>
>> 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.
>
Good points.
>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.
>
Decreasing the Q (by using low-Q conductor material) of the AL-80's
extant anode-resonance ar c. 150MHz is going to do such?
>You also decreased the lower VHF and HF Q, a place where we
>generally want higher Q.
>
Not unless Ni-Cr-Fe alloy has more RF resistance as freq decreases -- i.
e., it has reverse skin effect. Copper, aluminum, gold and all other
conductors in the known universe exhibit more RF-R as frequency
decreases. How is it that Ni-Cr-Fe alloy is unique?
- Also, by increasing the anode V, HF output RL was increased by c. 30%.
Thus, the stock HF tank L would have been c. 30% low, so HF tank-Q would
be increased c. 30%.
>>
>> 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.
>
Raising anode V raises HF and VHF amplification.
>> 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.
>
However, such gas is unlikely to be detected with a high-potential tester
due to the normal cold-gettering that takes place when the tube is
removed from the socket, not.
>> 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.
>
Or maybe the Fairy Godmother will drop by and suck the gas out of the
3-500Z?
>
-- Weird Science.
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
end
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|