friedman post phase master design

7704A said:
Around the 50 minute mark I think Dave states the circuit for it. Sounds like it's what @glpg80 said, if I understand correctly.
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Since Dave says he leaves the bias splitter resistors in then this would be a Fischer Type 1 PPIMV that Dave Bray uses or it could be a Type 3 that uses a single pot.
1717174384451.png
 
I've played a Type 1 Ken Fischer PPIMV and I remember that had more artifacts in the tone than the TYPE 2 Lar/Mar/Fisher. This Type 3 is quite interesting though. I may have to try it out and compare to my Lar/Mar Type 2.
 
From this pic of the PLEX it doesn't seem like there is enough room for a dual 250K pot in parallel? But Dave's description of shrinking the 220K bias splitter resistors would almost have to mean a Type 1 unless the Type 3 also has the effect of shrinking the 220K resistor values.
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Found these pics on another site. Supposedly the Master vol on a Suhr SL68 MkII.
Looks like the pot is shorting the bias splitters.
This would increase the negative bias voltage to the power tubes as you turn it down, gradually cold biasing the tubes and slowly turning them pretty much off. It would also be dumping more AC signal to ground at the same time as you turn it down. I guess that's the "loading the PI" part.
Surprised that this would actually sound good, but what the hell do I know......
Apparently it does

SL68MkII Master vol 1.jpeg


SL68MkII Master vol 2.png
 
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fusedbrain said:
Found these pics on another site. Supposedly the Master vol on a Suhr SL68 MkII.
Looks like the pot is shorting the bias splitters.
This would increase the negative bias voltage to the power tubes as you turn it down, gradually cold biasing the tubes and slowly turning them pretty much off. It would also be dumping more AC signal to ground at the same time as you turn it down. I guess that's the "loading the PI" part.
Surprised that this would actually sound good, but what the hell do I know......
Apparently it does

View attachment 315156

View attachment 315159
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So if the 250K pot is installed in parallel then the signal gain would be getting shunted to ground as GLPG80 explained already which would cause a decrease in volume. The difference being the Lar/Mar PPIMV replaces the 220K bias splitters with the 250K dual pot.

So the Suhr is a kind of modified TYPE 1/TYPE 2 from what I can see.
 
glpg80 said:
No DC current flows in grid bias but the pot shorts the bias splitter when the value is less than the the total resistance in parallel with the pot, which sends more negative voltage to the grid, biasing the amp hard class B, reducing volume. Unless I’m misunderstanding something here that’s exactly what’s going on and why he has to use a dual gang pot.
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So if the 250K dual pot is wired in parallel with the 220K bias splitter resistors then basically what is happening is the signal is being shunted to ground as the dual pot is turned down as as Dave Friedman described it since the 250K dual pot in not wired in series taking place of the 220K bias splitter resistors then if the 220K dual pot goes bad then all that happens is the full signal is sent back through the 220K bias splitters as if the 250K dual pot wasn't there is my rudimentary understanding of his description and yours.:unsure:

As a PPIMV it still messes with your NFB at less than 1/2 full volume and with the Lar/Mar Type 2 PPIMV if the pot goes bad it is like your 220K bias splitter resistors burned up, but those 1 MEG resistors are supposed to act as a fail safe.

THe TYPE 3 Fischer is interesting but I don't know if having the the two out of phases of the signal canceling each other out for volume control would produce a great sounding master without some strange effects, but I kind of want to try it out to compare to my Lar/Mar Type II which has served me well over the years.
 
Most designs used a combination of resistors/pots such that the bias splitter value remained close to the stock 220k
 
I found this evaluation of the friedman post phase master: "Dual 250k pot, fed from bias, each side to Pin 5 of each PT incl 2 x 2M2 Rs in parallel. 2M2 on PCB, close to sockets, no grid Rs. Pretty much a LarMar w/out screen Rs - as far as I can see from top of the board." Then he mentions something about the design is not used as a "divider" but as a "resistor" He says this "The pot is wired like a variable (dual) resistor and not like a (dual) divider". I believe this might be enough info to have it figured out... a variation of the Lar/Mar
 
It seems to be that the 2M2 resistors replace the typical 220k resistors on the board, and the dual 250 pot is wired like on the pictures above.
 
A little detour but, no matter how you slice it, all of these PPIMV designs simply turn the fixed PI output resistors (bias splitters, aka grid leak resistors) into a variable resistor (pot). This is the reason some might notice certain hot-rodded amps sound louder or fuller or more meaty than another. For example, older Ceriatone Chupacabras used 87K, newer ones use 110K, the Yeti uses 120K, Fortin uses 150K, some Marshalls with 6550s used 150K, EL34s were 220K, etc. Care should be used when messing with these resistor values and/or using a PPIMV because the total resistance the tube grid can handle should not be exceeded. This INCLUDES any grid stoppers. Although I doubt anybody is gonna go highter than 220K + grid stoppers.
 
glpg80 said:
Honestly you’ll get better tube life out of his amps so my original statement was a bit extreme but I prefer not to be messing with the bias as a volume taper. The reason is because as tubes age, their bias drifts, and your volume could actually change as the amp warms up which doesn’t happen with normal designs. I don’t think it’s a completely awful design but I do think it leaves a lot of room for improvement.
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I haven't seen the schematic, but if it's in parallel with the splits, I have 2 concerns. I think there's risk of someone not familiar with the design not properly biasing the amp. The amp would need to be baised with the master volume at full. I also see an issue if a pot fail open, the bias splitter resistance would raise causing less -DC voltage to the supply and the tubes would then be biased way too hot. Does anyone actually have the schematic?
 
I am new to amp building and I am still learning but could this Ceriatone JCM52 all accsess PPIMV be the Master Volume you are talking about? Probably not - but would be kinda funny ;-)
IMG_4144.jpeg
 
RealAmpsAreDead said:
... but could this Ceriatone JCM52 all accsess PPIMV be the Master Volume you are talking about?
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I don't think that the Ceriatone's LAR/MAR PPIMV is the new style that's available with the new Friedman Plex. Mainly because Dave Friedman said that his new MV doesn't remove the grid bias feed resistors of the original circuit. But the LAR/MAR style definitely removes them from the board. I'm thinking that the Friedman might be copying the Suhr SL67 and 68 MkII PPIMV. Judging by photos posted by @fusedbrain (post #27), I came up with the following illustration:
Suhr MV.png


I wish the photos from post #27 had a better resolution, or a link to where they came from.

EDIT: Edited my original illustration because it showed the dual pot upside down... :doh:
 
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