gk mb fusion 800 preamp high end distorting

[adube810]

I have a gk mb fusion 800, been serving me well for almost 5 years...I had put it in my storage closet. for a few months in its gig bag case. seems like the high end is starting to distort lightly at any volume and any gain. when I plug into the effects return it's crystal clear , so I can safely assume its something with the preamp. Is this a case of a failing tube? and if so, which one cause there's 3! any input would be appreciated !

 

[groovaholic]

The way that GK implements their effects send/return jacks, they can oxidize and degrade the signal path if they aren’t used regularly.

Your comments about the amp sitting in a storage closet for several months suggests that oxidation is a likely culprit.

The fact that plugging into the effects return gives you clean sound reinforces that notion.

So, take a guitar cable and plug/unplug into the effects send return jacks 10-15 times each.
For extra credit - rotate the plug back and forth during each insertion/withdrawal.

That might resolve the issue, and nothing you’d be doing would complicate troubleshooting or repair if it comes to that.

 

[B-string]

Not just a GK thing. Any amp head with a series effects loop can and may suffer this trouble.