Bad PS hum on Ultra Linear Twin

[wizard333]

Getting a bad PS hum on a UL Twin. Happens even if all preamp tubes including PI are removed. Replaced all electrolytics, tubes, no change.

With PI removed, showing 3.5mv of AC on the grids. Plates on one pair show a bit over 1vAC, but plates on the other pair show a bit over 2vAC.

Not sure where to look; the ripple seems about as clean as I'd expect with new caps, etc.

Any ideas?

[txstrat]

Can you rule out the power tubes themselves (tried known good ones)?
All groundings ok?

[wizard333]

Replaced all electrolytics, tubes

So yes I can rule out tubes. Which grounds were you referring to exactly? Everything I can think of tested fine.

[wizard333]

Nevermind I found it, bad screen grid resistor. Duh, I feel stupid.

[txstrat]

How'd you find out? Did it look burned?

[wizard333]

Nah it looked perfect which is why I missed it. I decided to check bias on all 4 tubes instead of 2, and saw that on that one there was 500v of plate voltage and 0 cathode current - which screams bad screen grid resistor. Tested it and sure enough. If I had had my bias probe on that tube to begin with I'd have seen it right away. I guess that speaks to having 2 of those things for 4 tube amps!