Re: 4-1000A Screen Biasing...

["Ed Phillips" <evp-at-pacbell-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Tesla list wrote:
> 
> Original poster: "Ray von Postel" <vonpostel-at-prodigy-dot-net>
> 
> Dan:
> I am not sure which suggestion you liked. Please don't consider
> connecting the screen to the plate through a resistor  as a
> recommendation!!  That is just a quick and dirty method that sometimes
> works with some tubes. Your best solution is to build a separate supply
> for the screen.  Transmitter tubes can run a long time if they are not
> abused.  My preference is to use good design practices and follow the
> mfg. spec.  

	Don't have an Eimac (?) data book handy, but looking at my RCA
transmitting tube handbook, DATA 3 for the 813 datged SEPT. 1, 1954,
screen voltage supply by the use of a series resistor of 46000 ohms to
the plate supply is specifically mentioned as one of the acceptable
biasing methods.  The particular operating condition was Class C
Telegraphy with a plate voltage of 2250 and a plate input of 600 watts. 
The only caveat is that such a supply NOT be used where thie circuit is
keyed.  [Presumably the reason for that is that if a large bypass
capacitor were used on G2 the voltage would rise key up and perhaps
result in excessive screen current key down, although they don't say
so.]  ALL of the data sheets for ALL of the power tetrodes/pentodes have
similar notes for ALL operating conditions.

	Bottom line is that it is a simple way of getting the bias, and is
automatically interlocked.  Question really is one of efficiency rather
than design practice.

	By the way, the use of a triode connection for tetrodes/pentodes has
been mentioned several times.  Before doing this you should look at the
tube data sheets carefully to avoid possible disappointment.  For some
tubes the two grids should be tied together, while for some others the
screen should be hooked to the plate.  Depends on the grd-screen grid
amplification factor among other things. Think some guys have used 803's
in one of these connections, but don't remember if the actual circuit
was mentioned here.  


>If you need more power, get a tube with a higher ratting.
> Of course that can get expensive!!!.
> 
> Lots of luck
> 
> Ray

	For projects such as these your time is the biggest expense.  Nothing
like honking big power triodes for simplicity and reliability, starting
out with babies like the 833's and working up to something really
interesting.

Ed