Fender Twin possible output transformer fault?

[Jazz P Bass]

Pretty cool the way you went about troubleshooting this beast.
"It's now permanently OC on that side"
That'll learn it!

[Prairie Dawg]

I wonder if you could have sussed this out with an insulation tester?

[Mick Bailey]

Well, a replacement OT fixed the amp and a postmortem on the old one revealed that the internal joint between the winding and connection lead was poor - possibly a defect from when it was made. I discussed it with the owner and the amp has a history of blown fuses and replacement output tubes going back at least 7 years.

[Tom Phillips]

Mick,
Thanks for reporting the final resolution.
Was the fault you found inside the OT repairable?
Tom

[Mick Bailey]

It may have been, but finding the problem caused some damage to the insulation of the outer windings, so I wouldn't trust the long-term reliability compared to getting it rewound - especially considering the time I've spent on it already. I was surprised how they're put together - pretty rough really. The lead insulation had also gone really brittle inside the bells.

[Alan0354]

I did not read through every post. But if you power up with no signal input. Assume no oscillation, the voltage across the secondary is actually very small. Remember the center tape is say 400V, the plates of the the two tube is only going to be a few volts below 400V because it's only have drop due to the plate current with the resistance of the transformer.....Because you are not supposed to have AC signal!!! Unless you have arc from primary to secondary which I doubt. You don't have any high voltage in the OT secondary. If you raise to 300V and start arcing, put a scope probe to verify if there is oscillation. That will answer most of the question. My bet is not on the OT. Monitor the screen grid voltage as you raise the voltage. Use a scope to see any signal also.

[Steve Conner]

Wakey wakey! He already cut the old OT to pieces and verified the fault.

[Alan0354]

I don't like the way this forum posting, when there are more posts, you don't see the latest and you have to look around all over.

[Steve Conner]

Well, before replying to a thread, you really ought to read all of the posts that were made since you last looked at the thread. The forum software offers a choice of linear, hybrid or threaded mode, I prefer linear.

[Chuck H]

+1
Linear.