Rich Measures wrote:
>>Hi
>>
>>I recall a recent very good article about tetrode screen supplys
>>in QST or QEX. Cant recall which one or the issue.
>>
>>Anyone have that info handy
>>
>For small tetrodes with less than roughly 800v screens, a zener-string
>shunt regulator works best.
GW4FRX's measurements quoted in my article (October QEX) show that a
zener string emphatically does NOT "work best" from the viewpoint of
IMD!
> For larger tetrodes, a 723-based positive
>floating series pass regulator has advantages. You can find both types at:
>http://www.vcnet.com/measures
But note also that a series regulator cannot handle reverse screen
current, which many tetrodes can generate. You MUST use a shunt resistor
from screen to cathode, that permanently passes the maximum negative
screen current that the tetrode can generate. Under conditions of
maximum negative screen current, the series regulator is almost cut off
but it is still controlling the voltage. When it is completely cut off,
the voltage will start to rise. This is dangerous because increased
screen voltage causes more anode current, more reverse screen current,
higher screen voltage and a runaway situation that ends with a Big Bang!
Both series and shunt regulator circuits can be made to work in this
application, but you may find a shunt regulator more convenient.
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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