Weird hum

[Leevi]

I'm analyzing a low level hum which comes on volume.
When I measure the AC from speaker output I get different values depending on the amp position.
The hum almost disappear when the cabinet (of combo) is laying. If I return the cabinet to its normal position
the hum starts again. I have tried to power it from different wall sockets but no difference. Also with external speaker.


/Leevi

[Paul1453]

It has a tube rectifier I suspect.

If so, that is a likely source of the problem.

Some tube rectifiers are not recommended to be mounted horizontally.

Just the change in the symptom when laying down would make me look at the tube rectifier first.

60 or 120 Hz hum?

60 Hz would point more at the heater wiring.

[Leevi]

Some tube rectifiers are not recommended to be mounted horizontally.

I tried to use 5Y3 instead of GZ34 but it didn't help.


It's 100Hz hum.


I got rid of it by adding more filtering.


/Leevi