> Add another twist or two...
>
> There is also a sub-harmonic being generated at exactly 1/2 the tx
> frequency. It comes and goes exactly in sync with the 1.5x
> 'harmonic'.
>
> Now the real kicker! I swapped the al-1500 out for an hf-2500, and
> the 'harmonics' are still there. And they respond to the tune control
> on the hf-2500 exactly the same as on the al-1500.... so now I am down
> to the 'some other problem'... still related to amps, but apparently
> not specifically the al-1500. This makes the problem much more
> important to find and fix as the last resort plan was to move the
> al-1500 to 10m, but obviously this is not going to fix the problem
> now.
It is virtually impossible to have an HF oscillation problem in a GG
amplifier with a very stable tube like the 8877. You also indicated
it was level sensitive with a fairly high power threshold, which
indicates it is outside the system.
All that considered, I'm not surprised you have found the same
problem with another amp.
I'd go through the system using standard troubleshooting of
disconnecting things one at a time, while seeing what happens. It
most likely is in a radio at your station. I assume you swapped
exciters to another type of radio, as well as the "signal checking
receiver" you are tuning around with.
Several years ago, I was called to find a similar problem at a
Government site. It turned out to be a radio that was OFF but was
left connected to an antenna. When the radio was turned on, the
problem vanished. The front end of that radio was mixing a signal
from a BC station with the local SW signal!
While I doubt you have something like that going on, you can have
some very weird things happen when field strength is extremely
strong. I have never, at HF, seen a "sub-harmonic" being generated
without mixing or unintentional spurious signals from the transmitter
itself.73, Tom W8JI
W8JI@contesting.com
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