lespaul6
Well-known member
Does anyone have a schematic for this? I would like to try it out in my 1959hw.
M
mixn4him
Active member
220k inline from treble pot, 1M to ground…
SpiderWars
Well-known member
I think that’s the pre-phase inverter MV. The post-phase inverter MV is just a dual 250k pot but leave the 220k bias splitters. So each of the dual pots is in parallel with each bias splitter.
It’s just like other post-phase inverter MVs except it keeps the 220k bias splitters and uses 250k dual pot. Most designs used a combination of resistors/pots such that the bias splitter value remained close to the stock 220k, for example the LarMar used a dual 250k pot but used 2M2 resistors from wiper to ‘ground’ (‘ground’ is the bias supply here). Or you could use a dual 1M pot and leave the 220k.
For some reason the Friedman MV sounds a little better when turned down. Not sure why.
glpg80
Well-known member
It’s because the shunt input impedance has a maximum tolerance for the power tubes and is typically 1M. The tubes are happier and there’s also less white and shot noise being amplified.
glpg80
Well-known member
If these designs are messing with the bias supply voltage, how are they assuring no crossover distortion as you turn it up or down?
lespaul6
Well-known member
I wish I knew… I thought it was messing with the nfb loop like that but not bias?
SpiderWars
Well-known member
I don't think they are messing with the -DC bias voltage, that stays pretty much the same. The bias splitter resistance is at both ends of the pot so it doesn't change. It's just the AC signal that is getting shunted to 'ground' (which is the filtered bias supply).
glpg80
Well-known member
Nope. The 220k are apart of the grid bias that sets idle plate current. Friedman’s design just turns your amp into a hard improperly biased bi-phase class B amplifier and gradually becomes class A/B as you turn it up and then depending on where bias is initially set, could become hard class A. Honestly I think that design is total shit if he is in fact adjusting the bias like that and calling it a volume pot.
glpg80
Well-known member
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.
fusedbrain
Well-known member
I was wondering if he has the PI coupling caps connected to the wipers of the dual gang pot.
That would keep the effective bias splitter value constant at 117k and the bias stable.
Dave has mentioned that when you turn the master down, it loads down the PI.
That would keep the effective bias splitter value constant at 117k and the bias stable.
Dave has mentioned that when you turn the master down, it loads down the PI.
SpiderWars
Well-known member
I just assumed they were connecting the pot end lugs where the old bias splitters were and then running just the signal to the wipers. So the pot is kind of backwards than the typical schematic.
EDIT: yeah @fusedbrain said essentially same.
lespaul6
Well-known member
I thought it was something like this but, of course, I could be persuaded by either argument not knowing enough about it honestly... very good discussion
napalmdeath
Well-known member
7704A
Well-known member
Around the 50 minute mark I think Dave states the circuit for it. Sounds like it's what @glpg80 said, if I understand correctly.
glpg80
Well-known member
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.
M
mixn4him
Active member
Any pics of this design yet?
psychodave
Well-known member
I’m more curious about Suhr’s PPIV master design on the latest versions of the SL67 and 68.
J
jski59
Active member
In a discussion about the plex, John replied on TGP that both master designs were basically similar. (See below)
Yes, basically it is causing the phase inverter to distort very similar to the power tubes. Dave and I are basically doing the same thing, it really works nicely and IMO negates the need for more drastic measures to bring the volume down.Saw a video where Dave mentioned something about bias resistors…is that closer
Not that I know what that even means.
syscokid
Member
I'm very impressed with the demos I've seen and heard of the Plex's master volume performance. It has become an obsession with me to find out more about this MV circuit. This has led me to the world of the Suhr SL Mk2's. Since the the Suhr's are built more traditionally, I keep hoping to one day see a good gutshot pic of any of the Mk2's. It would be much easier for me to understand the circuit by viewing the Suhr than the Friedman for obvious reasons.
H
harddriver
Well-known member
Here is a Suhr SL68 MK1. It's basically a Lar/Mar using a PEC pot that George Metropolous started using years ago with the Lar/Mar PPIMV. I've compared the PEC pot to an Alpha and the PEC has less artifacts in the tone making it less grainy at lower volumes. The Lar/Mar still neuters the NFB at volumes less than 1/4 to 1/2 up but using the PEC pot does improves things.
What Dave Friedman and John Suhr have done to evolve the design is unknown to me other than what you guys are discussing in this thread.
What Dave Friedman and John Suhr have done to evolve the design is unknown to me other than what you guys are discussing in this thread.
