Odd issue

[lowell]

Got a Kustom '36 Coupe here. Came in with bad hum, even in standby...although less so. Found that the power tubes were no longer matched. Got 42ma and 28ma. I had pulled the PI to make sure the hum was in the power amp and it was. I retubed it, bias looks good and matched, but hum was the same. I began testing a bunch of things, then thought well lets double check the PI voltages... so I reinstalled the tube. HUM GONE. Very odd. I'm sure it's something to do with the NFB loop. Could someone please explain, if you know why this is so? Amp sounds great so it seems to be working fine.

http://music-electronics-forum.com/a...m-72-coupe.pdf

[Lucaslz]

That is a hard call, but it sounds like a contact problem since fiddling around with it fixed it temporarily. Try cleaning all the socket pins of the phase inverter tube, make sure the soldering joints around that area are good, do a chop stick test on other components in the area, and try a new PI tube. Also clean all switches and sockets, and you will probably be fine.

[The Dude]

Dirty/Stretched/Corroded socket pins?

[olddawg]

I've never understood how a speaker can hum with the standby switch open interrupting the B+ power supply? The only way I can see it happening is inductively coupling the PT and the OT?

[Enzo]

Look at the schematic, the B+ is only removed from the screens and downstream in standby, the plates remain hot.

I cannot imagine how the NFB would cause hum.

Cause of hum? Broken connection to or inside a filter cap.

[olddawg]

Look at the schematic, the B+ is only removed from the screens and downstream in standby, the plates remain hot.

I cannot imagine how the NFB would cause hum.

Cause of hum? Broken connection to or inside a filter cap.

Yep... another lesson in not making assumptions. I didn't notice J10 and assumed the plates were disconnected from the B+.