Smoke on my power amp

To illustrate what I was referring to in my previous post, in the first of the following photos I’ve uploaded the two transistors with the grayish blobs straddling two of their leads are shown surrounded by a red rectangle. The second photo is a blowup of that area:

https://www.flickr.com/photos/32959731@N04/49419393081/in/dateposted-public/

https://www.flickr.com/photos/32959731@N04/49418924433/in/dateposted-public/

Although the limited resolution of the original supplied by the OP makes it hard to tell, I wouldn’t be surprised if those transistors were what smoked.

Regards,
-- Al

@ihcho ,
I did the same thing on my old DNA-1-inadvertantly, of course.
The only thing that happened was that the fuses blew.

I would be surprised that your amp wasn't equally protected.
Did any fuses blow?
Bob

Al, I agree. I see a collector to emitter short from what looks like heat damage. The smoke is from the case melting (concave damage in middle to right).

All I know is that the vast majority of amps I own or have owned have a huge warning in the owner’s manual that states not to operate the amp without speakers being hooked up. And I think you now know why. It is NOT safe to operate so don’t even think about it...

I thought I’d weigh in here. Basically this means the vast majority of your amps are fundamentally unstable. An open circuit is the easiest load on the planet - no current flows -- on the condition that the amp is stable. Inherently stable, not "stable when driving loads we kinda hope are there". It usually means that am amp is not inherently stable, **and** that the speaker load forms part of the feedback loop. Note this is a problem with opamps, and is corrected by either "compensating" it (which has side effects) or operating it only within certain parameters - typically not unity gain. Ive designed dozens of amps of various types (chip based, BJT, FET, valve/tube...), some for myself, some for production, some for others to produce, or derive something from. Every single one was unilaterally stable. Open circuit? Might as well be off. Happy as a clam.
So the above may well be true, but in my opinion its a sad situation. I think I’ll go upstairs, unplug the speakers and turn the volume to 11. Just because I can.
So, in the end read the owners manual. but also wonder why the condition is so. To be totally fair, my stuff has some odd requirements too (so read my manuals!) - mostly because i typically leave certain low-power-draw parts operating 24/7 for a variety of sonic reasons - and without on/off cycles they last longer, not shorter. Odd but true.  I used to tape the fuse to the part of the owners manual with warnings. People woudl call up and say "it wont turn on". Did you read the manual?  Humm? Notice anything?  :-)  G

@itsjustme, thanks for your comment. I would add, though, that in the specific case of tube amps having output transformers it seems pretty well established that such amps should not be operated unloaded while they are processing a signal. The concern being that "inductive kick" occurring in the transformer in that situation may damage the transformer and/or the output tubes. And I suppose the same may apply to the many McIntosh solid state amplifiers which have autoformers at their outputs.

Regards,
-- Al