> To: amps@contesting.com
> From: Pete Smith <n4zr@contesting.com>
> Turns out that a non-standard 1-ohm resistor in one anode lead had failed
> open.
Pete, if you have a small one ohm resistor in series with an anode
lead in the path of RF, GET RID OF IT.
The circulating currents on ten meters will kill the resistor!
Perhaps you just stuck the resistor in the PA to indicate which tube
was shorted?
> 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.
New parasitic suppressors won't fix bad tubes.. hi
> 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.).
It the bright color all around the anode's circumference? Maybe the
anode is mis-aligned, or a grid wire has broken.
> 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.
New suppressors, even good ones, won't fix bad tubes.
> Finally, my questions: Is this imbalance perhaps a function of something
> happening during the glitch?
Could be, when the tube arced, despite all the "grid fusing", a
grid wire could have been destroyed. Or, a grid wire could have came
loose and caused the arc!
You'll never know if the chicken came before the egg, because the
physical evidence is identical!
> Should a 3-500Z show color while idling at 70
> ma (140 ma for 2)?
At how much voltage Pete? The metallic anode tubes show color at a
few hundred watts of dissipation. It's more important that the color
is fairly even both in one anode and between the two anodes , rather
than if an anode has color.
> 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?
All this stuff about parasitics causing arcs in healthy tubes is sheer
nonsense Pete. If the HF peak voltage doesn't cause an arc, the tube
is probably never going to arc from a parasitic.
I've actually removed suppressors completely form amps like the AL-80
and SB-220 and tried to make them arc, and was unsuccessful. All the
parasitic did was cause grid current to show up with no drive. No
explosions, no destroyed tubes.
It simply sounds like you have a bad tube. As tough as that may be
to swallow for those who make part of their living selling parts,
tubes actually just fail. Ask any TV repairman how many times a CRT
faults and blows a circuit component, or how often low power tubes
under light duty operation short or fail.
What about the color Pete, is it uneven around the anode?
73, Tom W8JI
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