On Mon, 15 Mar 1999 14:29:34 -0800 Vic Rosenthal <rakefet@rakefet.com>
writes:
>
>I'm contnuing to plan my CW-only amplifier. I've taken one
>irrevocable step and
>bought a DTR-2000L tank circuit with the switch, caps, coils, etc.
>The rest of
>the parts will dribble in over time, unnoticed by the XYL... I would
>appreciate
>comments on the following if anyone thinks I'm on the wrong path, or
>whatever.
>
>Goal: 1200-1500 watts CW output, 160-10m. Reliability under CW
>contest
>conditions is the first priority. Low cost is the second.
>
>Tubes: I like the 8877, but pulls are getting hard to find and more
>expensive,
$190 for a guaranteed full output pull is expensive ?? If you really
intend to run at low voltage and 1200-1500W the tube may outlive you.
>and I feel (wrongly?) more comfortable with new tubes. New 8877's
>are
>outrageously expensive. I am thinking about a pair of 3-500Z's: they
>are
>available, not too costly, rugged, and instant-on. NOS Eimacs are
>available for
>$330/pr and RF Parts has their own brand of Chinese tubes for $250/pr.
> Any
>comments on the RF Parts tubes?
I wouldnt buy them...but I'm biased. I sell what I consider are even
better at $230 shipped, a full year no questions asked warranty.
However...stick with the 8877.
>
>The 3CX800A7 and 3CX1200A7 are expensive and not instant-on.
OH ! You're only 50% correct.
Also
>they have
>somewhat low grid dissipation ratings and I'm afraid of damaging
>them.
OH, again.
>
>I'm planning to run them class AB2. I thought about class C, but a
>single
>3-500Z would have to be run at very high plate voltage to produce the
>output I
>want, and the grid dissipation would be pushed to the limit. It would
>need too
>much drive to be cathode driven and too little grid-driven...and the
>harmonic
>problems would be something else to wrestle with. So it looks like
>two 3-500Z's
>in class AB2.
>
>The input circuits will be on a little board, right at the tube bases,
>switched
>with reed relays. Various people here have mentioned that this will
>improve
>efficiency and use drive more efficiently, and my exciter is just a
>bit marginal
>for two 3-500Z's (TS850s).
Yeah, real marginal, you should only expect about 1800W with a 3400V
supply. The 8877 will do it at 2300V. Do the math on the PS and RF
components at both voltages.
Your remaining ideas are good, but for Y2K style construction " Glass is
Out" unless you get can lots of freebies parts.
Even this old fart is convinced that the 8877 is such a nice tube to work
with at those power levels.
BTW, that DTR-2000L is now running 4KV on 6M and really does some serious
RF !
Still no need for a parasitic suppressor.
73 Carl KM1H
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