Leigh wrote:
>
>I guess the biggest question I have is in regards to the
>requirements of the screen supply. There appears to be no screen
>by-pass resistor in the RF deck so I'll assume the builder (have
>no idea who it was) included that in his screen supply. The
>Eimac data sheet states 70mA for this resistor so I guess its
>value should be around 5k ohms? What is the recommended size of
>this component. I would guess 5 watt would be large enough but
>maybe I am not considering possible failure modes? Some
>references I have seen state that HV should be applied before the
>screen voltage so I'll assume it is a good idea to apply screen
>voltage with the TR relay activated. Any thoughts on a
>recommended screen supply circuit. I wouldn't think with the
>screen bleeder that regulation is required and I was thinking of
>using a choke input filter (anyone have a ~4-5 HY 350mA 400VDC
>filter choke lying around?).
>
The Eimac advice assumes that you are not using a regulated screen
supply, only a "dumb" bleed resistor - but for good linearity and tube
protection you MUST regulate the screen voltage closely.
When you use active regulation of the screen voltage, you no longer need
a 70mA bleed. 10mA is plenty, and its only function is to clamp the
screen voltage while the T/R relay contacts are actually changing over.
There is a QEX article on screen supplies, downloadable from my web
site: http://www.ifwtech.demon.co.uk/g3sek/boards/tetrode/tetrode-3.htm
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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