[Amps] tetrode reverse screen grid current

[John Lyles]

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 at tampabay.rr.com>
> Subject: Re: [Amps] 4CX1000A
> To: amps<amps at contesting.com>
> Message-ID:<4F245924.2040702 at 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.