Need help on an AA764 Vibro Champ

[StratTone]

I do a little work here and there to help out a buddy of mine and he had a buy come in with a '73 Vibro champ that was blowing fuses.

Took it home and there was a HUGE fuse in there. (no surprise) Anyway the 6V6 was toast but the stock RCA pres and 5Y3 was good. I replaced the cathode resistor and the shorted cathode cap and it sounded beautiful. I played it for like an hour. Took it back to my buddy, left his shop and my phone rang like 15 minutes later. He said it sounded great until it stopped working. He said he was playing it pretty hard when it quite.

I go back get the amp and it is just like he said. I took it home, powers up fine. Cathode Resistor is fine as well as cap but no output at all. The main voltage I noticed that wasn't right was the 21V cathode is 11V and the Pin3 has no voltage at all really. Pin 4 is fine. So I took the tube out and unsolderd the red primary and it is open. It never gets hot or anything but how in the world did the OT just frie like that? He said it didn't smell like anything burning but jsut quite. I kept playing with it thinking it may not be the OT and finally burnt the 10k 1watter right off the rectifier. I'm thinking it is just the OT? Anybody got any ideas for me? The only other thing for sure that is bad is the .02 cap in front of the 220k needs replacing and I orded that tonight.

[Bruce / Mission Amps]

Originally Posted by StratTone

I do a little work here and there to help out a buddy of mine and he had a buy come in with a '73 Vibro champ that was blowing fuses.

... Pin 4 is fine. So I took the tube out and unsolderd the red primary and it is open. It never gets hot or anything but how in the world did the OT just frie like that? He said it didn't smell like anything burning but jsut quite. I kept playing with it thinking it may not be the OT and finally burnt the 10k 1watter right off the rectifier. I'm thinking it is just the OT? Anybody got any ideas for me? The only other thing for sure that is bad is the .02 cap in front of the 220k needs replacing and I orded that tonight.

If the OT's primary winding is open, you have no simple choice but to replace the output transformer.
Make sure you don't have a shorted 6V6 tube which could damage the new output transformer.
Use a bigger SE OT as these were pretty small compared to older and or more vintage single ended 6V6 amps from other companies.
With an open primary all you are doing is drawing screen current so it doesn't surprise me that you could over load the 1 w screen node resistor.
Unrelated, but how did you arrive at the conclusion the .02uF cap is bad?
I don't think I ever seen a bad one in these amps.

[StratTone]

Originally Posted by Bruce / Mission Amps

If the OT's primary winding is open, you have no simple choice but to replace the output transformer.
Make sure you don't have a shorted 6V6 tube which could damage the new output transformer.
Use a bigger SE OT as these were pretty small compared to older and or more vintage single ended 6V6 amps from other companies.
With an open primary all you are doing is drawing screen current so it doesn't surprise me that you could over load the 1 w screen node resistor.
Unrelated, but how did you arrive at the conclusion the .02uF cap is bad?
I don't think I ever seen a bad one in these amps.

I was checking DC voltage on V2 I beleive and saw some moving DC so I starting probing around. haha Didn't realise at the time is was the vibrato tube. Anyway I just pulled the .02 out and checked it on my Sprague TO-2 and it wasn't stable. Figure I will replace it anyway. Thanks for the advice about the resistor burning up. That had me scratching my head. I have another 6V6 on hand since my good NOS one I put in there got shorted, I just want to make sure that I get everything changed before I put the OT in. Don't want anything to mess up the new OT.