> A few months later my 2nd Alpha99 I had on order came in.
> They are about 3 months apart in age.
> I said to myself OK... lets see if this new one does the same thing.
> "Trip" ! It did it again. Wow this is driving me nuts.
> So I reduced power, and kept using it with reduced output.
> Like a few 100 watts are going to make a difference anyway..
>
> So one day during a contest I switched from 80 to 40, and later
> back to 80, and noticed my problem was gone.
> It was the ATR-30 Tuner.... Maybe?
I'm almost positive the Alpha 99 is one of the amps that have a tendency to
oscillate at very low frequencies when running into certain loads OUTSIDE
and far below, **not inside or near**, the band you are operating on.
Looking at ETO drawing SXX-X164 rev L there is a VLF feedback path through
L1, the parallel combination of R33 and D4, the parallel combination of D7
and R11, R10, and L3 directly from amplifier output to amplifier input. The
shunt impedances are virtually all capacitive reactances, and that forms a
low-pass filter!!!!
This forms a feedback loop directly from the amplifier output back to the
input through RF chokes in the switching system. The chokes and bypass
capacitors, in essence, form a low-pass network that connects the output
directly back to
the input.
When used with certain rigs that terminate the amplifier input in a high
impedance BELOW the operating frequency, and when the output of the amp sees
an almost open circuit at very
low frequencies, the amplifier actually oscillates at a few hundred kHz!
This causes problems similar to what you are describing. It also causes
spurious
signals on SSB and spurs and keyclicks on CW signals. There was discussion
about this on the topband reflector, after a contest. I had severe problems
with a W4 who use a PIN switched Alpha amp into a "T" network tuner.
If you add a 1/4 wl shorted stub for the band you are using on the PA
output, or eliminate the C-L-C tuner, the output of the amp is terminated in
a low VLF or LF impedance. This loads the feedback path, and the LF or VLF
oscillation stops.
This is a common problem with PIN diode switched amplifiers or amplifiers
that use RF chokes from output back to input, even when those chokes are
bypassed.
VLF and LF parasitics are actually more common than people imagine, and are
what cause the worse problems. The grid chokes in SB220's, 30l1's, and
TL-922's set the amplifiers up for such problems.
73 Tom
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