Paul,
With negative screen current the current thru the zener will DECREASE.
You need to have enough current in the zener string at zero screen current
so it will always be greater than the maximum negative screen current.
If the negative screen current goes greater than the zener current it will
go out of regulation and the screen voltage will rise.
73
Gary K4FMX
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of N1BUG
> Sent: Monday, October 17, 2016 2:48 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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