Jeff Seal,Lew... Marshall PIMV Pot Question,Concerns?

[STRATDELUXER97]

I'm using the PIMV #3 like Ken Fischer showed in his Trainwreck Pages...This would be the Phase Inverter MV that uses either the 100k or a 250K dual ganged pot and removing the "splitter" resistors from the board...

My question is regarding the balance of the pot's both sides and the effect it has on the bias and the amp as a whole?

I measure this one dual ganged pot and it's very close when at it's maximum resistance reading on both sides of the pot of 240K.....

However....As you measure both sides of the pot upon rotation,these values are quite a bit away from each other...

Is this ok and is it the norm? Is this something to worry about as far as what the phase inverter and the bias circuit see in my NMV 50 watt Marshall?

The MV is cool but am I safer and better off just sticking with the matched 220K splitter/bias resistors instead of the pot?

I'd love some info on these findings and whether it makes a difference? Thanks guys....

 

[STRATDELUXER97]

Re: Jeff Seal,Lew... Marshall PIMV Pot Question,Concerns?

Bump for Jeff

 

[Jeff Seal]

Re: Jeff Seal,Lew... Marshall PIMV Pot Question,Concerns?

Hey bro... The method you're describing does work but it has some drawbacks. The bias will have to be set with the pot all the way up, since it will shift the bias voltage lower when the pot is decreased. Although, inherently not detrimental, the imbalances you describe in the pot can be.

A much better method would be to add an additional pair of coupling caps and simply use one 1m pot between the legs of those. This way bias doesn't shift, and what attenuates the signal is simply "phasing" the signal to itself since the PI has split the signal into opposing phases. So basically, add another .022 between the plate side of the the PI and the existing .022 in series....so you should end up with 4 total. Where the two caps meet from one side run a wire to the wiper of a 1m pot and do the same with the other but attach the wire to the pot on the outer lug so that there is no resistance between the wiper and the lug when the pot is turned fully CCW.

Like so..... forgive the "paintbrush" drawing but it works...

hope it helped bro....

Jeff Seal

 

[Lewguitar]

Re: Jeff Seal,Lew... Marshall PIMV Pot Question,Concerns?

Sounds like Jeff answered that question way better than I could!

But I will add that I tried all of the master volume mods described in the Trainwreck Pages and none satisfied me.

Lew

 

[STRATDELUXER97]

Re: Jeff Seal,Lew... Marshall PIMV Pot Question,Concerns?

Sounds like Jeff answered that question way better than I could!

But I will add that I tried all of the master volume mods described in the Trainwreck Pages and none satisfied me.

Lew

Thanks Jeff and Lew.....I'm kind of agreeing and maybe I'll just go back to the 220K splitters and pedals....This type of PIMV mod tends to be a bit "splatty" to me and it sometimes adds this crappy harshness over notes that I don't like...

I'll study the drawing Jeff.....Appreciate the time brother!

 

[STRATDELUXER97]

Re: Jeff Seal,Lew... Marshall PIMV Pot Question,Concerns?

Hey bro... The method you're describing does work but it has some drawbacks. The bias will have to be set with the pot all the way up, since it will shift the bias voltage lower when the pot is decreased. Although, inherently not detrimental, the imbalances you describe in the pot can be.

A much better method would be to add an additional pair of coupling caps and simply use one 1m pot between the legs of those. This way bias doesn't shift, and what attenuates the signal is simply "phasing" the signal to itself since the PI has split the signal into opposing phases. So basically, add another .022 between the plate side of the the PI and the existing .022 in series....so you should end up with 4 total. Where the two caps meet from one side run a wire to the wiper of a 1m pot and do the same with the other but attach the wire to the pot on the outer lug so that there is no resistance between the wiper and the lug when the pot is turned fully CCW.

Like so..... forgive the "paintbrush" drawing but it works...

hope it helped bro....

Jeff Seal

Jeff It's cool to have the PIMV but not at all a necessity in my case...I Knew about setting the bias with the pot on 10,but other than the drawbacks you mention,is your method tonally better is what I'm asking buddy?