[Amps] LF oscillation caused by defective plate choke?

[N1BUG]

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