> overall with it. About 8 or so years ago, I decided I wanted to "soup
> up" the HV PS to a bit shy of 4 kV to see just how much smoke a single
> 3-500Z could generate. I did enough checking to determine that the
> components in the amp could handle this, but that I would be much
> closer to the limits of various switches and the tuning capacitors.
You are now far over the safe limits of the bandswitch and tuning
capacitor. The bandswitch is reasonably reliable up to about ~3200
volts DC anode voltage with normal conditions of mistuning. Same
for the tuning capacitor.
Best of luck!
> During this time, I also followed some of the suggestions of AG6K: I
> installed a new nichrome/MF resistor plate parasitic suppressor
> consisting of about 3 1/2 turns of nichrome wire wound about a 100 ohm
> MF resistor spaced about 1/8 inch the surface of the resistor.
You now increased the operating Q of the tube anode system at
VHF, near the frequency where parasitics occur.
You also decreased the lower VHF and HF Q, a place where we
generally want higher Q.
>
> Through this all the amp has remained quite stable, with no surprises
> or scary fireworks. At least, until a bit over a week ago. I was
> tuning it up into a dummy load on 20 m and, just as I was satisfied
> with everything, some fireworks went off. Visible blue flashes in the
> cabinet, lots of popping, and then the fuse blew. Strangely, there
> was no smell of anything burned or burning. Visual inspection showed
> that the grid current meter was bent against the negative pin, and
> fuses were blown. A 1.5 ohm 1% grid current meter shunt resistor had
> more or less exploded and was open, while the other 0.6 ohm 1% meter
> shunt resistor was fine. I straightened the meter needle and, to my
> astonishment, the meter still works. A bit more inspection showed
> that the Zener diode providing bias voltage to the tube was shorted.
> The original Eimac tube shows no grid-to-anything shorts. All else
> seems OK.
That's no surprise. Making everything worse is bound to cause an eventual
failure.
> rush I'd think the offending lint would no longer be in evidence. So,
> I'm assuming that I've been witness to a parasitic oscillation.
Probably not. Most things blamed on parasitics are not from
parasitics. More than likely the tube gassed up from sitting, and
now with a lack of headroom things exploded.
> now. More turns on the parasitic suppressor? I have no more
> nichrome, but have plenty of 0.028" stainless steel wire (aircraft
> safety wire). What else?
Put the original suppressor back in, and lower the voltage. You
also might need a new tube now, although you might getter the
tube by heating the anode red at low voltage for a few hours.
73, Tom W8JI
W8JI@contesting.com
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