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.220k inline from treble pot, 1M to ground…
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.
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).If these designs are messing with the bias supply voltage, how are they assuring no crossover distortion as you turn it up or down?
I wish I knew… I thought it was messing with the nfb loop like that but not bias?
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).
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.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.
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 discussionI 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.
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.
I’m more curious about Suhr’s PPIV master design on the latest versions of the SL67 and 68.
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.