fender concert 81, clean gets dirty after on awhile

[griiff-cal]

My concert has a problem on the clean channel. When turned up past 6 and after its been on awhile, lower notes distort- they almost have a double tone the clean sound and then a distiorted sound. somtimes that distorted sound lasts longer than the ringing of the string. If I swicth to the lead channel its gone. I tried it with different speakers same issue. It's never been recapped. But has 6 month old JJs in it. Any thoughts?

[techineer]

That sounds a lot like a tube getting weak... have you tried replacing any preamp tubes?

It's not inconceivable that the output tubes are the cause, too. But less likely.

It could also be a power supply filter capacitor... or any other cap for that matter. (or any other component, too!)

It's probably going to need some detective work with some test equipment and experience.