I recently bought a Marshall TSL60 head and have been really enjoying the sound I get out of it for both punk and metal, but recently it has developed a bit of an issue.

Despite warming it up for a fair amount of time when I play anything with distortion I get a high pitched squeal coming over the whole thing. It's not feedback and I've checked all the connections and cables etc so has anyone any idea's what's wrong with it.

When it's not been used it's either been in my room or in my van, could the recent cold weather have affected it whilst in my van?
Did you try changing guitars? Does it do it on the clean channel too?

If so, my guess is a dying preamp tube.
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