It is (except for a few coupling cap values) an AB763. It has a constant low-passed white noise. I have it down to V4B (removing V1, V2, V3 have no effect. I swapped in a known good tube, replaced the plate and cathode resistors and the cathode bypass cap (which was only a year old), replaced the 3M3 and 10pf on the grid, and replaced the output coupling cap. I lifted the board to check for foreign matter beneath it, and I heated it up to see what happened (nothing). Grounding V4B's grid to ground stops the noise. Help please.