Manfred,
Thanks for the work and reply on analyzing my problem! Sorry I got
behind on correspondence this week. Now I'm catching up.
From the number of turns, length of the winding, and the dimensions of
the rod, it's possible to calculate the inductance with reasonable
accuracy, even without knowing the exact ferrite material.
OK I will have to look for the math on this. Next time I have the
amp out of the rack and input compartment open I will measure the
dimensions and count the turns. It would be interesting to know what
I have there.
I have attached a quick hand-drawn schematic of the circuit, seen only
from the RF point of view.
Thanks for that an the following explanation. Now I see the feedback
path! I am always forgetting the RF path through the power supply.
I did actually make one big mistake in my earlier post: the HF does
not ahve RFC3 and C18. It just has the 225 uH plate choke RFC2 and
1000 pf by-pass C9. This of course just makes the situation worse.
Interesting about the phase lag. I suppose this probably changed
with the new plate choke, which could be some of the reason it
stopped oscillating (or at least any visible evidence of oscillation).
OK, so as soon as possible I will add some 4700 pf or .01 uf by-pass
capacitors in parallel with the 1000 pf doorknob that is now at the
end of the plate choke. I will also add some larger capacitors in
parallel with C6 (or C6 and C7). I have some .01 uF at 1 kV, and
some .1 uF at 1 kV ceramic discs on hand. I'm not sure how much I
trust the latter as I got them really cheap and no idea who made
them. I will have to get a 1 uF or more electrolytic rated at 450V
or more to add there.
This all brings to mind a situation I had with a 4CX1000A amplifier
on 144 MHz many years ago. That amp was built by someone else. The
basic circuit configuration was the same as my current HF and 6
meter units. I noticed some very strange thumping and low frequency
howl in audio of various devices around the house - stereo, TV and
so on - when I was using that amp. I do not recall all of the
details but I remember somehow arriving at the conclusion (or more
of a guess at the time) the amp was oscillating at a very low
frequency. It had only 500 or 1000 pf by-passing at the points we
are talking about. I added larger capacitors in parallel. I didn't
know anything in those days, but I took a shot in the dark and
decided to improve low frequency by-passing. I believe at the
cathode by-pass I added a .01 UF, a .1 uF, and some electrolytic of
unknown value. Anyway the strange audio phenomena around the house
stopped.
All of this in mind, I think I will improve the by-passing in my 6
meter amplifier at the same time!
73,
Paul N1BUG
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