[Hallicrafters] SX-101A oscillator mods

[Peter Bertini]

I'm thinking about modifying my SX-101A oscillator, and I am looking
for some ideas on replacing the 12BY7, which seems to be problematic
in this circuit.  I'm thinking of going over to a 6AU6 with regulated
DC on the filament. What was the reason for the DC bias voltage on the
12BY7 filament?

This is set is a low-value junker, I'm not modifying a particularly
collectable example.  I'm also tempted to replace the RFA with a
higher transconductance tube and changing the mixer to a 6ES8 or 6J6
Pullen.