This characteristic is quite normal for many tetrodes, as others have
suggested here. Sometimes it can be inferred from
looking at the characteristic 'constant current' curves, when the screen
current lines are have ripples or turnarounds in them. Many of the
curves on datasheets are redrawn and smoothed from the actual test
curves. I have seen original Eimac data (drawn with pen on graph paper)
that show noticeable humps or 'islands' where the screen current may be
turning back for a portion of increasing plate voltage, on some tubes,
medium and large sized. The Burle 4616, a well constructed high power
UHF tetrode with very well aligned screen and control grid wires,
exhibits reverse screen current too. There is thermal (primary emission)
and secondary emission. Primary is detected usually by pulsing a tube
and noting that the screen current changes (droops) on a scope as the
pulse is widened and the thermal loading increases.
The coating on a screen and control grid can alter the work function and
make a big difference. Modern high power tubes (with handles and
eye-hooks) from several of the major mfrs have
gone to pyrolytic graphite for grids, which reduces the problem to only
very extreme temperatures. I like using these tubes since it essentially
eliminates the need for heavy bleeder resistors on the screen supply,
tubes are well behaved with regard to minimal secondary emission. I
don't like using wire grid tubes anymore, having been spoiled. Ham
tubes, unfortunately, aren't so lucky. Adding heavy shunt bleeder
resistance is the usual solution.
73
John
K5PRO
> Message: 1
> Date: Sat, 28 Jan 2012 15:23:00 -0500
> From: Ron Youvan<ka4inm@tampabay.rr.com>
> Subject: Re: [Amps] 4CX1000A
> To: amps<amps@contesting.com>
> Message-ID:<4F245924.2040702@tampabay.rr.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> I'm not mike, but I think the reverse screen current is not strictly
> normal, in a perfectly built
> tube you should see none.
> These tube are built with the screen grid components hiding in the electron
> shadow of the control
> grid elements. When the alignment isn't perfect the electron beam strafes
> the side of the G2
> elements which produces emission via secondary emission.
> I suspect most new good tubes of this kind show negotiable emission. Those
> that do emit
> significantly may find their way onto the hands of HAMS more easily.
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