Fried something in a Deville 410. Ideas?

[Tiedyemike8]

Hi all,
My name is Mike. I've been referencing this forum for a while, it's been a great help with my fairly recent JTM45 build. Thanks to all of you for that. The wealth of tube amp knowledge and experience here is humbling.

Last night at a rehearsal, we accidentally connected my JTM45's speaker output to the speaker output of a Fender Hot Rod Deville.
Before realizing this, I powered up my amp and started to play through it. The deville was still turned off. Long story short, something fried in the Deville. It now has very low output volume and a distorted sound.
Can anyone point me to what may have failed (I realize it may be obvious to someone more experienced than me), or have an idea of where in the circuit the damage may be? Any help with diagnosing the problem would be greatly appreciated!

[sixpakldp]

Not sure I understand correctly, but it sounds like you connected to the 1/4" jack meant for the speaker on the deville, basically the plug for the devilles speaker.

Even though it seems useless, the first thing I would do is try plugging the amp into a different set of speakers and see if you get the same result. Next step I would do would be to try swapping tubes for known good ones.

At that point you've isolated it for sure to something inside the amp. Point being that while none of these things are in the path of what you did, in the event of a freak failure that happened to occur at the same time as plugging into the speaker out, you've at least rule them out.

After that, I'd take a look inside to see if you can physically see (or smell any damage).

Take a good look at the screen resistors on the power tubes, sometimes when something gets past the fuse these will go next. I had a filter cap that had a bad solder joint on a Fender Twin, I physically saw smoke coming out of the external speaker jack and thought for sure it was dead. When I opened it up it turned out that the screen resistor went even though the fuse was still in tact.

[neikeel]

So you effectively put the secondary of a switched on JTM45 into the secondary of a switched off Deville.

You might be lucky and fried the output tubes but they were more likely to have gone completely rather than gone quiet and distorted.

Try a pair of known good 6L6s first.

Sorry to say most likely casualty is the OT of the Deville you will have created some pretty spectacular voltages inside the OT for a short while.

[Roe]

yeah, you may have blown the output transformer