Ampeg J-20 Bias

[Leevi]

I installed a VVR from Skipzcircuit to Ampeg J-20 and made a mistake.
I forgot to measure the value of the fixed bias voltage before installation. Does anyone know how much
it should be? I'm currently using -40V with full B+. When using lowest B+ the voltage decreases to -65V.
Does this sound correct?

Vari-Watt:
http://www.skipzcircuits.com/VariWattFX.html

Ampeg J-20:
http://www.prowessamplifiers.com/schematics/images/J20_Schem_w_transformer2.pdf

/Leevi

[HotBluePlates]

I'm currently using -40V with full B+. When using lowest B+ the voltage decreases to -65V.
Does this sound correct?

No, not correct.

The kit you linked claims tracking bias, which would hopefully make it a licensed version of London Power's Powerscale circuit. The bias board should track the reduction in B+ so that as the B+ is reduced, the bias gets smaller (not larger). So your bias might reduce to something like -25v a minimum B+.

Check the interconnections to the bias board, and check for proper orientation of your transistors and MOSFETs.

[Leevi]

So your bias might reduce to something like -25v a minimum B+.

The kit seems to work correctly. It was my fault again because I measured the bias voltage
from wrong point. When measuring on the grid the voltage goes close to zero on the lowest B+ which is almost zero then.
There is a trimmer pot on the bias adjusting board where you can set the base value for the bias.
In my case with full B+ the maximum value I can set is about -42V. Hopefully this is OK for Ampeg J-20?

/Leevi

[HotBluePlates]

Sounds reasonable. You could measure the tube plate current to find out for sure.

I don't normally endorse the transformer shunt method, but if you clips the meter leads in place before powering the circuit, you can do it without upsetting the circuit or your nerves.