David and company:
I seem to recall the 2M amp as one by W1SL. It works great. Much
better than the "plumbers special" (*QST, Dec. 1961)
For frequencies up to 175 MHz., figure output at about 55% to 60%
efficiency for AB1 SSB service and about 70% to 75% for class C CW
service. Any more than these numbers and you're on the edge of a serious
flash....bang.
According to Eimac Technical Data for the 8957/4CX250BC for AB1 service:
2000 VDC on the anode, 500 mA for 2 tubes anode current, screen current
4 mA per tube @ 350 VDC. The G1 current should always be 0. This
should net about 550 to 600 watts out.
The 7580W/4CX250Rs have a bit more gain but IMD will suffer. Your
neighbors on the band during a contest won't be friendly. The R is a
ruggedized version and is suitable where severe shock or a high-level
and high-frequency vibration is expected. i.e. aircraft operation.
For class C operation: 2000 VDC on the anode, 500 mA anode current for 2
tubes, screen current +25 mA @ 250 VDC. Control grid current (2 tubes)
25 to 45 Ma. This should net about 700 to 750 watts out.
For operation at voltages, currents and powers above these......consider
yourself a very underpaid design engineer. :-)
In actuality, I run my amp to 2300 VDC at idle (200 mA) in AB1, droping
to about 2000 VDC on peaks. Key down with carrier, but no G1 grid
current my amp puts out about 600 watts. Screen is 350 VDC at about -2
ma swinging to +5 ma on each tube. In class C, I drop the G2 screen down
to 250 VDC, drive it to about 25 to 30 mA of G1 current for 2 tubes,
load it for a G2 current of 20 mA to 25 mA for each tube and I get about
700 watts out. Driving power, considering loss in grid circuit is about
10 watts for 2 tubes. Heater voltage is 6.0 VAC at the sockets +/- 5%
regulated via a constant voltage transformer.
The secret to keeping tubes alive and happy at these levels is blow them
at least as hard as Eimac recommends. More air is better and usually a
bit noisy. Mine runs about 1.5" pressure drop/water between the grid
compartment and the anode compartment. (I'm at 2000 ft above sea level.)
See my call on QRZ.com for the "old amp" in the tall rack behind me.
The new amp is 36" high and sets beside the desk.
73
Bob K4TAX
> Thomas haglund wrote:
> >
> > hello! i just bought a 145 mhz,Built acording to k2riw. tube amp with dual
> > 4cx250b,is there any GOOD internet sites with info and mods for those amps ?
> > what is the maximum power output for it and so on ?
> >
> > //Thomas
> >
> > SM7TVJ
> Unless there is obscure amplifier design by K2RIW for 2m which I don't
> know of, the normal K2RIW is for 70 cm, not 2 m.
>
> The 4CX250B is rated for 300 W up to 175 MHz, if memory servers me
> correct. 2000 V at 250 mA per tube should give an input power of 500 W
> and 300 W output. My own experience of a dual 4CX250B amp on 2m was very
> close to this. There is a data sheet for the Eimac 4CX250B on my web
> site - see below.
>
> --
> Dr. David Kirkby Ph.D,
> email: drkirkby@ntlworld.com
> former email address: davek@medphys.ucl.ac.uk
> web page: http://www.david-kirkby.co.uk
> Amateur radio callsign: G8WRB
>
> --
> FAQ on WWW: http://www.contesting.com/FAQ/amps
> Submissions: amps@contesting.com
> Administrative requests: amps-REQUEST@contesting.com
> Problems: owner-amps@contesting.com
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