Fender Twin changes volume

[jl_lensman]

I have a Fender "The Twin" that plays fine for about 20-30 mins then gets wacky. All of a sudden it will boost the volume a lot along with a lot of white noise. If I change the "output" setting form Hi to Low then everything is fine for about 10 mins and the the problem comes back. Switch the output setting the other way to Hi from Low and problem goes away for a little bit then comes back.

So are my tubes going bad or is something far worse going on with this beast?

[PRNDL]

are my tubes going bad or is something far worse going on?

A bad preamp tube is the most common cause, although it could also be the electrolytics or power tubes. One simple test is to tap the tubes - a bad one will make funny noises. It's easy to swap a new 12AX7 to see if it fixes the problem.

Here's a great site
http://www.geofex.com/ampdbug/ampdebug.htm