----- Original Message -----
From: "Ron Youvan" <ka4inm@tampabay.rr.com>
To: "amps" <amps@contesting.com>
Sent: Saturday, January 28, 2012 5:14 PM
Subject: Re: [Amps] 4CX1000A
> Carl KM1H wrote:
>
>>> 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.
>
>
>> I suggest reading page 4 and 5 in the below link.
>> http://www.g8wrb.org/data/Eimac/4CX1000A.pdf
>
>> And then tell me its not perfectly normal. Most external anode tetrodes
>> I'm familiar from the 4X150A
>> to 4CX1500B can exhibit negative screen current.
>
> I stand by my statement, "in a perfectly built tube you should see none"
> and "is not strictly normal"
> You say: "Most external anode tetrodes I'm familiar from the 4X150A to
> 4CX1500B can exhibit negative
> screen current."
>
> I agree, "Most" and "can exhibit" but not in 100% of them, in a beam
> power tetrode or pentode the
> screen grid is supposed to be in the shadow of the control grid and when
> it is not completely out of
> the stream the electron beam strafes the screen grid a glancing blow, the
> is prime angle to create
> secondary emission, which is the reverse screen grid current. (emission)
> I have worked with many RCA (now Burle) (like 4CX5000R) and Thomson beam
> power tetrodes (like
> TH-331) that did not exhibit any noticeable screen emission.
> I am sure lining up the screen "cage" with the grid "cage" properly, is
> the hardest part if
> constructing this kind of tube. (to make as many as possible that work
> well)
> --
> Ron KA4INM - Did you know ...
> ... that no-one ever reads these things?
We are discussing medium power tubes, not ones with handles. If you check
the specs carefully you will see that reverese screen current isnt even
mentioned with the big tubes. I specifically listed a range of tubes for
that reason.
With ham level power tubes its a perfectly normal event in the real world of
acceptable tolerances....but not on paper.
Carl
KM1H
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