[Amps] LF oscillation caused by defective plate choke?

[N1BUG]

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