Edward Knobloch ) writes:
I would simply use a divide by ten oscilloscope probe
connected to the plate of the S-108 local oscillator tube, feeding
a frequency counter. The divide by ten probe provides isolation,
to avoid pulling the oscillator off frequency.

If you just want a 100 KHz calibrator, I recommend an old Bud Radio
model "FCC-90 B". It has it's own power supply, and requires
no connection to the receiver. (Just uses a loosely coupled
antenna wire near the receiver input). It has two tubes,
a 35W4 and a 50C5, in a transformerless circuit.
The older Bud model FCC-90 used octal tubes.

73,
Ed Knobloch

Nobody has to go back that far. You can go back thirty to thirty five
years, and crystal calibrators were solid state. Not just that, but
they had a divider so you could get multiple outputs. This is
real useful for the especially ill-calibrated receiver (start witha
1 MHz output because they are easier to count), and those 25KHz outputs
were useful even for the better calibrated receivers, to find those subbands.

And realistically, once transistors and then digital ICs became available,
crystal calibrators were amongst some of the first station accessories to
be turned solid-state. And they were applied at the time to many a
real boatanchor.

Michael VE2BVW

Allen McBroom wrote:
I've had an old Hallicrafter's S-108 since the early 70's, and trying
to tell what freq I'm on is impossible. Does anyone know of a method for
installing a crystal oscillator on this old tube rcvr, or some other method
of acurately determining which frequency the radio is receiving at the
moment?
Many thanks for the help!