Hum in Triode mode

[drewl]

Got a hacked up Marshall 100w with an added pentode/triode switch that hums in triode mode, but okay in pentode mode.
The switch and wiring all seem okay.
Tubes appear to be matched alright, anything to look at before I dig into it?

[Enzo]

Try this: check to see if they wired the switch to simply short screen and plate together. if they did, I;d say that is your problem.

If you short plate and screen together, it makes the tube a triode, but it also shorts the plate and screen B+ nodes together. Instead or two filter stages at the screen, you wind up with one filter stage with a larger capacitance.

The screens have to be disconnected from the other circuitry when conected to the plates.

It is just a thought.

Her is another thought.

When in triode mode, check right at the tube sockets to make sure no screen wound up not connected. If one of your screens is not powered, then that tube won't conduct, leaving the power stage unbalanced. Unbalanced, it cannot cancel hum as it normally would.

[drewl]

Well that's a correct thought, that's how it's wired.
Aren't the marshall Jubilees etc. wired the same way for Pentode/Triode switching?
I'll have to to double check.
Thanks as always.

[Enzo]

Triode mode means connecting the plate and screen together AT THE TUBE, but not conecting the power supply nodes together. the pentode/triode switch should connect he screen to EITHER the plate or the screen supply

[drewl]

It's a shame really, this is a 2150 that had the pc board hacked up and modded to be like a 2203.
Got that all sorted out and it's sounding alot better but I can see the power supply and filtering have been messed with so I'm gonna have to return all that to stock and wire the Pentode/Trode thing correctly.......we need a facepalm smiley!

[Enzo]

Well, maybe you will find one

But sometimes getting what you want is an uphill battle

[drewl]

well with the correct switch installed and a new Filter cap for the first node which was failing it's fine now in both modes.
I've rebuilt hacked up Marshall pc boards before, but this one was one of the ugliest.
just buy the amp you want instead of mutilating a collectable one

[Enzo]

I'll have you know, that was the first time in my life that I ever posted a smilie face anywhere. I hope you appreciate my sacrifice.