Another options is a frequency selective voltmeter, they used to be real
cheap but now not so because I think the VLFers have their eyes on them. I
still see the odd one at hamfests for about $20.00.

WOW! I have a HP-312/313 pair that I paid a couple hundred for, but I
wouldn't let them go for twice that.

Yes I agree that a transistor osc. or a 6BE6 would do the trick.
I kept one HRO for sentimental reasons and to waste gravity and somewhere
in a box I have stored some odd ball coils. I'll have to see if I have
LF. I might because interest in LF and VLF is one of my chronic diseases
that is currently in remission.

If I want to use WWVB, I can use the 312 (With a surplus filter to keep the
59 KHz from the video monitor 2 floors away out of it) or a little TRF/PLL
RX I built for that maybe 20 years ago. Interestingly, my loop, (100 turns
on a 32 inch bicycle rim) resonates at 60 KHz with a big ARCO trimmer, but
resonates right at 24 KHz for NAA with just a alligator clipped on 1890 pF
silver mica. I monitor that with a simple DBM/crystal oscillator (with a 96
KHz oscillator and a divide x 4 74AC74). Lots of signal. I used to monitor
with an old Rustrak paper tape recorder, but a few years back, Radio Shack
had a special on for Metex Voltmeters that had an RS-232 output and a piece
of software that would send a reading to a file on the computer. Now I just
throw away the parts of the file that are uninteresting and save the SID's.

Interesting part of the spectrum, I also did a BC-453 conversion solid state
and to 136 KHz several years back, that was a learning experience but not
terribly interesting.
W4ZCB