>> From: "Ian White, G3SEK" <G3SEK@ifwtech.demon.co.uk>
>> Subject: Re: [AMPS] L-4B grid ckt shorts
To: <amps@contesting.com>
>> Date: Sun, 21 Sep 97 21:49:35 +0000
>
>> The same conclusion still applies, though. It seems incredible that so
>> much grid current could be flowing in an oscillation condition. I mean,
>> if that was the grid current, how big would the cathode current have to
>> be... and where would it come from?
>
>That's a good question Ian, since even 2000 volts of positive grid
>and anode voltage wouldn't drive that much current through the tube.
>
>The peak emission current of an ideally constructed thoriated
>tungsten filament tube is just over 100 mA per watt of filament
>power.
>15 amperes at 5 volts is 75 watts. 75 watts times 100 mA is is 7.5
>amperes.
After looking over the characteristic curves for the 4PR400A ('PR'=pulse
rated), I pretty much agree with thee. . . However, what if the tube is
drawing minimal current, and a cloud of surplus electrons are gathered
around the cathode? Might it not be possible to produce a one-shot
current pulse of something greater than 7.5 amperes? -- perhaps the
approx. 15 amperes necessary to melt the copper wire in the grid to
ground RFC?
- The bent filament helix shown on page 15 of the September, 1990 issue
of *QST* can not be explained by ordinary means. Shortly before the 'big
bang' this 3-500Z was functioning normally. Subsequently, the 3-500Z was
found to be grid-to-filament shorted, as can be seen in the *QST*
photograph.
- What does it take to produce such a short? In our centrifuge, to
move a 3-500Z filament this much, it takes approx. 6v on the filament and
70 seconds at 10g -- when the sun is out.
>
>It's obviously not oscillations.
>
>73, Tom W8JI
>
However, Mr. Rauch's conclusion ignores the presence of damage to Rs in
the VHF suppressor. Admittedly, this is something that is easy to
ignore. ...... Unsoldering one end of Rs from a heavy silver-plated strap
(Ten-Tec, Henry, Kenwood), is not easy. And, since VHF parasitic
oscillation damage to Rs typically does not affect the Appearance of Rs,
only the most wary are likely to discover the unique internal damage that
intermittent VHF parasitics typically produce in carbon-comp. resistors.
'Obviously not oscillations'? .......... Murphy said 'nothing is as
easy as it looks'.
Hunting for problems in amplifiers is like hunting for Easter Eggs. One
has to look in all of the hiding places if one wants to find all of them.
Rich---
R. L. Measures, 805-386-3734, AG6K
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