Removing cathode bypass brings 120Hz hum

[Smokebreak]

I was fiddling my first Expresss build today, and when I took off the 2nd stage bypass cap, I got a pretty annoying 120hz hum. Put the cap back on, no hum, only the usual hiss. This is the opposite of what I figured would happen in that department. This amp has been stable for a while now, been gigging with it, etc.
Any ideas? The only thing I can think of is that I've got a bad connection/ground somewhere in power supply, or bad filter cap and that cathode cap was filtering ripple ? I built it with organ amp iron but everything else was new when built 6mo ago
Preamp gain control does not affect hum. PPMIV does! Pulling any of v1,2,3 kills hum.
I swear, it never ends with this amp...just when I thought I had it licked...

[JazzGuitarGimp]

Are you sure it's 120 and not 60Hz? Are the preamp heaters elevated?

[Smokebreak]

Not positive, but it's a new hum that is different fom the usual 60hz single coil hum, and happens when nothing is plugged into the amp, as well. It's inbetween a B and Bb, too, 6th fret low E string, which I've read is 120.
My heaters aren't elevated by the winding has a center tap grounded .Would it make a difference to do both?

[pdf64]

I've noticed that with B+ hum (ie 120Hz for you) occasionally.
Never reached a satisfactory root cause but my guess is that it's due to poor grounding and being 'one of those things' in which 2 sources of hum can cancel out if in opposite polarity, eg cascading stages. Less gain from an unbypassed cathode may have resulted in less hum available to cancel out other sources.
If it's 60Hz then try a different tube - it could just have imperfect heater/cathode insulation but elevating it may well help.
Pete

[Smokebreak]

Thanks guys well I tried a ton of things and finally just took the cathode cap off 1st stage , kept 2nd stage stock and the hum is nearly gone. I really just wanted to make this more tele friendly for roadhouse stuff up the dial. I put in 50k trimmer w/ 68K inbetween stage 2/3, and 2n over plate resistor stage 2, and no cathode cap on v1. Now it's a little country/little rocknroll. Happy for now...

[Merlinb]

It could also be ripple from the PSU. Removing the bypass cap reduces the PSRR of the stage, so more PSU ripple gets fed to the following stage. I'm not familiar with that amp though, so I don't know how likely this is in practice.