Hello Jean:
I do not have a TL-922 and have only briefly looked at the schematic, so
these are just my thoughts to consider.
1. The filament transformer in the 922 is a combination unit that also
provides voltages for the relay and lamps. I assume there will be no room
for a separate filament transformer, so that would need to be external.
However, the 10 volt filament transformer is center tapped, so you can use
the center tap and one side of the filament transformer to get 5 volts out -
for one tube. You would then use either one side of the filament winding as
the B+ return or you could artifically center tap the filament by using two
resistors that are sufficiently high that it will not drain much current at
5 volts, but not be so high that it would impede the B+ current. This is
commonly done.
The next concern is using one tube, the input and output impedance will be a
mismatch. It may not be significant enough to worry about, but you may have
to deal with those matters. A non-inductive resistor across the input is
the simplest solution. Tuning will change in the output and ideally you
would change the output pi-network. Also, if the air is across the tube
rather than through a pressurized socket, you will not need to plug up the
missing tube hole.
Good luck, Colin K7FM
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