Re: 4-1000A Screen Biasing...

["Area31 Research Facility" <rwstephens-at-hurontario-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

I'd like to add one more comment to what Malcolm has said.  You should make
sure that under NO circumstances can the screens be energized with no normal
plate voltage present.  If the plate supply is shut off the screens will become
the new plate, for a few seconds, turning incandescent and burning out thusly
ruining the tube.  I recommend staying away from tetrodes entirely for TC and
self excited RF power oscillators.  They're too much trouble to implement.  Too
many 'odes
 
Robert W. Stephens
Director
AREA31 Research Facility
<http://www.area31-dot-org>www.area31-dot-org
>
> ----- Original Message ----- 
> From: <mailto:tesla-at-pupman-dot-com>Tesla list 
> To: <mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com 
> Sent: Wednesday, July 26, 2000 22:46
> Subject: Re: 4-1000A Screen Biasing...
>
> Original poster: "Malcolm Watts"
> <<mailto:M.J.Watts-at-massey.ac.nz>M.J.Watts-at-massey.ac.nz> 
>
> Hi Daniel,
>
> On 26 Jul 00, at 17:22, Tesla list wrote:
>
> > Original poster: "Daniel Boughton"
> <<mailto:daniel_boughton-at-yahoo-dot-com>daniel_boughton-at-yahoo-dot-com> 
> > 
> > All:
> > 
> > I have two 4-1000A tetrodes which I am using for a
> > VTC. Does anyone have experience biasing the screen
> > relative to the plate? The 2000 ARRL handbook lists
> > typical screen current to be 146 mA for class CT
> > operation and 145 mA for class CP operation(not much
> > difference) with a screen voltage of 500V and plate at
> > 3KV. Does this imply that that the screen bias
> > resistor will have to drop 2.5KV WRT GND and limit the
> > current to 145mA? This would require a 362.5 Watt
> > minimum resistor! This obviously doesn't make
> > practical sense so something is wrong. What is the
> > appropriate means to bias the screen?
>
> This one comes up a lot in high power audio amps. There is 
> really no substitute for a well regulated standalone screen 
> supply which is usually derived from a separate transformer 
> winding where screen current is high, or in cases where it is 
> required to be half the anode supply, the half point of a full 
> wave voltage doubler.
>      The screen current should be self-limiting if the correct 
> anode supply and operating point is used.
>
> Regards,
> Malcolm
>
>