[BULK] - [Amps] HF and 4cx250b's

[Ian White, G3SEK]

R. Measures wrote:
>On Jul 15, 2004, at 7:23 AM, Steve Katz wrote:
>
>> 4CX250Bs require a regulated screen supply, making the PSU design more
>> complex.
>
>ð  However, the screen current is so low in a 4cx250B that it often 
>goes negative, so the tube  is ideally suited for using a simple 
>zener-string shunt screen regulator connected to the LV end of the HV 
>bleeder-R.
>http://www.somis.org/D-a-10.GIF
>This circuit happily protects the tube from any possible failure mode.

BUT - the 4CX250B is very sensitive to variations in the screen voltage.

See http://www.ifwtech.co.uk/g3sek/cleansig/imd1.htm
for results of GW4FRX's practical measurements of IMD with different 
kinds of screen supply. A zener supply was notably worse than a true 
stabilized supply, especially for the higher-order products that make 
your signal 'wide'.

(Disclaimer: Yes, I do sell stabilized screen supplies commercially... 
but that's because I believe they make a difference.)


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek