Marshall JCM2000 TSL feedback on clean channel

[Edward1971]

I am having an issue with feedback on the clean channel.
At medium to high volume I am getting a weird feedback problem on the clean channel.
I can cause the feedback to start by tapping on the cabinet or the back panel of the amplifier, with an imput signal or without. It is independent of the footswtich.
When I change to the other two channels I can tap on the cabinet and hear no response in the speakers. On the clean channel you can hear each tap, sometimes building to a feedback howl.
Any suggestions?:confused:

[Mark Black]

While there could be other causes I would say by far the most likely is a microphonic preamp tube.

If you take the back cover off and either tap on or grab (to dampen) each preamp tube you may quickly find the culprit. Of course this should be done with the amp set to the mode where the feedback happens...

[Edward1971]

Thanks, I will give that a shot this evening. If that is the culpret, what is the remedy?

[Enzo]

If the tube is microphonic, you replace it.