>> 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!
But does the Rauchian-decree check out? ... ... . . There are two 1 ohm
MF resistors in parallel [Ra], each of which is able to carry 2.5a for
1-hour with a less than a10% decrease in resistance? At 29MHz, the 5pF
anode C has a reactance of 1097 ohms. With an anode supply potential of
3000V, the RF potential across the tune-C is roughly 3000 -300 =
2700V-peak, or 1900V-rms The current through the paralleled 1 ohm
resistors is 1900V/1097 ohms = 1.73A-rms, or about 0.9A per resistor.
Sure, RF amps cause more heating than DC amps. However, the safety
factor appears to be adequate except for RTTY broadcasting on 10m. .
.......
>> 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
True enough. BTW, the telegrapher's indication of laughter is "hi hi".
A 'novice accent' does wear well on us old guys, Mr. Rauch.
>
>> 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.
>
I have never encountered one with a broken grid wire. If a wire was
broken at one end, it could not bend enough to cause trouble. If a grid
wire broke at both ends, it would fall to the bottom of the envelope, out
of harm's way.
>> 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.
zzzzzz zzzz
>
>> 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!
Mr. Rauch confidently asserts that the tube may have arced. How does he
know this?
>
>You'll never know if the chicken came before the egg, because the
>physical evidence is identical!
Not quite. If there was an arc, chances are there would have to be gas
present, which would not disappear. If there is no gas present, measure
Rs.
>
>> 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.
At 500w dissipation, the anode of a 3-500z emits a fairly bright
yellow-orange light.
>
>> 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.
I agree, Mr. Rauch. Such a statement is nonsense.
- I have never stated that a parasitic can cause an arc in a 3-500Z.
However, in a tube with a gold-plated grid, intermittent VHF & UHF
parasitics can evaporate gold from the grid, that condenses into loose,
gold melt-balls, which may precipitate an anode to grid arc.
- Anyone who is in the neighborhood is hereby invited to look at the
grid of a gold-sputtered 8877 through my 50x microscope. It's fun. It's
educational. ....... and I have more gold-sputtered tubes that we can cut
open to have a look at.
...snip...
>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.
A bent filament-helix in a 3-500Z, IMO, indicates a VHF-oscillation
situation. To find out if this is the case, unsolder one end of Rs, the
VHF parasitic-suppressor resistor. Measure the resistance. If the
resistance of Rs is above the marked value by a substantial amount, I
cast my vote for Mr. Parasite.
Rich---
R. L. Measures, 805-386-3734, AG6K
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