[AMPS] High Power on 70cm - Possible?

[Ian White, G3SEK]

Noel Willis wrote:
>
>Hi to all
>I am wanting to build an amp (wishful I think) from scratch for 70cm
>(432MHz) to be used for EME work.
>As I am just looking around for designs, what is a reasonable scheme (and
>tubes) for something up to, or over, 1KW.
>I know that is quite a range but in VK land we can apply for a special
>permit for super high power EME work (our normal limit is 400W PEP) so the
>upper limit is constrained by cost and practicality of the design and
>supply.
>
>I am hoping for something that could be driven with about 70W maximum, and
>would work at something like AB1 or 2 (not Class C) for SSB work.
>
>Is there anything commerically available that does this sort of output that
>would be worth considering?
>Or would it be specialised enough to have to be built from the ground up?

A bit of both. 

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 Lunar-Link (K1FO -
www.lunarlink.com). These generally use 3CX800s at 2-2.2kV, and 70W
drive is plenty for 1kW output. A single 3CX800 will give 750W, or 1kW
if you push it, so good amps like the Lunar-Link use two.

For a homebrew solution using a low-cost Russian GS35b triode, check 
www.qsl.net/dl4mea/2g35.htm  If you have enough anode voltage (eg 3.5kV)
you'll easily see 1kW output from 70W drive.

These and other QRO VHF/UHF amps for 432 can sometimes be bought ready-
made. They come and go, depending on who is "in business" making them
from time to time.

>Is grounded grid with Pi-L output still practical at 70cm?
>
Grounded grid is the only way at that frequency (except for the K2RIW
design for 4CX250Bs) but you can forget about Pi-L and similar "DC band"
output circuits. Output circuits are always based on some kind of tuned
transmission line - the only coils you'll see are RF chokes.


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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