Friedman/Suhr PPIMV

[physics]

Hi!



I was wondering if anyone had any thoughts on the "new" master volume that Friedman and Suhr amps seem to be using, as seen on the Plex and SL68 amps. Bias grid-leaks are replaced with 2M2 resistors, and each resistor gets one gang of a dual gang pot (each gang wired as a rheostat) in parallel with it. As you turn down the pot, you reduce the grid-leak value and load down the phase inverter, or such is my current understanding.

I'll post a sketch after some sleep.

Thanks,
          physics

Edit: sketch
   

[K O'Connor]

Hi Guys

The 2M2 is a safety resistor to guarantee a connection between the bias supply and the tube grid in case of pot wiper discontinuity. Of couarse, if the the pot track is simply wired in parallel with the 2M2 and wiper linked to the top end, then the 2M2 is redundant.

As Physics (and physics) points out, dialing the pot to zero will tie the coupling cap to the bias supply and load the splitter while reducing the drive into the tubes to zero. This overall effect is the same as with the crossline MV inasmuch as drive to the tubes is reduced while the loading on the splitter is increased. In both cases the differential gain of the common Schmitt splitter is reduced and its distortion may rise. When feedback is applied to the power amp, as is the case with high-power amps, it can be difficult to retain a clean sound when the MV is reduced.

[physics]

Thanks for the info Kevin!

Edit: Kevin pointed out the circuit description that follows is confusing. It was meant to describe one of the DC-coupled PPIMV's in TUT4, not the F/S PPIMV. I'm leaving the description as-is to not mess with history/remove context for post #4.

The F/S PPIMV looks similar to the design you put in TUT4, with pot wiper to top of grid leak, the bottom of the pot to -Vb, and the top of the pot to the PI coupling cap. That one reduces the grid-leak to zero as well, but it keeps a relatively high impedance shown to the PI. It also has actual voltage-divider action happening with the pot instead of loading down the PI, if I understand it correctly. Would there be any reason to prefer the TUT4 implementation over the F/S implementation?

Thanks,
physics

[K O'Connor]

Hi Guys

I just lost my entire post...

Physics added the sketch to post-1 after my comments were posted. The sketch shows rheostat-wired pots, contrary to his description in post-3, which describes a standard DC-coupled
post-PI-MV.

For the two standard DC-coupled post-PI-MVs, the version with the wiper as the signal input loads the splitter, where the version with the wiper output does not. Neither effect the bias voltage to the tube. Any splitter that DOES change grid-bias voltage should be avoided.

The AC-coupled post-PI-MV can be wired in the same two ways as above, but is ground-referenced instead of referenced to B-. The two forms behave the same as their DC counterparts.

The design in the sketch has pot-X tied to B- if CW is supposed to be the loudest setting. The 2M2s are redundant.

[physics]

In retrospect, I see how post #3 was confusing. Sorry. To clarify, it was intended to describe one of the DC-coupled designs in TUT4. Here is my description reworked:

The F/S PPIMV looks similar to the design you put in TUT4, which has the pot wiper to top of grid leak, the bottom of the pot to -Vb, and the top of the pot to the PI coupling cap. Both your design and the F/S design are strapped across the grid-leaks.

Thanks for the additional info in post #4 Kevin. To your comment about changing bias voltage being a bad idea, am I correct in saying that the F/S design messes with bias voltage since it varies the grid-leak values, and thus it's not ideal?

Thanks,
physics

[K O'Connor]

Hi Guys

There is no MV design that I know of that will upset the bias condition of the tubes, assuming the tubes are not exhibiting high grid leakage currents - definitely a tube to be discarded anyway.

have fun

[physics]

Got it, thanks! I'll likely try at least one of these PPIMV in my Marshall 4104 as a stop-gap until I can do power scaling, and will post clips for reference.