[MrFixr55]

Hi Glyphmon,

Re the screen of the 6F6, it depends on which 6F6 you are measuring the screen on. On one 6F6, the screen connects to the same B+ that the output transformer center tap connects to. However, the other 6F6 screen connects to B+ through a 3.5K resistor. This is done so that the varying voltage from the screen of that 5F6 represents a signal 180 degrees out of phase with its input. This is the inverted signal sent to the other 6F6. This is in lieu of using a phase inverter stage; a Philco trick in a lot of the 1940-1942 sets that generally works. (Later Philcos used a genuine Cathodyne phase inverter.

The varying plate voltage on the IF 6K7 may be rather worrisome. Measure the B+ to the transformer. If this is stable and the plate voltage varies by that much, then the transformer may be at issue, maybe a bad solder joint or bad socket connection. the transformer may ohm good under no current but with the tube plate current as a load to the transformer, the issue may manifest itself. there could be issues with the tube itself. Swap the tube with the RF 6K7.

Suppressors of both the RF and IF 6K7s should be the same. -3V does not seem outrageous. note the 25 Ohm resistance between chassis ground and the next resistor, the 168 Ohm portion of resistor #68. the other end is tied to the HV center tap. This is how negative grid bias is taken.

What happens if you pull the 6Q7? does the noise quit? If so, check the following parts of the 6Q7 circuit:
Plate resistor (50), Coupling cap (44), resistor (51). Check the following parts of the 6Q7 and 6F6 grid bias circuit:
Resistors 54, 56, 57, 68 (the wirewound, tapped resistor).

Hope this all helps.