weird hum problem

[audiopro]

Hello all,
I have a williamson style stereo power amp that is showing some strange behaviour.

With only one input connected (doesn't matter which one) both channels are dead quiet.
Connecting both inputs causes the amp to hum - 60hz. The left is worse than the right.

Also:
With the left input connected to a preamp, touching the ground sleeve only from the right channel preamp cable causes the hum. However it doesn't do it the other way round. The left input needs to be connected all the way. Just touching the left's ground sleeve, with the right connected doesn't make it hum.

Also:
I inadvertantly grounded the right channels output terminal and both channels oscillate at a high frequency. The shorted channel being worse. So I tried this with the other channel. No oscillation at all. The other interesting thing, is that the oscillation only occurs when using the ground connection right at the right channel jack. No other ground point causes the instability.

The amp is grounded using a separate buss for each pair of input tubes, a separate buss for the outputs and then each is run to a star ground right at the first filter cap. The input grounds are tied together and one wire from the left input goes to the star.

This is the second incarnation of this amp on the same chassis. The previous circuit was more complex, but had the same hum problem. Thus the re-build.
Both channels work fine and except for the hum sound quite good. All the parts are new manufacture, no recycled parts. Transformers are Hammond.

Sorry for the long post.

PS: I've tried every ground scheme I could think of.

Thanks
Dan

 

[Wavebourn]

Looks like some ground loop.

 

[chrish]

Sounds like a similar problem I had with a solid state amp I built. Had sep power supplies for each channel and totally sep inputs etc. There appeared to be a loop in the grounds between the two sep channels connected at the common safety ground and the pre-amp. The pre amp has a common ground for both channels, this caused the loop. Now with all amps I build, I tie the two signal grounds together at the RCA inputs. It cured the problem.

 

[Wavebourn]

I would suggest to rethink the ground scheme since what you describe affect a stereo imaging even when you kill thge oscillation.

 

[chrish]

I have a Rotel surround sound processor that I use as a pre-amp. It has the signal ground for all the channels common. For me, keeping those signal grounds common by tying them together at the RCA inputs and after that splitting them to the channels fixed the loop. Have I missed something?