bias voltage

[JC@]

Hello, I must be stuck or maybe I just don't know how to do it.
I have a transformer that has a 60 volt bias winding, and no matter what I do, I get a 64 to 66 negative bias, too cold to bias the tubes (6L6 type).
Can I do something to lower that to -45 volts or so?
A zener to ground right after the diode maybe?
Sorry for asking, and thank you.
JC@

[Paul P]

You need two capacitors and a resistor to remove the ac ripple from the DC
produced by the diode, then a voltage divider to get the voltage you need.
The circuit used in the Marshall 1987 is considered one of the better ones
by a lot a people. Take a look at this schematic :

http://www.schematicheaven.com/marsh...d_50w_1987.pdf

With your 60v tap you won't need the 220K resistor in front of the diode, or
at least a much smaller one (the Marshall takes its voltage off the power
transformer's secondary). After rectification and filtering you should end up
with something around 80v dc which you'll have to divide down with a
resistor and a pot.

Changing the caps to 50uf 100v (electrolytics) would provide better regulation.

Paul P

[Enzo]

Right, you are leaving out the voltage divider that takes that 66 volts and knocks it down.

[JC@]

ok I'm ready. Thank you..!

[JC@]

Well, I have it now. Believe it or not, it was a new trimpot that came bad or that I made to go bad with the solder heat.

The thing is that the pot was working to some point, I remember I checked this at the begining of the troubleshooting but the fact is that the pin that was tied to ground was unconnected internally so there was no end of the circuit for the voltage divider... I feel good now, but really stupid for spending some of your time and so many hours of mine with this.

Thanks again.