>> 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.
? Good point, Mr. Rauch. A friend runs an amplifier analysis program
for an engineering company. He plugged in the feedback-C spec and other
constants for an 8877 and he found that it could only oscillate above
100MHz -- so HF is a safe zone. The sticky wicket is that with HF
amplifier layouts, the tube's C-out (10pF), the lead from the anode to
C-tune (typ 100nH), and C-tune itself (typ c.100pF) form a VHF resonant
circuit. Whenever the DC current in the anode lead changes, the tuned
circuit rings like a bell - much like a spark-tranmitter rings it's
high-Q tuned circuit with a DC pulse to make RF. If the frequency and
the feedback-C are right, regeneration is occasionally possible. Since
VHF energy can not pass through the HF tank, it has no place to go, so it
can run amuck and damage stuff. Since ringing-V in a tuned circuit is
proportional to Q, the work-around is to reduce the feedback-signal by
lowering the Q of the VHF-resonant circuit with R. This can be
accomplished with a VHF suppressor. Mr. F. E. Handy discusses using
resistance-wire in parasitic-suppressors on page 72 in the 1926-edition
of *The Radio Amateur's Handbook*: ³The combination of both resistance
and inductance is very effective in limiting parasitic oscillations to a
negligible value of current.²
> 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
? "sub harmonics" (with the exception of grandkids) are yet another
porcine myth.
>...
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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