Pretty basic questions, but wanted to run this by you guys. This is a Channel Master 6601 integrated amp from about 1962. I have the SAM's but no manual, and expect the biasing adjustment is pretty standard, but don't want to make any assumptions.

Assumption 1: The "balance adjust" pot is my common bias adjustment for all four 6BQ5's.
Question 1: what is the best way to adjust the bias on this amp? I assume I'm looking for a reasonable cathode current under 65ma.

Question 2: For the hum balance adjustment process... volume down, scope across the output (with speaker load attached) and adjust for minimum AC trace?

see second post

R33 indicates 1k @ 2 watts in the parts list.

So - what is the preferred way check/adjust bias on this circuit?

Didn't Sams include a procedure for adjusing those controls? Usually this information is part of the Photofact folder, sometimes on the front page under the description there will be a sentence or two telling how it should be done. I wouldn't want to second guess this one since that is a totally unconventional setup, looks like there isn't any direct way to change output tube bias without altering the circuit. They have the 12AX7 filaments connected in series used as the output tube cathode resistance, with that 1K 2watt resistor in parallel to keep it around 24 volts. One would reasonably assume since the 12AX7's have 150 ma filaments in series, and there is a parallel resistor, that the total cathode current for all four tubes was purposely made to be something higher than 150ma.

If I'm reading the schematic correctly, the preamp tube heaters are part of the cathode bias circuit to give them a DC supply, and the grid returns are through the two Hum Balance controls. It seems to me that the Hum Balance controls are really adjusting grid bias by varying the return between about 18V and 6V.

Am I missing something?

-David

R33 is only part of the bias resistor. It is in parallel with the heaters of V1 and V6, in an arrangement that provides DC for those heaters, without a separate power source, other than the cathode current of the output tubes. The effective cathode resistor is lower than that 1K ohms.

I didn't see anything in the Sam's regarding adjustments.

Schematic:
http://www.oldtubes.net/library/free_manuals/Channel_Master_6601.pdf

The hum comtrols rarely make a differance unless you have very high sensitvity speaker , i would center them and leave well enough alone unless you can hear hum they are fine , the bias has no ajustment . They are using the fiament voltage of the driver tubes to derive the bias voltage .

So if this is not the bias adjustment, what is the purpose of the "Balance Adjust" pot (lower schematic)?

It rasies the filament voltges above ground by the amount of the bias to prevent heater cathode failure on the 6BQ5's , two fixed resistors would have been just as good for the most part .

If you do want to adjust the hum adjust pots disconenct the source from the amp then connect a speaker of the proper impedance to the speaker terminals. You will then connect a voltmeter (DMM or VTVM) to the speaker terminals and adjust the hum balance pot to get the lowest ac voltage indication on your meter.