Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: OwnGoal on March 12, 2013, 01:56:34 am
-
Hi guys. I recently acquired a barely running '73 Princeton Reverb. I changed the cap can and all the electrolytics, confirmed all resistor values, cleaned all the pots, and tightened all screws, retainer nuts etc.
The amp is much improved, but I still get no sound at all between 1 and 2.5 on the volume setting, and the amp is breaking up at 3.5. Seems kind of off.
I checked for DC current across all pots, and found that I have between -15 and 30 VDC on the speed and intensity pots. Is this normal? Everything I have read indicates that the only pots that should see DC voltage are bias pots.
If this is DC current means bad things, should I assume that I have a faulty capacitor somewhere? Keeping in mind that all the electrolytics are new (but untested - I lack a capacitance meter), how should I proceed?
-
To be accurate, I'm going to make the assumption that you didn't check for DC >CURRENT< across the pots, you checked for DC >voltage<.
The intensity pot definitely sees the bias voltage, so yes, its wiper should sit about -30 to -34 volts per the AA164 schematic and -30 per the AB1270. The speed pot shouldn't see any DC and if you had leaky caps it would tend to be positive volts. That (what you are reporting) isn't normal.
Get the correct circuit revision from the paper tag inside the cabinet, eg "AA1164", "AB1270" and get the proper schematic from Doug's library or elsewhere on the net if the exact version isn't in the library of doom. Poke further. There are plenty of voltage measurements you should be able to check.