Adding gain pot introduced hum that goes away at full gain, why?

[CJ]

try swapping the blue and purple lead just for the heck of it,

that is, swap the wiper with the top of the pot,

 

[rock soderstrom]

Maybe a grid stopper directly on your first tube socket could help you.
http://www.aikenamps.com/InputRes.htm

 

[PRR]

> buzzy harmonics (which weren't there before)

The buzz was there all along. It harmonizes with the hum, so your ear did not realy notice it until the hum was *gone*.

Yes, long grid leads suck up all the buzz in the room and amplify it. You need the cover tightly closed to verify buzz/humm measurements. If the wall-power supply is in the same box, it becomes a difficult problem. Shorter grid leads, shielded grid leads, are often necessary.

The other side: just like proper this-to-that grounding of input is needed, proper bonding around the power supply rectifier is needed. Rectifier should go RIGHT to first filter cap *before* any wires to the amplifier. I once saw (one of my ideas) with the rectifier circuit run all around the long side of the PCB to the first cap, and signal grounds tapped along it. It buzzed. X-actoing that trace and running a short fat wire overboard RIGHT to the cap helped a lot.

 

[Bowie]

Thanks again for the responses.  I ended up ditching the leads from the pcb to the first tube's socket and just soldered a socket directly to the board as it was designed for. Inexplicably, it made things worse!  The noise is no better and now I have a random garble noise that comes every few minutes and stays for about 10secs before disappearing.  It's like an Atari video game noise, strange blips and pops with a ticking in the background.  It's not radio interference.  I can't tell where it's coming from and no amount of jiggling will make out go away.  It just comes and goes every so often.  It never happened when I had many leads hooked up.

  Any other ideas on grounding would be greatly appreciated too.  I've spent many hours experimenting but made only moderate progress.  I'm not sure where I'm going wrong or if it's just this PCB design. There is still a buzz that is problematic. I'm noticing a bit of hiss now too.  Incredibly frustrating as it's so close to being a great pre but this noise is just not acceptable. 
Would creating a false heater center tap using 100ohm radiators to ground have caused any problems?  It worked in my other builds but they weren't as noise sensitive as a mic pre.

try swapping the blue and purple lead just for the heck of it,

that is, swap the wiper with the top of the pot,

Thanks for the idea. Unfortunately, no change.

 

[JohnRoberts]

Go back to what was better... 

JR