PI hum

[NSky]

Hello everyone. I've just finished building PI&PA part of LiteIIb. It sounds great but there is 60Hz hum. It's not loud, it's rather subtle, but when I'm alone in a silent room it's just enough to be annoying.
That's my first tube amp, so I'm not even sure it's a layout problem, may be I'm just nitpicking as I'm a damn perfectionist. So if you tell me this actually a "feature" of tube amps I'll live with that
Anyway if there is a little chance to get rid of it I'd like to. What I've done so far.
1. Took out the PI tube and the hum gone. So I found that it's coming from PI. (input is shortened)
2. Tried disconnecting PI heater for a sec - hum fades out smoothly as the tube cools off. So this is not the heater hum...
3. Tried disconnecting PI's cathode/grid R (47k) and C(.1u) from the ground - hum's gone. So... pretty sure it's ground loop.
4. Tried connecting the mentioned R and C to different parts of the chassis, input jack, etc and found that I already had it in the most silent place. Other points give me same amount or more hum.
Now I start guessing... So looks like the "hum receiver" (i.e. PI) is wired properly as I get the lowest possible hum. So may be I'm able to reduce the amount of hum emitted by the PA?
I'm a tube noob so I'd like to know if someone have hands-on experience with this and if it works:
1. Put a screen on the part of the PT which is inside chassis? (PA is Hammond 1750 btw)
2. Cut off PA wires to shortest possible. (I now have rather long wires (braided though) and don't want to experiment yet as it's a draft design and I'll likely put it into another chassis)
3. Bigger filter caps in power supply?
4. In most builds looks like people don't put anything above PT. I got rather tight space, do I need significantly bigger chassis to reduce hum?
4. Anything else?

[NSky]

And this is my ground "star":
- red dot connects to chasis
- blue dot is heater center tap
- yellow dot is plate supply center tap
- pink is shielded input wire (screen is only connected to the "input jack" (return jack actually, dummy load resistor is connected to the input to shorten it )
- white is PI's ground (again, I tried connecting it to different spots: the input (return) jack, the left part of the chassis - won't help).
Anything else I can try?

[JMPGuitars]

Let's start from the beginning.

Did you try pulling the preamp tube first? Did that change anything? PI as the source of the hum sounds like a jump. As you said, it might be normal noise. However...

If you want us to diagnose noise, you need to make a sample of the noise. We need to hear the noise with and without guitar playing.

I also suggest reading the soldering and ground scheme threads in my signature. Watch the linked soldering videos too.

Thanks,
Josh

[NSky]

Thanks for the quick reply. There is no preamp yet, PI and power tubes only. I have an external preamp, but for the test I just shortened the input jack.

I'll try to make a sound sample later, it's pure 60Hz, very low sound without audible higher harmonics. Not even sure my phone can record it, but I'll try (too noisy now).
With guitar playing there is no hum as the guitar is gazillion times louder and it sounds great, may be I just should not stop playing

Soldering in real life is actually looking better than the photo I've been experimenting with different values, now need to make my soldering great again. Connections are ok though, chopstick test passed.

Grounding as far as I can see conforms with other layouts, except I'm not sure about grounding the return and speaker jacks together, but I saw it in one of your layouts so decided it's ok... and as I said I tried grounding in different spots and this is the best I could get...

[NSky]

Finally I fixed this hum and it was the most counter-intuitive thing that could ever expect. Hope this can save someone a few hours/days of troubleshooting:
1. I pulled the 22uF cap on C power bus (the green one on the photo) feeding the PI to test its value.
2. The cap was in specs, but I still tried it replacing with other caps and bigger values caps without luck.
3. Then I happened to turn the amp on without that cap at all - hum was gone!
That's the weirdest part, the cap which job is to reduce hum did the opposite.
4. I tried connecting and disconnecting that cap with the amp on just to make sure I wasn't going crazy (in that case I could use some electric shock for therapy ). The result was the same - the cap was making hum.
5. I removed that cap from where it was and soldered its negative directly to the 32uF cap can negative and the positive where it belongs. And so there is no annoying hum anymore (unless I put my ear within 1 foot from the speaker, there I can hear something which is totally fine).

[Daviedawg]

It sounds like you were creating a ground loop with the neg connection in original position. In any case power amp grounds are better separately connected from other stages. So grounding at the cap can is fine.
Since you do not have a full circuit installed yet, is it possible you made a ground loop incidentally with the signal insertion?

Dd

[bradfinamore]

Also consider this:
Hum on the entire +B will be cancelled by the PI as it is fed to the output if it is "in" phase. (Not sure if I worded this correctly)
I thought of this because I was fighting a similar, if not the same problem in a build. The problem went away when I connected the rest of the circuit.