Super Reverb AB568 Bias Question

[mikeboone]

I have this amp on the bench and see there is about 20v difference in the bias voltage to each tube, one is at -45 and the other at -25. The net result is about 40mA difference between the two. I looked up the schematic and it's kind of confusing me. It doesn't take much.

Any of you old pros have any insight into this?

I searched the circuit number for other threads and didn't find anything directly addressing this.

[JoeM]

SF with bias balance rather than bias set? When you move the bias pot, does the relative voltage on each tube change? If so you can re-wire to the earlier BF setup.

Check the grid resistors have the same value too.

[mikeboone]

Yes, Joe. The bias change is relative as I sweep thought the range of the bias pot. It just seems like 40mA is a huge disparity.

I am concerned something else is going on that I am not seeing because I am focused on this weirdness.

I changed the tubes with a known good set. I also swapped sockets first to see if the problem followed the tube or the socket.
The resistors are very close to 1500.
The plate voltages are the same.
The big honking 150 ohm resistors hanging off the cathode are close in value.
The 100K resistors are close to spec and not too different form each other.

[JoeM]

OK, I recommend re-wiring the Bias pot so you can set bias voltage, rather than the relative -V on each tube. As it is now, can you get to a setting where the voltages equal? Set that way does the current through the output tubes more or less balance?

[Enzo]

Pull the power tubes out, now just monitor the voltage at pin 5 of each. The schematic for AB568 shows a balance type control. AS I see it, one side will have a steady voltage, and the other side will be variable with the control setting. Is that your case too? Can you adjust them to be the same?

If the difference in voltage exists and cannot be adjusted away, then perhaps you have a leaky coupling cap from the phase inverter.

[mikeboone]

Thanks guys for input. The whole balance thing was new to me. I did just as you suggested, Enzo. I took the tubes out and adjusted it until I got the same voltage. However, I could not get it past -23 leaving me at 75mA per tube. I could however get to about -46 on the wiper. I moved the 100K's into a fixed bias config and landed the -46 in between them as other amps are configured. Now I have them set at 40mA.

Thanks again for the nudge in helping me understand that circuit.

Mike

[Bruce / Mission Amps]

Note the resistor that is grounding the balance pot to chassis ground.... the larger the value, the deeper negative the bias voltage will go... cooling down the tubes.
Also, while you have it apart, replace the bias filter caps... sometimes just a little leaky cap will cause the bias voltage to go closer to zero then negative, creating an underbiased condition of the power tubes.

[mikeboone]

Thanks Bruce...

I talked to the owner and he is OK with it being reconfigured to the conventional fixed bias setup. I already wrote down all the tips you guys gave me in my handy-dandy notebook for future reference.

Mike

[Bruce / Mission Amps]

Thanks Bruce...

I talked to the owner and he is OK with it being reconfigured to the conventional fixed bias setup. I already wrote down all the tips you guys gave me in my handy-dandy notebook for future reference.

Mike

That's good... next time try a little 1/2 W, 25K trim pot, wired as a variable resistor
in series with a 6k8 to 10K 1/2 W resistor..... used in place of the 15K-22K fixed resistor and then you can have both, .... balance and level setting.