>
> I wonder about going the 4CX-250 route, or 4-250. Apparently my Eimac hand
> book is out in the shop so I can't look up the numbers for the 125, 250
> and
> 400, but I do have a commercial 2-meter amp using a pair of 4-125s and I'd
> guess the 4-250s may go that high. the 4-400s? they are as large as a
> 3-500Z
> and maybe a tiny bit taller. If they'd go to 50 MHz (which I think they
> will) they might make a whale of a 6-meter amp in an SB 220 case, but as I
> recall the 4-400s are a bit more pricy than the 125s and 250s or even the
> 4CX250s and a *lot* more than 3-500Zs . They'd fit with the possible
> exception of the 4-400 and are of a relatively good price. the 4CX 250s
> were
> widely used in VHF amps as well as HF (re the old KWS-1 which I had and
> the
> collins 6 & 2 transverter) IIRC the 4CX-250s will mount in any position
> and
> should give 600 to 800 out for a pair. Yes they would take a screen supply
> but a relatively simple one that should easily fit in the space freed up
> by
> going to the smaller tubes. There'd also be space for a plenum for the
> tube
> sockets and a blower. Of course I remember the old KWS-1 was a great
> space
> heater for my shack in the winter, but a good part of that came from the
> 75A4 and exciter portion of the KWS-1. . OTOH my old Henry 2K-4 really
> throws out a lot of heat. With the 3-500Zs having a typical gain of 10 db
> I
> keep wondering how some are claiming 1500 out with only 75 watts of drive.
> It takes me a 100 to get 1200 out and I could use it for a coffee warmer
> while doing it. Coffee warmer? I could cook dinner on it.
>
> I realize I've gone from a tank circuit conversion to a major change, but
> isn't that what ham radio is all about, or is for at some of us.
>
> 73
>
> Roger (K8RI)
Why on earth would anyone want to replace 3-500Z's with 4-400's in a
SB-220????
First of all they have worse IMD in a GG configuratiion. Secondly they will
not be cooled properly by the existing fan.
I really dont understand your logic.
The 4CX250 family is another very poor choice. They were not developed for
SSB use and the linearity is only in the mid 20's under the best of
conditions. This includes the 250R. Ian, G3SEK/GM3SEK, has written
extensively on that subject and even developed a set of bias regulators to
at least improve the IMD a bit.
You also have the problem with secondary emission which is prevalent in a
goodly number of the used 4CX250's available. As Im sure you are aware hams
rarely buy new tubes if they can help it.
A pair of 3-500Z's will easily do 1800W with 100W drive with enough HV. The
AL-82, PT-2500, SB-200 with Dahl outboard xfmr are just a few examples. A 6M
monoband amp requires tuning just once if you stay within the lower 400 KHz.
With the proper capacitors that tune up takes only a few seconds. Adding
bias control might save pennies on your yearly light bill and do nothing for
longevity since a 6M amp is not used nearly as much as an HF version.
I built a single 4-400A 6M amp in the 60's and drove it to 600W with under
10W. I also have PP 4-125A's that was originally on commercial high band
VHF on 2M FM. However those old technology tetrodes have nowhere near the
IMD performance of modern tubes and would probably be no better than a used
4CX250 in the hands of most basement builders. I can guarantee that a
SB-200/SB-220 will be much better.
Carl
KM1H
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