[old_coot88]

Originally Posted by Tennangler

First, is it ok to use the 35c5? Second, if it is does the hum indicate a grounding issue?

35c5 is a different pinout and wouldn't work without rewiring the socket.

https://www.radiomuseum.org/tubes/tube_35c5.html

35f4 would be a drop-in but most likely unobtainable

https://www.radiomuseum.org/tubes/tube_35f4.html

Hum is bad filter capacitor(s).