120 Hz hum: symptom of crappy filter caps?

[rfgordon]

So I built this amp--as I've built many--based on the TW Rocket, with some Matchless Lightning features thrown in. It works great, except that it's got persistent 120Hz hum.

It uses a choke. B+1 & B+2 are on a JJ 50/50 cap can, and each of the three preamp tubes is on its own 20 mF Illinois cap. The screens are on another cap.

Lead dress, etc. is just like other amps I've built. I'm out of filter caps to try swapping out (I've ordered another JJ and some Spragues).

Is persistent ripple a sign of just a poorly functioning cap? I've not come across this problem before.

[Firestorm]

Definitely 120Hz? If it's in the PA, I'd suspect a screwy EL84. If it's in the preamp, could be a cap.

[rbryerton]

Where's the hum originating? Have you tried pulling V1 or 2?

You don't by any chance have reverb on that amp w/the tank unplugged, do you?

The reason I ask is I just built a 5E3 type amp but added reverb (first time doing this) and when the tank is unplugged, it has a nasty 120Hz hum. Tore my hair out trying to find it, and just as I was about to start swapping out filter caps, I plugged that tank in to check out the 'verb, and....silence!

Along the way I discovered that grounding schemes are not really as important as they're made out to be as I made several changes, to my grounds with little (or no) changes to noise level. Maybe because I'm working with such a simple circuit, but basically everything is sharing a common ground and is very quiet.

But I digress...

[rfgordon]

It is 120 Hz. The first iteration of the amp had Dana's VVR installed such that the power tubes and PI were scaled, but V1 and V2 were not. I recently built another amp that way, which behaved perfectly. This amp, however, was very noisey that way, so I changed it to scale the whole amp. This reduced the hum, but did not eliminate it.

The hum is in V1 and V2, but not V3 (the PI). V2 is a cathode follower, feeding the tone stack, then PI. Grounding the signal at the tone stack kills the hum, as does turning the bass/middle all the way down.

I've done tube swaps with no change, including the power tubes. The EL84s are funning a straight Matchless voltage profile, with 350 VDC on the plates and 240 VDC on the screens, so they are pretty happy.

I'm thinking that the caps feeding B+ to V1 and V2 must both be weak or leaky.

There's been much discussion on the Ampgarage about cheap/counterfeit caps, so I didn't know if anyone had encountered Crappy Cap Syndrome--if that's what this amp has.

[Firestorm]

V1 and V2 voltages okay? Maybe a leaky coupling cap? At the preamps, the B+ is already filtered so much, even a bad cap would almost need to be partially shorted to hum like that.