[VHFcontesting] Why the Old Fashined Screen Supplies "Just Don't Cut It"

WBR/WF4R wbr at verizon.net
Wed Mar 9 17:34:52 EST 2005


>Hello all,
>I've been doing some reading in the VHF/UHF DX Book.  I have ahead of me, a
>homebrew amplifier and a power supply to build.  In reading some of the
>various power supply details in the book, they mention that the old screen
>supplies used for Class C (modes AM, FM, and CW) won't work for Class-AB
>service in SSB.  
>I'm still learning more about tubes and amplifiers here.  I would like to
>hear some discussion on why, when building a amplifier screen supply, that I
>can't use the old screen supplies?  I assume regulation is at the heart of
>this statement.
>Also, when I am reading some of the older literature, at what year in the
>anthologies, should I assume that an amplifer was NOT designed for SSB
>operation?  All I know is that SSB was around when I was born.  Anything
>before that is beyond my recollection!
>VHFcontesting mailing list
>VHFcontesting at contesting.com
Jason, et all,
                   A good way to derive screen voltage in via the plate 
supply.  One thing about tetrode power tubes is that you do not ever 
want to apply screen voltage without plate voltage being applied.  
Assuming a plate supply of 2500 Volts, a series dropping resister of 
about 125K ohms (125 W can be derived using 5 x 25K/25W power resistors 
in series on a heat sink) with a string of Zener diodes of about 20 or 
30 Watts of disapation to give you the necessary screen voltage works 
very well.  This all assumes of course that you wish to use tetrode 
tubes like the 4CX250 variety or even the newer 4CX400.   I think that a 
nice grounded grid design using the 3CX800A7 triode would be even 
better.(No screen to worry about)  You may want to refer to the April 
1984 issue of QST which featured such an amplifier on the front cover.  
Tweny Watts of drive will give you 700 Watts out.  Another advantage is 
that the socket for the 3CX800 is much cheaper than for the 4CXxxx 
series of tube.  Good luck.  73, Bill, WF4R

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