David Kirkby wrote:
>
>I perhaps did not make it clear that each tube should produce around 300
>W in class AB1 - ie 600 W for a two tube amp. In class C each tube
>produces around 380 W if memory serves me correct. Obviously, if the
>tubes are pushed beyond their ratings (not exactly unknown in amateur
>circles), you will get more power.
>
... and more IMD. AFAIK, the most complete information on linear
operation of 4CX250Bs is the chapter by GW4FRX in 'The VHF/UHF DX Book'.
Very briefly:
Va 2000V, Vg2 350V, Vg1 about -55V (adjust for ZSAC 100mA per tube).
Maximum conditions for good linearity: Ia 450-500mA (2 tubes), Ig1 zero,
Ig2 - adjust loading for +2mA per tube, power output 500W (2 tubes).
Under these conditions, Ig2 goes to about *minus* 5mA per tube at
intermediate drive levels (most negative at about 300W output) and then
goes slightly positive when driven to 500W output.
There is much more information in the book, which is available from RSGB
and ARRL. There are links from my web site to their ordering pages.
Dave is right: there is no such thing as a K2RIW for 2m.
The one and only one true K2RIW is for 70cm, and uses 2 tubes with a
half-wave line. Somebody did design an amp for 2, which was based on the
K2RIW with a half-wave line (and you can imagine how long that was) but
much more common on 2m is the W2GN design which uses the original K2RIW
12in box but with a quarter-wave line.
You may also hear of a "K1RIW" which is a joke name for a 70cm K2RIW
with only one tube and a narrower line. One F station has also
successfully built a "K3RIW".
In many ways a better design for two 4CX250/345/400s on 2m is the W1SL
push-pull amp, which John describes in the book.
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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