In the R390/R390A series of receivers a current
regulator is used to regulate the heater voltage
of V505 (BFO) and V701 (PTO) tubes. This was presumably
to minimize frequency drift when the radios were used
in a military environment where power supplies were
unreliable but in the average ham or DXers shack this
is hardly necessary. Besides which the 3TF7 tube is
expensive to replace. There are several ways to deal
with this problem.
1. If you are a purist you can replace the 3TF7 with
a solid state current regulator. There is an article
in Electric Radio on this subject (No 70, February 1995)
2. Replace the regulator with a 45 ohm 10 watt resistor.
3. This is the one I prefer. Replace the BFO and PTO
tubes with 12BA6 tubes. These are cheap and easy to
find. Then simply bridge out the current regulator.
I make a bridge out of a paper clip and simply
insert it in pins 2 and 7 of the tube socket. It
is a good idea to put a label on top of the RF
cover to remind you about this so you don't replace
one of the tubes with a 5749 somewhere down the track.