in factory condition. I installed tens in marshall amps, never get customer disappointed. Build an simple Airbrake atenuator and together with this MV can get any tones of a Marshall can do but at decent practice level.

[Diablo]

There is another effect involved with this kind of MV wiring.
As with turning down the MV the load resistance to the coupling caps strongly decreases, the lower corner frequency shifts from maybe 25Hz to e.g. 120Hz. So bass content reduces at lower settings. May be desirable - or not.

Thanks again for taking a look at the miswired circuit. It’s definitely not the way to hook up the MV control, based on the sonic results at low settings. Some folks claim the Lar/Mar PPIMV isn’t very good in an amp that uses negative feedback, but when wired correctly my conclusion in the 5E8A is that it works well. The same MV also works well in my Dizzy 30, which doesn’t have any negative feedback. So, I would recommend that folks try this style of MV in any push-pull amp circuit.