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
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