This is rare but does happen. There can either be a feedback path that
causes the problem due to the fact that by-pass capacitors
are chosen to bypass HF RF not low frequencies or the tetrode at times is
operating in the dynatron region, a region where the plate
impedance is negative. The difference in the chokes is likely to be their Q. If
the choke had lower Q the amplifier would be less likely
to oscillate. This is why the VHF parasitic chokes have a resistor. The extra
series resistance lowers the Q, introduces loss, in the VHF
parasitic resonance between the tube plate capacitance and the output matching
network.
I have even seen a problem with a LCt filter used 28 power supply to power a
300 Watt solid state amplifier.
We were getting a very low oscillation, around 50 Hertz, in a RF amplifier
powered by a 400 Hz generator. I had it on the
test bench and realized the nonregulated DC power supply output was
oscillating. It was simply a power transformer, full wave
bridge rectifier and LC filter.
I finally realized that the problem was solved by replacing the filter
capacitor. It had lost considerable capacitance over 20 years
and by doing so the resonant frequency increased in the LC filter which also
increased the Q of the filter. It was probably marginal
in the first place.
The reason was that the solid state amplifier had considerable feedback that
fixed its gain. When the collector voltage increased
apparently the amplifier became more efficient rather than less efficient and
the collector current dropped producing an negative
resistance. Could actually measure this effect with a variable regulated power
supply. The final solution was to replace capacitors in
both of the amplifier with much larger ones and then everyone was happy. The
problem did baffle our accelerator technician and
me too for a while.
It initially looked as if a passive circuit was oscillating and my first
thoughts, since it did have a filter choke was to think back to
the old magnetic amplifiers. But, it only oscillated when loaded with the
amplifier.
73
Bill wa4lav
________________________________________
From: Amps [amps-bounces@contesting.com] on behalf of N1BUG [paul@n1bug.com]
Sent: Tuesday, October 08, 2013 7:46 AM
To: 'Amps'
Subject: [Amps] LF oscillation caused by defective plate choke?
Several months ago I inquired about a problem with my 4CX1500B
160-10m amplifier. The initial symptom was a welded vacuum relay
while operating on 12 meters. I replaced the relay and subsequently
discovered a LF oscillation in the 400-500 kHz range. This would
happen on all bands with certain (very broad) settings of the tune
and load capacitors. The LF oscillation would vary over roughly a
100 kHz range with the tune and load caps. There wasn't any obvious
sign of the cause. The amp had behaved well for 9 years since its
construction.
My station had to be disassembled for home repair and renovation
before I got to the bottom of this. I recently got back to it and
discovered, after removing the plate choke, it has developed some
rather odd irregularities. The choke is a close copy of the
multi-section choke that is or was in the ARRL handbook. The first
and second sections from the hot end appeared to have small sections
of enamel missing from the wire. Additionally, there was some minor
blackening of the ceramic form in the gap between those two sections
of winding. Visually there did not appear to be any shorted turns
and I thought this didn't look serious enough to cause the amp
problems. However since the choke had visibly changed, I decided it
must be replaced.
While waiting for the new choke to arrive I was doing some testing.
While trying to tune the amp on 40 meters it developed a new
symptom. Upon reaching about 700 watts out, the power output
suddenly dropped off sharply to less than 200 watts. It wouldn't
cone back up without reducing drive to a very low level and then
bringing it back up. When drive was brought back to a level
sufficient to produce 700 watts out the sharp drop would happen
every time.
Yesterday I installed a new Ameritron plate choke. I tested the amp
on all bands. Power output was as expected with no apparent sign of
instability. I varied the tune and load capacitors through their
full range on every band, in particular testing the combinations
that had resulted in LF oscillation before. I could find no sign of
it. As far as I can tell, all is back to normal. One thing I did
notice is the plate tune capacitance is somewhat different than it
used to be on all bands - even 160. It isn't a drastic difference,
but very noticeable since I had all the old band presets memorized.
It is a vacuum variable with turns counter, so even small variations
are easily recognized.
I remain puzzled as to exactly what malfunction of the original
choke would have caused LF oscillation. I am somewhat paranoid the
plate choke wasn't the whole story, but I can find nothing else
wrong. Time will tell, I guess.
73,
Paul
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