Hum even in Standby

[Capel]

Scored a 6100LM recently, love it except for one thing - it has a hum (electrical sounding) when switched on, even with nothing plugged into the input or fx loop. The hum is audible even in standby, the only way to kill it is to disconnect the speaker cable (in standby, I'm not ever gonna do that when operational!!)

Suggestions / solutions??

[MKB]

If the amp has had a power transformer or output transformer replaced, it is possible that the tech installed the new transformer 90 degrees from how it was supposed to be mounted. This can cause stray field magnetic coupling from the power to output transformer, it will cause a tiny bit of hum in the speaker even if the amp is on standby and the power tubes are removed. The transformers in most amps are carefully oriented as part of the design to minimize this issue. The only fix is to reposition either the power or output transformer until the hum stops. Usually rotating it 90 degress will help the problem.

I have built several amps that had this problem, and it can be hard to eliminate. Also some power transformers emit more stray fields than others, and can cause more of a problem.

I would guess the same thing could happen if the 6100 was originally designed without a power supply choke, but a tech added one (a common mod) and did not carefully place it to minimize the magnetic coupling. But I would far more suspect coupling between the PT and OT.

[GuitarBuilder]

If it's not what MKB suggested, then a grounding problem is the next likely cause.

[zvenamps]

just tighten the screw's on the Power trafo....

sometimes I glue epoxy in the trafo and that's help a lot... (the windings is on a plastic bobbin and it can vibrate and create hum...)

its a mechanical hum..