>Greetings,
>
>I have started work on a new amplifier (80 - 10 meters) utilising the
>popular grounded grid design but with a slight difference than the norm as
>I use four valves in parallel, of the AMPEREX 4-250 type. The main reason
>is I have ample supply of 4-250's (as compared to the usual 4-400 or 4-500
>one reads about in designs). I still would like to break the 1 KW barrier
>on output.
The 4-250A uses the same 5v/14a cathode/fil. as a 3-500Z and 4-400A.
Peak emission is roughly 1.2A. Max anode potential is 4000v. For SSB
voice service, in g-g configuration, there is not much difference between
the 250 and its larger cousins. To be able to use a 4-250 at max
ratings, the anode dissipation must be temporarily limited to about 50%
during tuneup. This can be easly be accomplished by sending 50wpm dits
with a CW keyer. With max. drive applied, adj. Tune and Load for max
output. // Since the output Z with four valves is 1/4 of that of a
single valve, 4x as much Tune-C is Required to obtain a proper match.
Thus, the 28MHz multi-tube bugaboo is an old wives' tale.
>
>I have a similar working one I build in 1999 but although it is working it
>is not very efficient and prone to destroying chokes and by-pass
>capacitors. Power output is also way below what I expected .. and not
>breaking the 1KW barrier.
>
>I run the valves with +/- 3,400 Volts
3400v is good. Otherwise, it is difficult to obtain the needed ZSAC. I
would expect around 700w pep per anode at this potential.
>on the plate and the filaments have
>two transformers connected in parallel giving the 5 Volts at +/- 15 Amps.
>
>I need advice on how to improve performance and limit component failures.
>
>I know I have to look at the capacitance of the four valves in parallel as
>this will impact on efficiency especially at 10 meters.
Not true. You will need to add C to make the tank work properly on 10m.
>I AM JUST NOT
>SURE WHAT I AM SUPPOSED TO BE LOOKING AT.
>
>My plate choke will also have to be properly designed, as previously I
>took it out of a YAESU 2100Z and just transferred it to the homebuild
>version. It works but surely is partly the cause of the lower than
>expected power output.
>
>I also now want to add a tuned input circuit as previously I just put a
>ATU beween the rig and amp input.
>
good idea. . be sure to use a Q of 2 if you want a good match to a
solid state radio. Each cathode needs about 65w pep drive.
>Any advice,
? The 4-250 has been used successfully in 2m push-pull amplifiers, so
low-Rp VHF parasitic suppressors are probably a good idea. // For
operation in g-g service, I would connect two 5 - 10 ohm 2 - 3 watt MOF
resistors between the screen connections (pins #2 and #4) and the grid
connection (#3).
>especially on the use of the four valves in parallel be it
>practical or an indication that I should use two valves will be appreciated.
>
I would not hesitate to use (10) 4-250s in parallel provided that I had
650w of drive power.
>
cheers, Wynand
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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