Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: rastlr on January 29, 2012, 06:33:19 pm
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I have a 1966 Fender BF Showman AB763 that is blowing fuses. Here is the scenario. I’ve had this thing for almost 40 years. It always was finicky. I had to be careful not to push it too hard or the second tube from the left (looking at it from the rear) would torch the socket, arc the tube, and blow the fuse. As a result, I put it away for a LONG time and then have used it intermittently over the last 10 years or so with a Gibson EB-0. It was stored badly—heat, cold, humidity, etc., everything you don’t want to do.
So recently I dug it out and put new sockets in for the power tubes. I did not change the tubes (I had done so many years ago). Fired it up. It worked okay with my old Rick 360. Then my son plugged in his Flying V with a Blackstar HT-Dual pedal. We didn’t push it hard, but within 3 minutes the fuse blew. There may or may not be some arching on the second tube like the old days--I can't really tell.
My question is, any ideas what the problem(s) might be? The amp does need a cap job—no question about that. I have a fresh set of tubes, but I’m afraid to put them in until I get to the bottom of this. One further note, it has a 1X18 JBL speaker cabinet and the transformer is a 125P34A.
Thanks!
Larry
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Check and or replace all PS capacitors. Check and replace all rectifier diodes. The rectifier diodes are way outdated for there voltage rating from 1966 so change them but confirm if any of them are bad. This will tell you if this was your problem. And last check the power tubes on a tester. A tester is the only way to know for sure if they are shorted or comming apart internally and intermitantly arching inside the tube and over to other pins
When i test a power tube for a possable short i have found that sometimes it will not show any short during this part of the tube testing. This is why i also flick my finger against the power tube as i am doing the emissions test. If the tube is coming apart inside and i cant see it,then i can sometimes create a momentary short during emmission testing by shocking the tubes internals
It's important to know exactly what the problem is and to confirm weather the suspected parts are good or bad
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Is it still blowing fuses? If you put a new fuse in and turn it on does it blow the fuse immediately or play for 3min and blow a fuse? If it blows fuses immediately try taking the power tubes out put a new fuse in and turn it on. If it doesn't blow a fuse change the tubes. I would replace the filter caps while you are working on it.
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When i test a power tube for a possable short i have found that sometimes it will not show any short during this part of the tube testing. This is why i also flick my finger against the power tube as i am doing the emissions test.
I seem to remember reading Ken Fischer used a rubber mallet to test has el34s...
Ditto on the power caps if they are more than 10 years old, especially if the amp was put away for extended periods of unuse as described.
Between what pins was the socket arcing out? Always the same?