1969. Bassman Bias

[Indie]

Amp works grat but I have problem cause this model has no bias adjusting pot.....instead of it ....here is bias balance pot......
Reason why I need adjustable bias is cause my NOS Ei 6L6GC tubes have only 7 mA on pin 3 when i measured.
Now I am searching good schematic to solve bias adjust problem.....I know the way Gerald Weber doing it but ,if it is posible, I would reather keep bias balance and add some timer to be in position to operate with bias current....

I have done similar job on my twin......by adding one 47k trimmer and 4,7k resistor in series instead of 33k resistor for fixed bias....and it works well .....I have both......bias balance and bias adjust...
But now I have different circuit on Bassman and ask does anybody know the way to convert it on similar way on bassman...
???

circuit is AC568

[tubeswell]

Yikes, that is one nasty bias circuit!

If you want those tubes to run hotter, you could try tweaking that 15k-to-ground first. I would try decreasing it to say 12k in the first instance, then 10k, etc (JM2CW)

[Indie]

Ty Tubeswell

I would like them run hoter 7mA is to low...
I have 35mA on my twin reverb...

Do you think that adding trimmer instead of 15k is bad idea?

[Gibsonman63]

I wouldn't think it would be a problem. I would pull the power tubes and get my bias voltage close before putting the tubes in. You may want to consider putting a resistor in series with the pot and doing a little math to calculate the resistor and pot size based on your maximum and minimum bias current for the tubes you want to run + a little fudge factor to allow for tolerances.

[dynaman]

Yeah, I'd replace the 15K with a pot and resistor. You'll probably have to increase the value of the 1K before the diode in the bias supply to drop voltage and run the tubes hotter.

[Bruciep07]

Heres a good read from Doug Hoffman's site, may be helpful...

http://www.el34world.com/charts/bias_conversions.htm

[selloutrr]

how about...
something like this. subsitute resistor values for your needs.

This set up allows you to cancel out or double your resistance. while always having a minimum safe bias in the event of a pot failure.

You can easily mount it with an "L" bracket using a factory screw hole keeping your amp stock.