Here's the test condition:
No tubes installed, power on and warmed up for a minute.
The bias rail before the grid circuits measures -53 volts and maintains that value. However, from left to right as viewed from the top rear, of the 4 6550 tube sockets, the two inboard sockets slowly go down in bias voltage. This is after I have replaced all of the resistors in the circuit for good measure. This makes absolutely no sense at all, and lifting the cold side of the 1uf coupling caps to the grids confirms there is no DC leakage causing any problems. The only thing I can think is the opto couplers but it still doesn't make any sense. There should be no voltage drop without a load, and even with the tubes installed, there should be minimal if any drop with a bias supply so well regulated. I have slept on on it and I woke up without any answers. A real head scratcher.
Thanks for your response.