Since Eimac states that reverse screen current is "prominent" in the
4CX1000A I believe it is safe to conclude the other term would be "normal".
A negative screen current of up to -25ma will occur under some operating
conditions. Notice they say "operating"
Eimac also strongly suggests (they say "absolutely essentisl") a bleeder
current of 70ma when using a tube type series regulator and this was written
long before FET's were invented but the requirement is still there if you
dont want to destroy the FET.
Carl
KM1H
----- Original Message -----
From: "Hsu" <hsu4qro@gmail.com>
To: <jtml@vla.com>
Cc: <amps@contesting.com>
Sent: Sunday, January 29, 2012 1:41 AM
Subject: Re: [Amps] tetrode reverse screen grid current
>I have a thought about shunt bleeder resistance ,If we use a HV MOSFET
> seial a resistor to replace it, when the negative screen current
> happened,
> control the total resistance so the screen voltage still constant, Is
> it a good idea?
> 73!Hsu
>
> -----原始邮件-----
> From: John Lyles
> Sent: Sunday, January 29, 2012 2:28 PM
> To: amps@contesting.com
> 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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