Ampeg B-50R issue

[fcleffer]

Sounds much like a blown speaker, but speaker good. Raspy rattle occurs at low-mid volume; about 3 on knob. Resolves to normal when guitar cord removed from input jack and immediately reinserted. Then issue reappears after a very short time. Manually lightly vibrating the guitar cord ¼ inch male does not produce the issue, so i believe the jack wiring is not compromised. Anyone have history of this with B-50R who can suggest next level of troubleshooting?
Thanks.

 

[Lowbrow]

Turn it on, plug in, strike a note and then start turning each knob, full rotation. If you hear any crackle, that’s your problem—a dirty pot.

 

[anderbass]

I agree with trying Lowbrow's suggestion.

Another very-common possibility is a poor Shunt connection in your headphone jack. The purpose of that Shunt is to mechanically disconnect the speaker anytime you have headphones plugged in.

Next time the amp malfunctions, try cycling a spare guitar cord (or 1/4" headphone plug) in and out of the headphone jack serveral times. The act of plugging in & out like that will typically restore a good Shunt connection.

I've circled the Shunt in this Jack pic to help you understand how a slight amount of dust or corrosion might be causing your issue: