[Amps] LF oscillation caused by defective plate choke?

[N1BUG]

Manfred,

Thanks for the reply.

> Could it be that your plate choke was arcing over between those turns
> where the insulation flaked off? Or perhaps those turns tended to get
> shorted via creepage over those blackened areas, without visible arcing.

It is possible. I couldn't check for visible arcing without removing 
the cover, which would change stray capacitance quite a bit and 
upset things. Also I would lose forced air cooling since the chimney 
is inverted and attached to the amp cover. This is a somewhat 
compact layout. Or at least is is "squashed" in height to take up 
less rack space.

> Any such shorting of turns would result in a drastic reduction of the
> choke's inductance, bringing it into a range where resonance with bypass
> caps and the like makes the amplifier unstable in the low frequency range.
>
> It might even be that the negative resistance behavior of such an
> arcing or creepage between turns contributed some gain at the
> oscillation frequency!

Interesting thoughts.

> That's a classic sign of the amplifier breaking into oscillation.

Yes, and the fact it started behaving oddly on more bands and under 
a wider range of tuning conditions makes me think something was 
getting progressively worse.

> It's quite likely that the only other "fault" would be that the
> combination of bypass capacitances, stray inductances, and so on,
> including those in the tube socket, are giving you a rather marginal
> stability, so that even a small change in the plate choke's inductance
> is enough to make the beast oscillate.

Which leaves me wondering if other upgrades or changes would be a 
good idea. The by-pass capacitors surely are not much good at low 
frequencies, some maybe even a bit marginal on the lower HF bands.

> - Load down the grid with resistors, as much as you can until being left
> with just the necessary gain.

I forgot to mention this amp has only 13 dB gain which is just about 
enough as it is. The cathode is driven and loaded down with a 50 ohm 
resistor, with grid at RF ground through 6000 pF, and screen grid is 
connected directly to the chassis. I have of course inspected and 
cleaned the screen grid to ground connections, as I'm aware things 
can go very wrong if that is not a good solid connection!

> - If you happen to have another choke in the circuit, for example at the
> grid, or perhaps the screen, make sure that it is totally different from
> the plate choke, in terms of possible resonances by itself or with other
> circuit components.

There is a filament/cathode choke, bifilar wound on some unknown 
ferrite rod. There is no choke in the grid circuit, instead there is 
a 200 ohm resistor with a feed-through capacitor at its cold end.

> If you want, send me the schematic, just to see if I can catch any
> further reasons for oscillation.

I wish I had a schematic to send you. Unfortunately I was lazy and 
never got around to drawing up a schematic. Except for the tapped 
inductor in the output pi network and some L and C values elsewhere, 
the circuit configuration is identical to my 6 meter amp, which I 
did make a schematic for (I don't know, just have been bored that day)

http://www.n1bug.com/tech/4CX1500B-6m/6m4CX1500Bsch-hires.jpg

I can tell you many of the C values from memory,

C1 .01 uF
C2  6000 pF (3 x 2000 pF)
C6, C7  .01 uF (I think)
C8  .001 uF
C9  .001 uF

I haven't the slightest clue what the inductance of RFC1 is, 
unfortunately.

I think I'm not very happy with some of those. This was built from 
whatever parts were at hand at the time, and since it was stable for 
9 years I did not give it further thought until now. Comments?

73,
Paul