Biasing a 1966 Fender Bassman

[wrather1]

There is a single bias control for the 1966 Fender Bassman amp for the two 6L6 output tubes. I was told that you should set the bias pot so that the background hum is at a minimum point. I'm worried about overheating an expensive matched pair of 6L6's. Is this procedure correct?

Thanks,

Dave W.

[Albert Kreuzer]

Hi Dave,

if it has the AB165 circuit with the bias balance control, yes, the procedure is correct.
The tubes may still run hot (or cold), so I'd check the bias with a meter (or bias-rite or whatever).
Changing the bias may involve to change the 15k resistor from the bias balance pot to ground.

Cheers,
Albert

[wrather1]

Biasing a 1966 Fender Bassman

Thanks, Albert! Yes, it is a AB165 circuit as I can best determine. The new Electro-Harmonix 6L6's seem like they're running hot as I can detect a smell of hot phenolic after I've played it awhile.

Guess I need to check the current. Is 30mA about right?

DW

[Albert Kreuzer]

Hi Dave,

yes, 30mA per tube is fine. Assuming 450V plate voltage, everything under 38mA that sounds good will be ok.

Cheers,
Albert

[Regis]

Try using this to accurately bias your Bassman.