Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on February 18, 2014, 12:57:26 pm
-
I have a silverface princeton reverb on the bench that wont get negative bias voltage.I put in a new diode and cap but there is only -1vdc. I disconnected the intensity pot and no change.I have a good ground,I lifted the bias board up to make sure nothing was wonky but still nothing.
I have 200v on the leg that attaches to the bias supply so that's not it.I tried two rectifier tubes.Nothing.
Very odd indeed.Both resistors measure the proper values,diode checks out good and is properly oriented.
I believe i ran across something like this before but my old mind cant recall what was happening.
I'm at a loss right now.Anyone have any ideas?
-
Check the cap again. Measure resistance from the anode of the diode to ground. Should be about 27K.
-
Try removing all load(s) from the supply output, on the anode (the non-bar end) side. Got big negative volts? Not much to go wrong here.
-
My guess would be a leaky coupling cap between PI and power tubes.
-
Well maybe so,but its wrong. Changed all components in the bias supply now and double checked grounds.
I've got -3v dc
Totally bizarre :BangHead:
-3v with bias wire off trem pot. :dontknow:
-
the resistance from anode of the diode to ground is 26.9k.Another
cap makes no difference.Tried another meter.Same.
-
Found it!!! Seems the wire coming from the rectifier socket had a bad contact on the socket.I measured the voltage but i guess it couldn't carry the current. Bizarre but i switched the pin the bias comes from and voila! It works!
-
Damn kranky pins!