Will a PPIMV Affect Tone?

[The New Steve H]

Does a PPIMV affect the tone of an amp?

I'm asking because I've installed a few, and it may be my imagination, but there seems to be a small difference in the tone, which decreases as you turn it up to full volume.

[Colossal]

Performance depends specifically on the amp a PPIMV goes in.

With an amp with no feedback loop such as a Vox AC30 or a Rocket, it is quite transparent and useful throughout its range. Amps with a feedback loop such as a Plexi suffer some tonal variation that becomes more noticeable as the PPIMV is turned down because there is less feedback from the output stage going back into the preamp. Some of the tonal change perceived also comes from just not moving the speakers with volume.

[The New Steve H]

I put one in a 5F6A I built, and then I built an amp with a 5F6A front end and the output section of a Rocket. I love the PPIMV, but I can't help thinking it would be nice to be able to bypass it.

It adds no noise whatsoever. That's not the issue.

I stuck a pot on the plate resistor (first stage) of the phase inverter of the Rocket-style amp, so I could play around with the voltage, and it didn't affect the tone a great deal, but it did make a nice volume control.

[iknowjohnny]

Performance depends specifically on the amp a PPIMV goes in.

This^^^ big time. In some amps with cascaded preamps it can sound so bad it's unusable at stage volumes. I would assume they are best done to amps with clean preamps. But whatever the case, they definately work far better in some amps than others.

[Kagliostro]

As far as I can know you must adopt a pair of precautions as to avoid the PPIMV affect the tone

you must follow this schematics

in one you can see that as the capacitors results to be in series and decrease their capacitance you must increase their value

in the other one you see there is an in series resistor (to be chosen experimentally from 33k to 100k), this prevent the interaction between the pot rotation and the second capacitor, otherwise your tone can be affected rotating the pot

K

[sepulchre]

I've got a PPIMV in my cascaded JMP style 50 watter. Yeah, it effects the tone exactly as Colossal describes. Basically, the more you turn the PPIMV down, the less negative feedback you have. When it's all the way up it's transparent.

Personally, when I'm playing quietly (living room along with the stereo) having less feedback is not necessarily a bad thing - it breaks up quicker and not in a terrible way. It doesn't sound the same as when the PPIMV is dimed and the amp is cranked but it does have its uses.

So it does effect the tone; not like a guitar's volume pot - I wouldn't put a treble bleed network on it, but the feedback is there for tonal issues and removing it has an effect. That said, I find that if it's not turned down too much the effect is not too noticeable. 3/4 is hardly noticeable to me and even down closer to half there's not a huge difference. However, remember that I don't mind the change removing some feedback gives so keep that grain of salt in mind.

Like Colossal says: amps with no feedback circuit should have no tonal effect from a PPIMV at all.

[Kagliostro]

May be amps with NFB require to implement an adjustable circuit for the NFB using a pot in series with a lowered in value standard resistor to be used correctly with a PPIMV

K