Parts came in, time to report back.
Replacing the LM350 for the heater regulator fixed the hum problem. I mounted it off board and doubled the thermal insulation sheets as well. Now the unit is dead quiet, measuring the same if not better than some of the solid state designs I've built. I still need some replacement festoon bulbs for the meters as I managed to burn my out during initial testing. Then we can rack this up and call it done.
After spending some time with it listening to it and messing around with it, here are my thoughts for this circuit:
First, a quick review of how mine is setup:
Kubi mods - LL1521 1:2.8 input, EA 2503 Hi Nickel output 1:3, Stepped controls, matched/noise tested NOS tubes from Christian Whitmore, Dual VTL5C2/2 Vactrols (thanks again, Paul)
As others have noted, it's not intrusive on the audio it is affecting, especially in the extreme low end, which is something I have had problems with other compressor circuits. It also requires to really listen to what is happening as the meters only seem to indicate that something is happening and not a real accurate gauge of the gain reduction taking place, but this has been the case for as long as VU meters have been in existence, at least in audio. Using a 500R trimmer position leg of the vactrol (with the dual setup it is the leg closest to Vx02) I am able to match the gain reduction to each channel when linking them using tone and when listening to program material after this adjustment I can not detect any shift in the stereo image. The metering however is not as closely matched as discussed in this thread. It seems that replacing R121 with a 10k trimming would allow the metering to be adjusted for gain reduction being displayed, unless I am reading the schematic wrong, which is always a possibility. For my purposes, I think I'm just going leave it as I know the actual audio is matched.
Also with stereo linking, I found it useful to turn one channels threshold all the way or almost all the way off and then just use one threshold control. This seems to give a more detailed range over the gain reduction.
The dual vactrols seem to make the differences in threshold/slope mods less apparent. The no mod threshold is much lower and almost negates the need for any sort of additional mods. I was debating between the single or dual vactrol and I ended on the dual. I sometimes think having a board to switch between the two would be handy, but for this circuit having a bunch of options is not needed for myself when it's primary role will be for mastering. I do hear some changes with the threshold mods as posted at the beginning of this thread, but not nearly as apparent as I was expecting, so more tests will be in order down the road to see if they make any difference for my purposes.
I chose a 1:3 transformer but I think a 1:4 as prescribed might be more suited as I find myself wanting a little more gain at the top of the range. R125 might be adjusted for more output as this appears to be a feedback resistor. Someone please correct me if this is not true.
All of my listening tests were with program material and using the DAOC for individual tracks may produce difference opinions. I'll post a pic when the meter lamps are replaced.
After three years of collecting dust, I'm mostly glad it's just done.
Thanks!
Paul