Thanks to all who responded to my tale about the flash/bang/bad smell in my
SB-220.
Turns out that a non-standard 1-ohm resistor in one anode lead had failed
open.
Sorry for the bandwidth consumption to follow, but I really am an amplifier
neophyte, and I need some more advice before the CW Sprint tonight.
I tested the tube from the failed side in the non-failed side. The good
news is no evidence of a short or other terminal malfunction, but the
1-tube situation didn't yield much output, which contrasted a bit with the
situation right after the failure, when one tube continued to put out about
what it should have. I figured that with only one tube in its socket (vs
two before, abeit with no anode voltage on one) the input SWR would be off
- and it was, at 2:1 - and so low drive.
Anyway, next step was to replace both parasitic suppressors with ones
supplied by KM1H. With both tubes back in their sockets, I applied plate
voltage - no problems. I then keyed the amp's bias on. Trouble. At a
resting current of about 140 ma in SSB position (about 100 ma in CW
position), one of the final tubes (I *think* the one that didn't have the
anode resistor failure) is showing slight color. In the tune position on
my TS-930, in CW position on the SB-220, with about 25 watts indicated
drive, indicated output power is about 300 watts, but the same tube is very
much brighter than the other one. With 50 watts drive, in SSB position,
the power output is about 650 watts, but the suspect tube is a fairly
uniform orange throughout the anode, while the other one is only starting
to show orange near the center of the anode. Plate current is about 525 ma
and grid current about 115 ma.
Finally, I tried full key-down CW, with the amp in SSB position. Drive is
about 110w. Induicated output is about 1300W. Plate current is 750 ma.
Grid current is 215 ma. For all of these tests, output loading is just
slightly beyond maximum output. The suspect tube is still much brighter
than the other one, but doesn't seem to be getting any brighter than it did
with 50 watts drive and 650 indicated output (I have not kept it key down
more than 20 seconds, max.).
The amplifier appears to operate normally on all bands, except for the
unbalanced tube color. BTW, on 10 meters operation is very similar to that
before I replaced the parasitic suppressors. Output on the lower bands is
right where it was before.
It's possible that the tube color has unbalanced been this way since day 1.
I'm looking a lot harder, and the outer cabinet is off now, where before I
could only see them from directly above.
Finally, my questions: Is this imbalance perhaps a function of something
happening during the glitch? Should a 3-500Z show color while idling at 70
ma (140 ma for 2)? All other things appearing OK, will I do any additional
harm operating the amp in a short, intense CW contest? Should I be looking
for a new tube or pair?
Thanks all, for your help and patience.
73, Pete Smith N4ZR
n4zr@contesting.com
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