failing cap residual AC question and another buzz/humm thread

[Marc]

another hum buzz thread...
I have a Dr. Z mazarati on my bench that hums like the fluorescent light in my kitchen.
less hum with the preamp pulled, nothing with PI pulled.
the B+ is 340V and the residual ac on the first two nodes is 3.9v and below .05VAC after that....
Do failing filter caps usually impart a higher residual AC on the B+ or do they just drag the voltage down lower as they die?

This amp has two grounds and they are tight. I've tightened up the jacks.
It's a really nicely built amp that is untouched inside.

Any ideas?

[Chuck H]

Probably an insulting question but...
Did you try another tube in the PI socket? I've been getting some hummy preamp tubes lately from Sovtek. The first time it happened to me was a first time fire up on a new build of a known good design with all new tubes. I thought sure it was a mistake on my part. Looked for it for an hour and then tried a tube swap. It's happened on two of my last three builds now.

Other than that I would say try bridging a filter in place across the existing one. If the hum reduces replace the filters.

[Enzo]

Is your hum 60Hz or 120Hz? 120Hz is ripple - what your caps fight. 60Hz hum has nothig to do with those caps.

[Marc]

Chuck,
I tried both the tube swap from my known good basket and bridging a cap, neither seemed to change the sound.

Enzo,
I think it's 120hz..I got my tuning fork out and it buzzes 2 octaves below A440 on B flat.
I was thinking that the low AC residual was OK at ~4V...
I tried a different 5AR4 with no change...