Hot Rod Deville 212 - loud hum when treble dimed

[Maddus]

Today I did a full recap on my HR Deville, and also did the twin mod as well as changes in the tonestack (slope resistor to 100kOhm, bass cap to 22nF, treble cap to silver mica 500pF). Now I get a loud buzzing hum when I turn treble to 11-12, but only on the drive channel?!
The buzz sounds like when you’re touching the tip of your guitar cable when it’s plugged only in the amp.
I measured the treble pot and the taper seems to be intact.
The silver mica cap has a 500V rating, could that be too low for the Deville (stock cap had 1kV)?

[pompeiisneaks]

I would guess it's more likely you didn't get a good solder connection on one of the grounds for the filter caps... buzz/hum is of two types 60hz or 120hz, the 120hz is usually due to improper or lacking filtering.

If the buzz is more of a 60hz then maybe one of the new caps you put into the circuit could be related.

another possibility would be one of the mods adding a ground somewhere that's causing a ground loop.

It's hard to say for sure any more than that without photos of the amp/mods and more detail on what exactly the mods were in a schematic and/or layout.

~Phil

[pompeiisneaks]

Oh I should add, I've worked on several of these amps and the PCB's are super cheap, crappy quality with weak traces. It's possible you broke a trace somewhere for the ground... on a filter cap or elsewhere, thus causing the hum. Using your multimeter on continuity mode to ground/chassis through all the new soldered connections that should be 'to ground' and making sure that you still have that, would be a good step.

~Phil

[Stevem]

If the amp was not doing that before you worked on it the rule of Thumb is that the new issue is related to the work you did!

So go and back track and double check your work, and it's not unheard of that a new cap canted be bad with very high ESR!

[Maddus]

Thank you guys for the suggestions.
Long story short: I swapped the silver mica back for the stock ceramic treble cap and now the phenomenon is gone. Seems like the mica leaked some DC at full treble.

[ChopSauce]

Congrats & thank you: you make me think I may have some problem with a mica cap, myself...

[TUBEDUDE]

Micas seem to be sensitive to heat. Solder in carefully. Use a heatsink if necessary.