Hum disappears with dummy plug... but what does it mean?!

[Gaz]

I have a Red Bear MK 60, and when the amp is turned up to a certain volume a slight hum is happening. Without a guitar plugged in the hum goes away, and only the amp's 'blow' can be heard. With a dummy plug inserted into either of the inputs only the 'blow' can be heard as well.

What does this indicate? Is there a problem with the jacks?

BTW, I'm using humbuckers, and my guitar is quiet though other amps. I've checked all the grounds, tubes, and the grid wires are shielded. The hum is relatively subtle, but because annoying because I'm playing so loud.

Thanks.

[Chuck H]

If the hum doesn't occur with a shorted input (ie: dummy plug or no cable in either jack) then the problem is most definitely in your guitar or instrument cable. Thats just the logistics of the situation.

Even if the guitar is "quiet" through other amps. There are many things that could be different. Perhaps this amp has less top end bleeders or the other amps use some kind of diode noise gate or different gain levels. And "humbucker" is a reletive term, like "water resistant". There will still be some noise.

Does the hum still occur with the guitar plugged in and the volume knob on the guitar turned all the way down? If yes, ther problem could be the instrument cable. If no, it's noise from the room getting through the humbuckers to your amp.

Chuck

[Enzo]

What he said.

[Gaz]

Thanks, Chuck, and Enzo too for the verification. I was thinking it was on the guitar/cable end of things, but just wanted to check that it couldn't be a faulty jack, especially since I redid the tone stack, etc. and had to rewire the jacks.

The noise goes away when the volume on the guitar is turned down, so yes, it must be the guitar!... or as you said, other design elements which contribute to the audibility of the hum.

Thanks again.