>Ham Radio, June 1986, has a regulated screen grid PS article by Rich, AG6K.
>Uses string of zeners and a TMOS FET.
The regulation is not as good as the 723-based positive floating
regulator using a TMOS FET pass element.
>>Small tetrodes have the somewhat unpleasant habit of generating
>>potentially-fatal reverse screen
... ...
>Take a look at the constant current curves for the 4CX250B series. Even
>with no grid current flowing, in linear operation, there are 'islands' or
>families of current curves for screen current that are negative. And it
>appears that as the plate voltage swing gets closer to 500 volts, (near
>screen potential) the screen current starts turning around. This is the
>current in the screen connection of the tube. Obviously, internally there
>is some secondary electron emission from the screen. These electrons are
>then being attracted to the plate.
Nice explanation. I have heard that the reverse screen current is
usually less at UHF.
>A lot of the Eimac tubes are well behaved, according to the DC curves, from
>the 4CX3000A up to the 10,000. This doesn't guarantee that they won't have
>reversed current. The tube tested for the curves, for the data points
>recorded, did not show it. Another tube of the same type, maybe aged with a
>little cathode material bombarded on the screen, or with a warped grid, or
>at different voltages than recorded for the curves, or made slightly
>differently (production variations) might have it. It's best to assume that
>your screen power supply will have to handle it someday as you adjust an
>amplifier, so plan on it.
good advice.
... ...
> However, a series pass regulator with a
>>sufficient bleeder resistor will do essentially the same job.
>>- If the zener-string/chain is powered from the anode supply through an
>>appropriate resistance, the screen is automatically protected against
>>loss of anode potential -- and there is no chance of damaging the screen
>>by too light loading or by sudden loss of the antenna during a tornado.
>
>Also can use a sensitive adjustable relay in the plate voltage, at the
>bottom of a resistor divider, to determine that, yes, it is actually
>present at the amplifier, (not at the PS, but at the PA side of the HVDC
>connector!). This relay will allow screen power enabled only if the plate
>is up to normal voltage.
A simple solution for tetrodes with handles is to fuse the primary of
the screen supply transformer with an appropriate sized fast-acting fuse.
If the screen current becomes excessive, whether the reason be too
light-loading or sudden loss of anode potential, the fast fuse opens
quickly.
Rich---
R. L. Measures, 805-386-3734, AG6K
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