[Amps] Negative current with a Zener screen grid regulator

[Jim Garland]

Paul,
Positive screen current means that current flows into the grid, and negative
screen current means that current flows out of the grid. The Zener diode
string fixes the screen voltage, with a current-limiting resistor in the
string that biases the current at a fixed amount, e.g., 100 mA. Positive
screen current robs some of this current, so that if the screen current is
+25 mA, then the Zener current drops to 75 mA. Negative screen current dumps
more current into the diode string, so that a negative screen current of -25
mA will increase the Zener current to 125 mA. So your conclusion, below, is
correct. The zeners have to be sized so they can handle the excess negative
screen current, and the series resistor has to be sized to make sure the
bias current is high enough the positive screen currents won't starve the
zeners of current and drop them out of regulation
73,
Jim W8ZR

> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of N1BUG
> Sent: Monday, October 17, 2016 01:48 PM
> To: 'Amps'
> Subject: [Amps] Negative current with a zener screen grid regulator
> 
> Hey amp gurus,
> 
> I have been going around and around with a recently rebuilt VHF amplifier
that is refusing
> to work correctly. To help me think this through, I want to be sure I am
understanding how
> some portions of it are SUPPOSED to work, or the theory of them.
> 
> I have a 4CX1500B tube. Screen voltage is regulated by a string of five
high power zener
> diodes with the screen voltage tapped at the third or fourth zener from
the bottom of the
> series string. I think I understand how this works with positive screen
current but can
> someone nudge me in the direction of understanding what happens when the
tube draws
> negative screen current? My belief is that current through the zeners (or
SOME of them?)
> INCREASES in by the amount of negative screen grid current.
> 
> Is that correct?
> 
> Other than exceeding the power ratings of the diodes, are there any
special concerns here?
> Is there risk of the thing dropping out of regulation and allowing voltage
on the screen to
> rise significantly?
> 
> I feel sheepish asking this, as I ran this amp for many years with
negative screen current and
> it obviously worked. But now that I'm having problems with it I want to
understand things
> better.
> 
> I will ask my other question separately as it is not particularly related
to this one.
> 
> Thanks,
> Paul N1BUG
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