Gibson GA-15 RVT Reverb Hum

[Helmholtz]

Another question I'm left with. You indicated that the humdinger was something of a band-aid solution and didn't address the root cause. So now that we know this hum can be eliminated by manipulating the heater voltage balance, does it suggest a specific root cause that I could be investigating? I'm curious as well why the bypass cap didn't seem to help as you anticipated if it is in fact heater related.

If the humdinger helps, your hum was heater related. A humdinger can't compensate ripple hum because that's a different waveshape.

Reading up on heater hum in Merlin's book, I now understand that there are several causes, mainly:
1) Heater- cathode leakage
2) Capacitive crosstalk between heater and grid within tube
3) Capacitive crosstalk between heater wiring and grid wiring.

A cathode bypass cap only helps with cause #1. So my earlier statement wasn't quite correct.

[bobloblaws]

If the humdinger helps, your hum was heater related. A humdinger can't compensate ripple hum because that's a different waveshape.

Reading up on heater hum in Merlin's book, I now understand that there are several causes, mainly:
1) Heater- cathode leakage
2) Capacitive crosstalk between heater and grid within tube
3) Capacitive crosstalk between heater wiring and grid wiring.

A cathode bypass cap only helps with cause #1. So my earlier statement wasn't quite correct.

Interesting. I'm in inclined to think 1) and 2) are ruled out since a brand new tube did not make any change. So that would leave 3) which I assume we can call a lead dress scenario of some kind. The heater leads are not twisted, could that have something to do with it?

[vintagekiki]

Only for first (input) tube.
To avoid heater hum, one of the solutions is to ground the cathode, and the required bias voltage is supplied from a lithium battery, or use a 10Mohm resistor in circuit g1.
The cathode is at gnd potential, so the “leakage” between heater and cathode is negligible.

[Helmholtz]

Interesting. I'm in inclined to think 1) and 2) are ruled out since a brand new tube did not make any change. So that would leave 3) which I assume we can call a lead dress scenario of some kind. The heater leads are not twisted, could that have something to do with it?

1) and 2) are not defects but normal tube properties - though there is some variation.

Any serious literature on tube amp design recommends to twist heater wires, as this is a kind of self-shielding which can considerably reduce crosstalk.