[Milsurplus] BC-9: Answer One Question, Get Another.

[Hubert Miller]

As i recall, keying the set shifted the frequency down substantially. That's kind of to be expected for a vacuum tube VFO.
The fact that the 500 pF in the 120 volt supply has any effect at all on tuning means the filter F is not that effective -
RF is showing up at the +120. I wonder if in receive, the transformer winding + the meter winding inductance works to
choke RF. Then when the key is closed, these elements are short circuited, exposing the tuned circuit to the 500 pF. 
Ordinarily you might conclude this would detune the tuning yet more, on transmit. I "suspect" something is going
on with RF showing up across the unbypassed 4 volt filament supply and being coupled back into the circuit out of phase to 
the main filament to plate current. I am just thinking out loud and i invite anyone else to enlighten us on their theory.
I haven't carried it beyond that.

With the Q of the tuned circuit there can easily be 150-200 volts or more on it. Filter F can be "mostly effective" and
still have a substantial RF volts show up at the +120 supply. If my numbers are right, and they may be wrong, the
inductances are around 1000 ohms total and the capacitor bypass 50 ohms. E input x 5/100 still gives a substantial
voltage. 
-H