Need help changing an auto bias circuit to adjustable Crate BV50

[gtrplayr1976]

Hey folks . I have posted in another thread about a hum issue that seems to have been resolved. My problem now is the bias is too cold . It's auto bias ,but it's topping out at 20ma +/- 1. WAY too cold. Plate voltage is 463 ,and I'm using KT66s. I would like to change this to adjustable ,but I need help .
Any help much appreciated. Schematics attached.

Thanks

[pontiacpete]

I'm no expert, but I would try putting it either in place of R80 or R103. YOu could try paralleling a resistor at these points and see which one gives you the current changes.

[gtrplayr1976]

I'm no expert, but I would try putting it either in place of R80 or R103. YOu could try paralleling a resistor at these points and see which one gives you the current changes.

Thanks pontiacpete. I'll give that a try. If I could get the current in range of the KT66, I wouldn't mind just adding a resistor instead of a trim pot.

[vavlehead]

R103 is just a bleeding resistor across one of the bias voltage power supply caps.
But that voltage seems to also feed an op-amp... not a simple install it seems.

[gtrplayr1976]

Just my luck Thanks valvehead .

[gtrplayr1976]

I'm no expert, but I would try putting it either in place of R80 or R103. YOu could try paralleling a resistor at these points and see which one gives you the current changes.

pontiacpete..it is R80 . I just got the same info from Michael at Eurotubes , so that's where I'll make the change.

[pontiacpete]

That's good to hear you are on the right track now.

[gtrplayr1976]

We'll see...

[gtrplayr1976]

So, I have added a 10K bias trim pot in series with a 5.6K (closest I had to a 5k) The problem is I'm only getting 25ma max. Should I lower the 5.6 K, or make it higher to get to where I need to go ? Around 47.2

Thanks !

[pontiacpete]

I would try lower or just put in a 20k trim without a series resistor

[gtrplayr1976]

I guess I would have to parallel the resistor instead of series to lower the resistance . I may look for a different trim pot also. I have plenty of resistors though

[gtrplayr1976]

Ok...now I have a hiss that changes with volume ,and a noticeable buzz /distortion sound when playing at higher volumes. Also when I cut the volume back on the guitar it sounds tinny... ????

[pontiacpete]

Were you able to dial in the right amount current for the new kt66s?

[gtrplayr1976]

Yes , I did get the bias adjusted. I had to change to a 20k resistor with the 10K pot. I MAY have the pot wired wrong. I'm trying to find out now. Always something.