Different master volume schemes

[trevorus]

Having built a few amps now, I tend to like having a master volume to play with. On my plexi combo, I have a post PI master volume with the coupling caps and everything, and on another plexi build I did a pre PI master volume. The pre PI was quite a bit simpler, and much easier to find parts for without needing the dual gang pot and caps. Which do you prefer, and why?

[Hardtailed]

My Plexi style amp, which is a 1987/2204 hybrid (has a switch that goes from jumpered channels to cascaded channels) has both.
I much prefer the regular MV, as the PPIMV tend to limit bandwidth a bit.

I thought it had to do with the reduction of NFB that goes with it, but I've since started using the amp with no NFB at all (I like it like that!) and I still prefer the regular MV.

The PPIMV is the one known as "LarMar" on the web (with dual ganged pot connected to the bias supply)

That said, the problem is that in Plexi mode, gain is really limited unless maxing the MV and using the PPIMV. But that's only a problem if you need that much gain.

[daz]

No matter which amp i put one in, a PPIMV always sounds horrible for me. I never did figure out why for me only, but the PI distorts horribly, even with the exact same values as a stock marshall or even if i remove NFB, or you name it.....nothing helped. I thought i might have figured out why, but i'm not so sure anymore. High gain preamp drives PI too hard. But i think i may have put a voltage divider at the end of the gain stages and may have even removed one stage to lower the gain a lot just to see, and it still did that. I RECALL trying those things, but not sure tho....too long ago that i abandoned the PPIMV to remember for sure if i tried that. But in any case a PPIMV has always radically altered the tone into a thin ratty/fuzzy unusable tone reminiscent of the worse 70's SS amp distortion you ever head.

[pdf64]

The dual pot PPIMV is my preference.
When playing an amp with it, as it's turned down from '10', it's pretty much impossible to tell at what point the power tubes stop overdriving - the tonal balance and overdrive characteristics stay consistant.
The main difference is that at lower levels, the dynamic response doesn't get the compression effect from power supply sag.
However, it can't do that scooped mids thing, due to there being significant overdrive after the tone controls/loss of presence boost, so it may suit a BF Fender configuration better than a JCM800

[anson]

I think dual pot PPIMV's can sound great, but they cut out global NFB at low volumes, so the amps changes a bit at different volume settings. I prefer to have the presence knob useful at all volumes, and to not have the PI overdriven much, so I've now got a MV just before the PI, with an attenuated signal coming from a split load.

[trevorus]

I just recently switched my Plexi combo over to prePI master, and I think it really opened up the amp. I like the tone of the NFB working at all times the way it should.