>
>
>-----Original Message-----
>From: rich <2@mail.vcnet.com>
>To: Steve Thompson <g8gsq@qsl.net>; AMPS <amps@contesting.com>
To: <amps@contesting.com>
>Date: 30 January 2002 10:22
>Subject: Re: [AMPS] Shunt regulated high voltage psu
>
>
>>
>>
>>>
>>>
>>>-----Original Message-----
>>>From: rich <2@mail.vcnet.com>
>>>To: Steve Thompson <g8gsq@qsl.net>; AMPS <amps@contesting.com>
To: <amps@contesting.com>
>>>Date: 29 January 2002 22:03
>>>Subject: Re: [AMPS] Shunt regulated high voltage psu
>>>snip
>>>>>There doesn't seem to be any real need for an active series element to
>>>>>feed an active MOSFET shunt regulator. The loop gain in the shunt part
>>>>>is already very high, so the series feed can be just a plain resistor.
>>>>>
>>>>>Certainly, in that configuration. There's numerous valves where the
>>>reverse
>>>>>screen current appears to low or non-existent (4CX800 for example),
>where
>>>>>dumping 40+mA all the time in a shunt regulator feels wasteful.
>>>>
>>>>// Low screen-V tubes such as the 4cx800 (350V-sc.) suffer more from
>>>>reverse screen current than tetrodes with handles. However, even
>>>>4cx5000As and 4cx10,000s occasionally exhibit reverse screen current with
>>>>1200 - 1500 V on the screen. Bleeding 25mA seems to take care of the
>>>>problem.
>>>>
>>>>>This led me
>>>>>to look at series supplies, which led in turn to thoughts about effects
>>>>>under fault conditions.
>>>>>
>>>>// Did you look at the screen current curves for the 4cx800?
>>>
>>>
>>>Certainly did. The Svetlana data sheet shows no areas of negative screen
>>>current, even though the text discusses the need to cater for it.
>>>
>>// It looks to me like there should be some more curves to the right of
>>the 0 screen current line, like there are on the 4cx350 curves.
>>Obviously, at times the 800 has negative screen current or they wouldn't
>>have mentioned it.
>Possibly or probably - I'll find out when I fire one up (anyone out there
>save me the bother?).
// If the 4cx800 does not exhibit reverse screen current, Svetlana
succeeded in defying the laws of physics.
>They put -ve contours on the 4CX1600B so maybe they
>just cut and pasted the text. Happens all the time - or did when I worked in
>the semiconductor business. Anyhow, that's a digression. My guess is that
>any -ve current is likely to be small, and normal operation appears to run
>around 40mA +ve. It seemed to me to be wasteful dumping 40odd mA all the
>time in a shunt just to meet peak demand so I designed a series circuit,
>then got to thinking about protection which is where I came in.
>
// I would use a means of sinking c. 15mA of reverse screen current to
prevent the screen from rising to the anode V and destroying the tube.
However, if you have a free source of 4cx800s, no such precaution is
needed.
cheers, Steve
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
end
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