There is another solution?
Electronically regulated supply?
Paul K0UYA
----- Original Message -----
From: "John Lyles" <jtml@losalamos.com>
To: <amps@contesting.com>
Sent: Sunday, January 29, 2012 12:28 AM
Subject: [Amps] tetrode reverse screen grid current
> 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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