hum just started!

[soulfireband]

Hello everyone:

New to the forum. Seems like a great forum. My HT-5RS just started making a humming noise. The hum is not very loud and it is coming out of both speakers. No noise coming from the back of the amp itself. I changed speaker wires and it still does it. When I put the amp on standby, the hum goes away. Turning the volume up/down, turning the gain up/down, switching channels; none of these things changes the volume of the hum. It is just a steady, low volume hum. I actually thought it was a prop plane flying overhead when I first heard it.

Any ideas as to what might be causing this? I haven't changed out either tube or done any other kind of maintenance on the amp. It is just as it was when I purchased it.

The amp is a couple of years old.

Thanks, in advance, for your help.

[thephantum]

Since the volume control does not affect the hum, the cause is most likely after the volume control (thus after the preamp). That tells me power amp. Since the head is a few years old, the first course of action would be to change the power tube and properly bias the amp. Sometimes tubes can develop an internal hum as they age.

It could also be that bias has drifted as tubes tubes aged. Push pull amps use less power supply filtering because the output transformer normally cancels out most of the flutter in current flow. That cancellation can be negated if bias has drifted and different amounts of bias current are going through the tubes (or in this case tube). That can allow a low level hum to be heard.

My guess is that a new tube and a proper bias will get rid of the hum. If not, then it needs to go on a bench to look at things like filter caps, internal grounds, etc.

[soulfireband]

I actually changed-out both tubes and (so far) everything is working as it should.

Thanks for your reply, I appreciate it.