There's been so much discussion on this, I can't resist:
Last time I built a tetrode amp I used a 4-1000A grid-driven and put +550V
on Ig2, provided by a stiff power supply that had no electronic regulation
whatever. Just a 400V, 200mA transformer, full-wave bridge, ~160uF
filtering, and a 10k, 50W bleeder resistor dissipating 30W. Eg2 never
varied by more than 1-2Vpk key up, key down, with modulation, without
modulation, whatever. Seemed plenty stiff enough to me, and cost about $20
for the entire circuit, including the transformer.
Duh-?
WB2WIK/6
"Each success only buys an admission ticket to a more difficult problem." --
Henry Kissinger
> -----Original Message-----
> From: rich [SMTP:2@mail.vcnet.com]
> Sent: Wednesday, January 30, 2002 9:55 AM
> To: Steve Thompson; AMPS
> 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: 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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