MV + neg feedback mod for Custom Vibrolux

[the russ]

I like my Custom Vibro, but I want two things out of it that it doesn't deliver stock: a master volume control, and switchable negative feedback. The circuit changes necessary to implement such mods are relatively easy. A switch, a potentionmemer, and some resistors.

My question is - can I utilize a push-pull pot of the appropriate value to implement this on a single control? Or is having the preamp signal and the negative feedback signal routed through the same physical part a bad idea?

Would a regular old 1Meg push-pull be appropriate for this circuit? I'd be using a pre-PI MV type as present in the old silverface units.
http://img.thegearpage.net/board/images/misc/progress.gif

[donnyjaguar]

Given the phase of the NFB signal is the same as the input signal, I'd say you have a potential for oscillation - especially with the NFB disabled.

[the russ]

isn't it out of phase with the input signal?

"input" meaning input to the phase inverter

[hasserl]

I like my Custom Vibro, but I want two things out of it that it doesn't deliver stock: a master volume control, and switchable negative feedback. The circuit changes necessary to implement such mods are relatively easy. A switch, a potentionmemer, and some resistors.

My question is - can I utilize a push-pull pot of the appropriate value to implement this on a single control? Or is having the preamp signal and the negative feedback signal routed through the same physical part a bad idea?

Would a regular old 1Meg push-pull be appropriate for this circuit? I'd be using a pre-PI MV type as present in the old silverface units.
http://img.thegearpage.net/board/images/misc/progress.gif


A push-pull pot with an On/Off switch combined with a potentiometer is actually two separate components mated together, The electrical sections of the two components are discrete and there should be no problem with using one.

However, if I could make a suggestion, I'd go with a KF style post PI master, which requires a dual pot. I've not found a dual 250k pot that also has a push-pull switch combined into it, so I'd stick the switch somewhere else. The added benefit you'll get from the Ken Fisher MV vs any kind of pre PI MV or cross line post PI MV is worth the added component. Or leave the nfb switched on all the time.

[Structo]

Are you talking about Local Negative Feedback?
Like you see on V1 sometimes? I like that on a switch my self.

If not, the other is called global negative feedback.
You can use a fixed value but I prefer it on a presence control.

If you have room on the front panel I would put a presence control on it.
Or even on the back.
This controls the negative feedback and also acts as an adjustable brightness control.

[the russ]

I figured I'd make the switch engage a global, fixed negative feedback circuit like what was present on the blackface Fenders. I do like the richness no-negative-feedback provides, so I want to make it switchable.

I would rather not drill or remove any existing controls, so I believe I will actually sacrifice the benefits of a PPIMV for reversibility. I mean, I so *rarely* crank this thing up to where it's pushing the phase inverter anyway and I'm happy with the MV control on pre-PI amps I currently own. I can live without it.

Is a 1M pot the value I want for this purpose?

[hasserl]

It could be. It's really up to you since you're designing it. A higher value pot will load down the signal less but at the sacrifice of fine control; a lower value pot (i.e. 500k or 250k) will load down the signal more, but with better fine control over the volume.