[Tubesforme]

If I were working on this radio I would determine which path the hum is taking. Again assuming that both the tube and the socket are good there are only two likely paths . Either thru #19 cap or #20 resistor into the 27 tube.

The first thing I would do is to lift the #19 cap as that won't affect the bias of the 27 tube and will break the path from the IF detector. If the hum stops then we know it's from there. Hum still there then it's the bias line thru the #20 resistor.

If the hum is from the IF detector then as Steve mentioned the #14 cap and the #18 resistor should be checked.

I have tried various 27 tubes and each causes different amounts of hum. I have changed out part 19 twice, with different brands, NOS with no change. Part # 20 measures right on 470K. I have cleaned the tube socket, by hand and tightened the contacts, no change in the hum. I had connected only part # 20 to the grid of the 27, leaving part #19 disconnected. The hum was still present. Naturally when the grid is left open the hum is extra loud. I have tried bypassing part #20 with a small electrolytic, connected to the supply side of the bias. Still had the hum. The only way to eliminate it is to ground the grid of the 27. Part #14 and 18 have been changed out twice, and both test good. I do suspect feedback from part #21 even though it is NOS. Also I have a Philco speaker with the correct output transformer coming because I am using a universal output transformer and a different speaker with a higher ohm field coil. The hum might also be some feedback from that transformer. Once that speaker arrives I will retest the radio and if the hum remains then replace the interstage transformer. This job really needs a scope, which I don't have anymore.