Fisher P-22 6GW8 Circuit

[RudieCntFail]

Another old console followed me home. Seems to be a Fisher Philharmonic P-25, which is the P-22 chassis. I have some Hammond monoblocks amps that run 6GW8 output tubes, and for the price I thought that at least I could score some of those from this Fisher unit. Turns out, they were smoked. So now I have this console in my garage, but it’s in good shape and worth restoring. After replacing all of the paper caps and some electrolytics, checking the rectifying diodes, and looking at voltages I brought it up on the variac with some of my own 6gw8s in it. It sounds pretty good.
These output tubes are getting expensive and I want to make sure that I don’t burn up my stock. Can anyone explain the triode circuit design in this unit to me? Two of the 6gw8s have about a -1v bias cathode to grid, and the other two have no voltage on the grid and 40v on the cathode. Are they simply shutting this section of the tube off?

 

[gadget73]

This is the whole circuit so it makes a little more sense



Using the numbers for the left channel only
V6A is the voltage amp for the power amp section. Input to the grid from the volume control, input to the cathode is the feedback loop. Output from the plate goes to the grid of V7A via C42
V7A is a split load inverter. Grid is biased through R38 tied to the cathode. R38 is 4.7M so its effectively grid leak biased, meaning it generates its own negative voltage grid to cathode.
Inverted output of V7A is plate to V6B
non-inverted output of V7A is cathode to V7B

its a pretty typical setup other than the grid leak part of things. Normally with split load inverters its either direct coupled to the plate of the voltage amp, or there are 2 resistors in series on the cathode and the grid resistor ties to the junction to set the grid voltage slightly lower than cathode voltage

 

[RudieCntFail]

This is the whole circuit so it makes a little more sense

View attachment 3755129

Using the numbers for the left channel only
V6A is the voltage amp for the power amp section. Input to the grid from the volume control, input to the cathode is the feedback loop. Output from the plate goes to the grid of V7A via C42
V7A is a split load inverter. Grid is biased through R38 tied to the cathode. R38 is 4.7M so its effectively grid leak biased, meaning it generates its own negative voltage grid to cathode.
Inverted output of V7A is plate to V6B
non-inverted output of V7A is cathode to V7B

its a pretty typical setup other than the grid leak part of things. Normally with split load inverters its either direct coupled to the plate of the voltage amp, or there are 2 resistors in series on the cathode and the grid resistor ties to the junction to set the grid voltage slightly lower than cathode voltage

A thorough explanation, and much appreciated. I’m still learning tube amps, and have not seen a split load inverter before. I haven’t even heard of grid leak biasing before, either. The 40v cathode to 0 v grid had me concerned, but I’m starting to understand what’s going on here. Thanks!

 

[gadget73]

The grid voltage would be difficult to measure but it would probably end up ballpark 1 volt lower than the cathode voltage.

split load inverters are also sometimes called cathodyne or concertina. I have no idea why it has so many names but a lot of things use it. It has good balance so long as the plate and cathode resistors are equal and it doesn't need many parts.

 

[RudieCntFail]

What started this whole thread and inquiry is that with ground as reference, there was 40v dc on the cathode and 1v dc on the grid. That’s why I was concerned. I’ll have to check again..