Spent the better part of the weekend putting this pile of guts into a proper chassis (hoping that will solve at least a couple issues...).
Got a nice used 2RU chassis with plenty of room. Got everything in there nicely. Everything in the audio circuit except the transformers is on a piece of perf board. The power supply PCB, as mentioned earlier, is a repeat of an earlier build.
Had a bit of hum when I first powered it up, so took a closer look at the wiring. Made a couple changes regarding shielding that seemed to help, but still had some hum.
Here's how it looks so far....
The input XLR has pin 1 soldered directly to the shield lug, so the pin 1 connects directly to the chassis right at the connector. A length of Belden 8541 shielded cable connects that XLR connector to the primary of the input transformer. The drain wire for this run is only connected at the XLR, NOT the transformer. The shield of all three transformers is also connected to the chassis, simply by virtue of the fact that they are bolted to the chassis to keep them from moving. I ohmed this out to confirm.
The secondary of the input transformer uses another length of 8451 to go to the circuit board, but the shield is not connected at either end. Trying it connected didn't make any difference.
The gain pot is connected to the board using 8451, with the drain wire providing the signal ground, the black providing the wiper, and the red providing the top connection.
More 8451 makes the connection to the primary of the interstage transformer, again not shielded, and again, with no change either way. I may go ahead with connecting these shields anyway.
The secondary of the IS transformer is connected to the input of the PP stage using 8451, with the drain wire as the ground connection (secondary center tap).
Since the output of the PP stage is also carrying the plate supply, I have two heavier gauge wires going from the board to the primary of the output transformer. I figured I should run these twisted together, since that's what I did with the heater wires to both tubes.
From the output secondary, another run of 8451 goes to the output jack. As with the input, the shield is only connected at the connector end.
The three separate rails from the power supply go to a terminal strip right next to the supply PCB. From there, the 0V ground, B+ supply, phantom power, and phantom ground are accessed. I have a twisted pair feeding the heaters from there. A single ground wire and one B+ wire go to the board, and another single wire feeds the B+ to the center tap of the output transformer primary (for the PP tubes).
From the supply PCB, the power ground connects to a chassis lug along with the mains ground.
Still getting a slight hum. If I'm listening when I unplug it, the hum disappears even as the unit still passes signal until the supply caps run out. So I started to take a closer look at the power supply transformers. They are both toroidals. They are situated at the opposite end of the chassis from the input stage. The one thing I noticed was that the B+ output from the supply was awfully close to the second transformer (the one that provides the step-up for the B+). Just rotating the transformer seemed to help a little.
I got a little more help by simply unbolting the transformer from the floor of the chassis and placing it vertically against the side of the chassis. I've yet to try this with the other power transformer, but I can't see that it would hurt. I'm also figuring on moving the terminal strip, since I parked it right near those transformers.
The odd thing is that my last build uses the same supply, and in a much tighter package; and did not give me this same problem. On that one, the power transformers are stacked on top of each other in the bottom (horizontal), and the PCB is mounted over them in a vertical position, right smack in the middle of all the audio hardware. (That gadget is not in an RU chassis...)
It's looking a little better, but not quite there yet. Trying right now to hunt down any ground loops I've either missed or created.
Phantom power isn't even connected at this time.