>This morning, while tuning up my SB-220 on 15 meters, I did the
>flash-bang-bad smell number, followed by a sudden flare of white light from
>one of the 3-500Zs in my SB-220. As long-time readers may recall, this amp
>has the AG6K parasitic suppressors.
>
>After the event, the amp showed about 1/2 normal resting plate current and,
>when driven, approximately half normal output.
>
>Opening the case, I found that the low-value resistor in series with the
>anode of the other tube (the one that did not "light up") is shattered.
>
>As I understand it, the resistors are there as a fuse against plate
>glitches. My post-glitch experience suggests that the other tube is OK -
>normal resting current and output. Should I just replace the series
>resistor and try the amp again, or should I assume that the 3-500 on the
>damaged side is kaput? I ask because I'd rather not kill the remaining
>tube, if there's any risk of doing so.
IMO, there is no risk to the other tube.
- To find out why the anode fuse resistor let go, one needs a
high-potential tester.
. Measure the leakage current between the anode and grid. At 8kV, with
0v on the filament, there should be under 10uA of leakage. If there is a
bad seal, the leakage will be much higher and you may see a blue glow in
a darkened room. Another problem (Eimac only) is bad spotwelds between
the anode cooler and the anode stem. This sometimes allows the anode
cooler to come into contact with the grounded grid. Either problem can
blow Ra.
>
>Also, any idea what caused the episode?
Any of the above, plus an intermittent VHF parasitic-oscillation. I have
a TL-922 which makes up it's mind to blow the anode fuse resistors (Ra)
about once every 5 years. The fix: Replace Ra and replace D1 in the
high speed switching circuit, and all is well for another 5 years.
If your VHF suppressors use, U-inductors for Ls, it has been my
experience that, although U-inductor suppressors dampen the VHF
cavity-resonance in the 922's output compartment, U-inductor suppressors
are not easy to decouple from each other. It might be worthwhile to move
the suppressors farther apart. In some cases, it might be best to use
one coil-inductor and one U-inductor to discourage the suppressors from
talking to each other..
IMO, Pete, not everything that happens in amplifiers can be explained.
My helper and I are toying with the idea of supplying a 25mm x 30mm black
candle with each retrofit kit.
Rich---
R. L. Measures, 805-386-3734, AG6K
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