Is this a problem?

[gary sanders]

I was cleaning up the chassis and moving a few wires around and by accident when I was checking bias current I noticed when the presence control was turned all the way up the bias current goes up a few like from 25 to 30.It only does it when the control is turned from about 75% to 100%.I was wondering if thats normal or do I need to look at something.

[talbany]

Gary
Sounds like you could have some sort of internal oscillation happening when you decrease the GNFB..Could of been induced when you were moving those wires

Tony

[gary sanders]

Thanks man,I'll get out the chopsticks and see if it helps.

Was definitely a what the hell moment

[gary sanders]

Final Edit: 55v bias winding is shorted out......damn it.Continuity to ground.

Time to buy a PT I guess.

[BobW]

Gary, You don't need a bias tap to create a bias voltage. Just use the circuit shown here:
http://www.drtube.com/schematics/ampeg/v4bpwr-jp.gif

You may want to change the 75k to a 68k + 10k pot or similar to adjust the bias.

[Bob-I]

Final Edit: 55v bias winding is shorted out......damn it.Continuity to ground.

Time to buy a PT I guess.

You sure? The bias winding is just a few ohms, a continuity meter may beep even if it's good. If it's really shorted to ground you have no choice but to replace it, but I'd check more before spending the bucks.

[talbany]

Weird.. none of these problems really explains the interaction of the Presence control in the first post..

Tony

[jelle]

Weird.. none of these problems really explains the interaction of the Presence control in the first post..

Tony

+1. Check leaddress.

Jelle