Bob Marston wrote:
>Ian White G3SEK wrote:
>
>>For a ready made amp you can go to Henry, or you can go to a number of
>>hams who build amps for sale, For example (K1FO -
>>www.lunarlink.com). These generally use 3CX800s at 2-2.2kV and 70W
>>drive is plenty for 1 KW output. A single 3CX800 will give 750W or 1kW
>>if you push it, so good amps like lunarlink use two.
>
>
>Point of clarification. The 3CX-800A7 has maximum rated operating
>frequency of 350 Mhz. This tube is not suitable for use at 432 MHz.
That's too strong. Although it's true that Steve has now changed to the
3CPX800, for many years before the 3CPX became readily available he and
others used the 3CX successfully at 1500W on 432. Advice on operating
conditions is given in his detailed description of the amplifier in the
ARRL UHF/Microwave Experimenter's Handbook (the one in the black cover).
Even so, you're quite right that it would be foolish to buy new 3CXes
today for use on 432, when both new and used 3CPXes are so readily
available.
>K1FO
>uses the Eimac 3cPx-800A7 for his equipment above 220Mhz.
>
>Stephen (K1FO) also points out that Svetlana only manufactures the
>3CPX-800A7 but stamps 3CX-800A7 or 3CPX800A7 on the side of their tube
>depending on what you order. Lunarlink products will use either tube and
>present the option to the purchaser of the brand of tube they wish to
>select. Lunarlink also urges their customers to purchase replacement tubes
>through them as they are pretested. This would indicate that he is aware of
>the fact that he is tickling the limits of these tubes.
>
As I said, Steve has been clearly on record about that for many years.
>If one is building their own amp the GS-35b is a much better tube as it is
>rated clear to 1 Ghz.
Unfortunately it's not that simple. At 432, 1500W from a GS35b with
reasonable gain requires at least 3.5kV, and there are issues of tuning
drift. However, in terms of value for money the GS35b can't be beaten.
It's definitely the best choice for homebrew.
>If the drive requirements of that tube are too steep
>the GS-23b Tetrode is another option. Like the 35 it is also rated to 1 Ghz
>and Grid Driven with a Screen Supply will produce 1500 Out with 20 watts of
>drive.
Has anyone actually *grid* driven the GS23b on 432, and kept it resonant
and stable? Numbers of people have used cathode drive with good success
(with correct grid and screen voltages relative to the cathode, of
course) but the drive requirement then is greater.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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