[TenTec] Zero beating aids?

[CATFISHTWO@aol.com]

I remember back in tech school, (1966) in the air force at Biloxi, MS, we  
had classified "sets"  so we would go in the classroom and first pull the  bulb 
out of a couple of signal generators so they looked like they were  off.  then 
we would hook them up to speakers, and set one at 25,000 cps and  the other 
at 28,000 cps or so, you could not hear either of the signal  generators ( too 
high of a pitch, but you could hear the 3000 cps squeal from  their 
hetrodyning..  But you could only pull it once per instructor.. ..  same idea, different 
result..:)  tom N6AJR
 
In a message dated 6/28/2005 7:31:19 P.M. Pacific Standard Time,  
dsaukers at mn.rr.com writes:

The  other was an audio frequency generator set to 750 Hz. You would 
audibly  beat that against a received tone. If the tones matched, you 
had only one  tone to listen to. If they did not match, you would get 
both sum and diff  and an audible warbling.

I know I used them with my HW-101. Don't  remember if I used them with 
my Corsair or '745.

Any of you  remember something like this? (I no longer have my 
archived  magazines).
They were probably OP-Amp  circuits.