MC 240 ReTube or Replace

PM me to offload it cheap, and I help you out.
Alternatively it sounds like it could be a capacitor. Any decent tube person will likely be able to sort it out.
It could also be something with the speaker (like an XO cap), which is presenting a difficult load, or oscillation…
You could switch the RHS and LHS channels to ensure it is related to the amplifier channel.And also which the driving source like a CD player with different interconnects into the unit with LHS/RHS swapped.
I am not even 100% certain it is the amplifier. My amp was also bad until I found it was a bad set of interconnects… Not the ones that I had just soldered up, but the 5 year old store bought ones.

Thanks for the suggestion of lifts....I've used them over the years for several motorcycle builds :-)BUT you still have to get the amp onto the lift And THEY DON'T DO STAIRS WELL :-)

OK I swapped the output tubes side to side, reseated all the other tubes and replaced the internal b+ fuse.I powered it up and listened to it for 2 3/4 hours with no problems....
That has been it's history...  After 3 or so power ups .. a burst of static and a blown fuse.

We'll see what happens next :-(

It sure sounds like a valve or a bad connection at a valve. Clean and tighten the pin pockets. Then take the valves and plunge them into a cotton ball with WD40 a dozen times. Wipe everything off and insert the valves back in the sockets.. 101 valve maintenance..

On that PS rebuild, did you re-bias the the PS, did it come with a little pot? or did it have a fixed resistor?

7581 vs 6L6GC 23watts vs 30watts, your better off with 6L6WGC (military aircraft) or just GC 30 watt valve, tough too.

Regards

As to the biasing I have resistors on hand and used such to achieve proper biasing.The amp has over 1000 hours on it with no other problems.