hum problem in new homebrew build

[km6xz]

He did mention the transformers were "several inches" apart with a choke in between. Seems like a possible candidate. Personally I think this is being approached all wrong, shotgunning is not getting to the heart of the problem when taking some careful measurements would give enough data to reach a logical diagnosis. The frequency still has not been measured, nor ground currents or magnetic fields. That choke is likely not as well shielded as a transformer. Without some basic measurements, we can only speculate. Your transformer coupling comments were already dismissed above, probably prematurely.
Too many cooks and all, I will bow out....

[Enzo]

Did we ever ask if the PT and OT laminations were set at right angles or made parallel?

I guess the easy test is this: remove all the power tubes, connect a speaker, power up the amp. If you still get hum, it is likely the transformers coupling.

[km6xz]

Cross coupling is usually into the amp via feedback path so removing the tubes would only test extreme cases of cross coupling, which is clearly not. Think back emf being amplified out of phase for speaker damping and that would be the same mechanism.

[AcePepper]

Thank you all for the suggestions. After all I've tried I'm kinda back to thinking about the output tube's bias. Please help check my math on a couple things.

My main B+ voltage is 360vdc.

I have two 150 ohm cathode resistors, one for each pair of EL84s. I'm measuring 11.6v across each 150 ohm resistor.

If I remember right, to figure the current draw/bias: 11.6v/150 ohms = bias current for two tubes, .0773 Amp, or 77 mA. Divided by two tubes = .0386 A or 39 mA for each tube. Does this sound right to y'all?

According to the Weber Bias Calculator, EL84s, in Class A, at 360vdc should bias around 30mA. That would mean my amp is pretty much too hot/overbiased. Correct??

Whatdaya think??

[eschertron]

Thank you all for the suggestions. After all I've tried I'm kinda back to thinking about the output tube's bias. Please help check my math on a couple things.

My main B+ voltage is 360vdc.

I have two 150 ohm cathode resistors, one for each pair of EL84s. I'm measuring 11.6v across each 150 ohm resistor.

If I remember right, to figure the current draw/bias: 11.6v/150 ohms = bias current for two tubes, .0773 Amp, or 77 mA. Divided by two tubes = .0386 A or 39 mA for each tube. Does this sound right to y'all?

According to the Weber Bias Calculator, EL84s, in Class A, at 360vdc should bias around 30mA. That would mean my amp is pretty much too hot/overbiased. Correct??

Whatdaya think??

What's EL84's dissipation? Finish the math:

360v at the plate - 11.6v at the cathode = 348.4v across the tube
each tube conducts 38.6ma
348.4V * 0.0386A = approx 13.5W

is that where you want to be in class A? I don't have the data sheets handy, so I'm not holding back, I simply don't know the answer to that one. I only know 6V6s