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,
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?
The 3CX800A7 and 3CX1200A7 are expensive and not instant-on. Also they have
somewhat low grid dissipation ratings and I'm afraid of damaging them.
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). I'm going to use separate circuits for each band,
including warc, and they will have a higher Q than normal (I just need to cover
the CW band and this should require less drive).
I'm thinking about just buying the SB-220 capacitor/rectifier unit and metering
board from Harbach. A little expensive, but peace of mind. I will also be
looking at SB-220 replacement transformers from various sources. I'm thinking
about using solid-state relays for the power relay and the inrush relay. Is
this a good idea? I hate noise.
Cooling: Is it best to use sockets with chimneys, or to use the "SB-220 method"
as somebody called it, where a muffin fan blows on the tubes from the side? I
don't want to get cheap in this area. I would like it to be conservative and
also quiet, if possible. My SB-1000 uses the muffin fan method, and it's quite
loud.
For QSK, I will be using a vacuum relay in the circuit that appears on Rich's
web site. I built one into my SB-1000 and it has been working great for the
past year or so. It doesn't hot-switch even at 50 wpm dots. I will also
implement the cathode-bias switch in that circuit; I've had good results in the
SB-1000 with it also.
I guess I'll buy a plate choke from Ameritron. I found it hard to homebrew one
for 160-10, including warc.
Any comments or further ideas will be appreciated.
73,
Vic, K2VCO
Fresno CA
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