DUGAS, JASON A. (JSC-EP) (NASA) wrote:
>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!
>
>Thanks,
>
>Jason
>KB5URQ
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>
>
>
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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