Edward Knobloch wrote:
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

While all of the previous comments are good advice I suspect a counter
would drive a guy crazy on an S-108 with the constantly drifting freq
and oscillator pulling.
The 100kc calibrator will get you in the ballpark...at least to as close
as you can read the dial...I don't recall that there's a front panel
tweak to align the pointer or LO to have the calibrator line up with the
dial.
Nice little radio but if you want a better radio, well...
Its kinda like sticking STP racing stickers on a Chevy Vega.

-Bill