Ran 68 50w amp dimed, now loud pop on shutdown

[guilds100]

I was playing with my marshall 50 watt clone at full volume just to see what it would sound like and I think I goof up. I was holding some power chords on my 70s les paul to see how long it would sustain. After 5-6 times the amp started to hiss alot. I shut it down and let it cool for 5 minutes. I then turned it back on and it had alot of hiss that it never had before so i went to turn off the standby and POP! It was very loud, enough to get me to post this before I turn it on again. the tubes looked ok on the restart but I didn't leave it on long as the noise had me worried. My first thought would be to change tubes, but after that Im not sure whre to go. Any thoughts on what could have happened to cause this, or things I should check? Im running new tungsol preamp and mullard output tubes. The amp has been running great for months before today.

[Ydna]

Take a look at the fuses if you haven't already, if they're fine, look inside the chassis, check for burnt resistors. If everything checks out, check the bias of the amp.

[guilds100]

Thanks for the reply. I replaced the output tubes and it seems fine now