Speaker killing amp

The amp is a 410 HD 130W with SS in the pre amp.
The owner experienced that the amp stopped sounding for maybe three seconds and then started sounding again. He found that the speakers were really hot, hardly able to touch. And two of them dead. He ordered new ones, Jensen JCH 10/35. Same thing happened. Ordered again, same thing...
I have heard that the amp can start oscillating for some reason, and that this can burn speakers.
Could this be the case, and what would be the cause of the oscillating?

I don't think oscillation would be the problem but ANYTHING is possible. Most likely cause is a crapload of DC on the speaker output. Especially when you say they were hot. Where did it come from? Geez, a million places to look. Caps bleeding DC would be my first check. Check the big caps first in the PS area. This is just a guess. You need to see if there IS a boatload of DC on the speaker first. Mike.

There is no DC on the output. If there is in the moment when the speakers burn, I don?t know. But what way would that DC current go/come from? Somebody thought it could come from short circuit from primary to secondary in OT. Is it a good start to swap all electrolytes?

Found that a the screw and nut that holds one of the driver transistors to the cooling plate is loose, which means that the transistor has run overheated. Could that cause the problem?

Would answer why it runs for while and intermittantly goes boom. I would be surprised if it was that though. The emitters are tied together and dump into the tubes. I would be looking at the PS caps first, then OT. Somebody over on one of the other boards had a transformer tester, easy build to show shorted turn. Here's a pic of it. Might help in this case.

With that you assume that the amp doesn?t work? I play on it now, and it sounds good. Or could it still be working with a short in OT? And why would a short in OT fry speakers? Unless we are talking of a short from primary to secondary, with high voltage DC to speakers.
To me it looks strange that the intermittant silence and the speaker frying has nothing to do with each other, since they occur simultanously.

You are going to have to be able to look at the output while driving it. I would suggest a dummy load big enough to handle the output, push a heavy signal through it and watch the output for DC. Short of throwing parts at it, you need to catch it in the act. If it hasn;t been re-capped, I'd still be suspicious of that first. The driver transistor heating up wouldn't take out ALL the sound, it would PROBABLY just distort badly when it went thermal. It would take both of them at the same time. The only other thing that I can think of is the feedback loop that that ties in to IC 8. If THAT IC was bad and intermittantly shorted, it COULD POSSIBLY be sending 16V to the output side of the OT. But THAT'S a stretch. Mike.

A question. I?m a bit puzzled about one thing I haven?t thought about before.
There are cables attached to the metal frames of all four speakers, connecting them all together. And when measuring with the multimeter, it proves that these cables are connected to the minus poles of the speakers. Why? It means that all minus poles are one electrical point (language?) What does that mean in a series/parallell winding?

The voice coil should NOT be grounded out to the frame in any way. The voice coil SHOULD be totally isolated. The frames themselves are ok to ground to the chassis but NOT the voice coil. Do you mean WITH them connected to the OT? If that's the case then yes, they are connected to ground via the OT. One end of the OT is always connected to ground.

Forget about it, I was wrong. Yesterday I tested with a dummy load and a car speaker. No problem. Today I ?ve played it with a 2x12 cabinet made for a 100w amp. Covered the cabinet with cloths and blankets and played with pre and post at FULL for one hour. No problem. I?m seriously thinking the Jensen speakers can?t handle the power. If that is the case, what speakers to suggest?

Well, just about any good quality speaker would do but I AM partial to webers. They have the complete menu so you can choose between something ultra clean like my Cali's or the vega clones that I put in my little cabinets to something like celestions. The Chicago versions are getting pretty good press but I don't have any. The girl in my band does have ONE though and it sounds great. Try the weber site and look at the choices. They are reasonable and great to deal with. The only down side at all is they build WHEN you order so it's not like MF or Sweetwater. It WON'T be here in four days. Better plan on a couple of weeks. Good stuff though. I put the cerwin vega clones in my orange cabs and use them for my gigging amps. So far so good. Had them a year and love them Mike

Howdy!
The guy has now played the amp with other speakers cabinets without frying them. But now there is another problem, maybe connected to the other one. Ghost notes, you know. These notes are low and follow the actuel pitch. When a note is played and slowly disappears, the ghost note is more and more evident. That is, it doesn?t fade out in the same amount as the original note. Regardless what speaker is used.
Would you say filter caps?

Did you check the bias drive on the amp? I'm going to say caps first here but the amp could be in a wacky stage of crossover distortion if way too cold. Maybe causing some harmonic but I would start with the caps unless you have the ability to look at the signal with a scope and see where it starts and stops. What I mean, is it definitely in the finals? Coming from the preamps section? The ghost notes should show up on the scope AS a ghost to the sine wave.

The bias is OK. I?ve ordered electrolytes now.