Ampeg J12t cuts out

[Marc]

I am hoping this is not a case of bad OT as a tech buddy of mine has suggested...

The amp is a newish Ampeg j12t. It sounds pretty good most of the time, but it has an intermittent problem with cutting out when it is taking a large strum from my Fender telecaster humbuckers. It will come right back if I mute the strings. So it doesn't lock up completely. The guy who brought it to me just got it on ebay. He says the problem is happens more easily when he uses a pedal. I could only get it to lock up when it was wide open and I couldn't do it consistently.

I'm thinking maybe blocking distortion with a bit of ultra sonic oscillations and my friend (who repairs gear for a living) thinks it's a bad OT.

This amp has 22K grid resistors and a 250 pf cap accross the EL84 power tube inputs. I was thinking the grid resistors could be almost doubled and the cap moved (or added) to the plates of the PI as a starter fix.

It seems odd to me that the sound would cut out and that the OT could be 'bad'.

I have not tried new power tubes yet.
Anybody seen this problem and have any suggestions for a cure?

thanks,
Marc

[Enzo]

WHY does he think it is an output transformer failure? Specifically. if we think it is oscillating, then we need to scope it and see.

[Mark Black]

And being a "new-ish" Ampeg I'd say it's a candidate for a thorough solder touch-up at all the jacks, pots, connectors, etc. Kinda like putting new plugs in an engine before the tune-up...

Also a tube socket re-tension and "squirt & work" with Caig DeOxit in the switches/jacks & Caig MCL in the pots.

That would get you to a good starting point if it doesn't actually cure the problem.

[Marc]

WHY does he think it is an output transformer failure? Specifically. if we think it is oscillating, then we need to scope it and see.

Hi Enzo,
He did suggest scoping it out. But he also said to try tacking in a different OT to see if that would cure the problem.
What kind of pattern would I see on the scope? I have a scope, but rarely use it....
thanks
marc

[Marc]

And being a "new-ish" Ampeg I'd say it's a candidate for a thorough solder touch-up at all the jacks, pots, connectors, etc. Kinda like putting new plugs in an engine before the tune-up...

Also a tube socket re-tension and "squirt & work" with Caig DeOxit in the switches/jacks & Caig MCL in the pots.

That would get you to a good starting point if it doesn't actually cure the problem.

Hi Mark,
By newish it looks like it's been trated with care, Probably the ebay shipping was the hardest travel it had. It's circuit board looks very clean, no mods or repairs and the sockets seem tight. It has the flat ribbon cable connecting the tube socket board and the main circuit board. Is it possible this has to be positioned with care like some of the Bules Jr's do?
I'll try the cleaner, it can't hurt. If I get to pulling the board I may go ahead and up the grid leak resistors while it's out...
thanks,
Marc

[Marc]

update

I sprayed out the sockets, changed the power tubes to sovtek from the JJ's that were in it and changed the screen resistors to 470 ohms from 100 ohms. Now I can't get it to lock up.
So, If the owner can get it to lock up with his guitar and whatever pedal he was using then I will break out the o'scope.....

thanks,
Marc

[dporto]

Have you seen this thread?
http://music-electronics-forum.com/s...34846#poststop
Sounds like a very similar if not the same problem...Certainly could be many different causes, but I'll be interested to hear the outcome with your amp...Good Luck