Re: 4-1000A Screen Biasing...

["Malcolm Watts" <M.J.Watts-at-massey.ac.nz> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi Daniel,

On 26 Jul 00, at 17:22, Tesla list wrote:

> Original poster: "Daniel Boughton" <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