Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on September 08, 2023, 08:39:44 am
-
Hi all,
A friend of mine owns a Champ circuit with AB763 reverb I built a few years ago. He's had no problems with it until yesterday when he texted me and said: "The amp started squealing when I turned it on and then it started smoking."
Just looked at it this morning and immediately noticed the cathode bypass cap on the reverb driver (12AU7) blew out, spilling its guts on the positive end. Looked down at the driver tube and it's got a tiny crack on it, with the tell-tell white color around the end of a failed vacuum.
So I'm guessing the tube was hit and cracked, and possibly shorted internally, which could have caused the cathode cap to blow. Everything else appears to be fine: all power, output and reverb transformer coils have the continuity I'd expect, nothing else appears damaged. Only other thing is it looks like the 820 ohm dropping B+ resistor feeding the reverb transformer and 6V6 screen grid got hot. Tests out fine, but I'll replace that, and take a good look at the 6V6 to make sure it wasn't damaged.
Anyway, anyone have any thoughts on this? Is there anything else I should be looking for? Thanks!
-
ohm out the transformer, they're not known to like lots of current
-
ohm out the transformer, they're not known to like lots of current
So far so good. I checked it with my little setup at work when I got it, and the HV secondary seems good, about 106-0-98. Later I'll power it all up without the rectifier to see what kind of AC voltages are coming out of it. Hopefully all is good there.
-
Well I wish they were all easy. Tested all windings, replaced cathode cap and resistor, and the B+ dropping resistor that feeds the reverb transformer as it definitely got hot. Started up slow, watched for current draw and was rewarded with an amp functioning as it should. Took less than an hour, wish they were all this easy!