[HBR] Need advise: protection for my only 1600 kcs xfmr

[Ian Wilson]

Hi Cal,

The load resistance presented to a tube by a parallel tuned circuit is given
by Q times the reactance of the L or the C (same at resonance). A typical
455kHz IFT will have a winding inductance of something like 1.3mH and a
Q of 150. So the plate of the mixer or IF amplifier tube sees something like
90k load. The voltage gain of the stage is given by the gm of the tube times
the load that it sees. So if you were to use a 1200 ohm resistor (which
would
then effectively be in parallel with the tuned circuit), the gain per stage
would
drop by a factor of about 40! Also, the bandwidth of the tuned circuits
would
be reduced by a similar factor. (I used 455kHz for demonstration purposes
but any medium frequency IFT would have similar properties).

You could in theory use shunt feeding with an RFC in the plate lead. This
would need to present a large (compared to the LC) reactance to the plate.
So you'd be looking for a 300mH RFC at 455kHz with a self-capacitance of
less than 40pF. This (if it existed) would be a lot scarcer and more fragile
than your IFTs.

One thing you can do when powering up for the first time - having checked
with an ohmmeter that the IFT windings do not have unfortunate paths to
ground
is to apply a low HT to start with. The RF and IF amplifiers will probably
start
to work with as little as 25v on the plates (oscillators will probably need
more).
You can double check that the expected voltages, etc, are present before
applying
full HT.

My main problems with IFTs have all been mechanical: slug getting stuck in
the
threads, wire inside the can disconnecting itself from the pin when
soldering to
the pin, etc. Have not had (crossing fingers) any problems caused by using
them
in-circuit.

Good luck with the construction!

73, ian K3IMW