Cap can ground loop

[octal]

I just solved a problem that had been dogging me with one of my builds and thought I'd share what I discovered. I was using a brand new JJ multi-section can cap (I think it is a 40-20-20-20 @500V if memory serves) and had a persistent background hum in the amp. The only way of getting rid of it was to add a 220uf (!) cap in parallel with the final power supply node. Finally, it occurred to me that the high current pulses going though the first cap section might be 'polluting' the ground with ripple... any voltage drop across the ground lug of the cap can will be imposed on the later filter sections.  Replacing the first cap can section with an external cap (with the CT of the transformer connected to the negative of the external cap) and then running those to the can cap really quieted things down.

Nathan

[phsyconoodler]

Never had that issue after a few dozen JJ cans.I suggest maybe your ground location was doing it and not the can itself.

[DummyLoad]

usually, i use a dual can for the first CRC/CLC filter with it's own wire to gnd. never have liked having the preamp and output stage filters in the same can. i have used the JJ 40-20-20-20 can in several builds, however, usually, i'll bond 2 of the 20s for 40-40-20 and use the 20 for the PI or as such, or i'll bond 40-20 and 20-20 (60-40) and use it for both ends of the first CLC/CRC filter. i like having the pre-amp filters in close proximity to the associated ckts. with discrete paths back from the filter(s) to the star gnd.

--DL

[octal]

Well, I was using the negative of the cap can as the star grounding point, so I don't think there's any way I could have wired it differently (w/o splitting the cap up like DL suggested.)

Interestingly, the hum was inaudible when the amp circuit was in its first iteration, but after I converted to cathode bias (with a much higher idle current) and increased the gain of the PI, the hum became problematically loud. (note that I did not change any grounding during either of those mods.) I think the all in one cap can will work fine in many applications (as psychonoodler noted) but in higher gain, high current draw setups it has the potential to be problematic.