Date: Thu, 9 Nov 2017 18:01:17 +0000 (UTC)
From: Catherine James <catherine.james@att.net>
To: <Amps@contesting.com>
Subject: [Amps] Amplifier for 630 meter band
<I'm starting to think about building a transmitting station for 630 meters
next spring. I've filed my UTC request and expect it to be approved.
I think it will be much easier to start with an existing amplifier and modify
it rather than building from scratch. I have no sheet-metal-working experience
and I'm not interested in putting in the time and effort to learn that skill.
Something like a junk SB-200 with a good power supply and burned bandswitch
seems like a reasonable starting point. Good 572B tubes are once again
available from RF Parts. I'm sure there are many other models of tube amp that
would work. It will probably depend on what I can find available. Replacing
all of the filtering and tuned input/output circuits seems pretty
straightforward. Some pretty hefty capacitors and inductors will probably be
needed.
What are everyone's thoughts on a simple, inexpensive conversion of a 500 watt
amp for 472-479 kHz?
## I have zero experience on that band. Whats the normal tank output
config... IF a tube is used ? Is a PI net still used, or a PI-L ? If so,
the tune and load and main coil values...and also any small L coil, will be
almost QUADRUPLE the values used at
1.8 mhz.... which is a bunch. Still doable, except be prepared to pad both
caps. Dunno if a torroid is useable, or just a big chunk of ..small gauge air
dux. Plate choke would not be too big an issue. IF GG used, the bifilar
would have to be thought out. Even then, you can always do the Rich measures
trick, and resonate the smaller fil choke with a cap wired in parallel with the
C2 cap of any PI tuned input.
## If a KW SS amp used, it would still require a KW rated LP filter.
With a tube amp, you could probably get away with a simple PI. A PI-L would
also work, provide for somewhat better harmonic suppression, and a smaller load
cap, or less
padding required.
## 2nd harmonics would be 944-958 khz. Even the 3rd harmonics would be in
the AM broadcast band.. at 1416 – 1437.
## U may well have a tough time stuffing parts into a SB-200 chassis. This
is where a simple, bigger box would help out. Something like a single
3-400Z / 3-500Z would do the trick, dead simple. Then u could also stuff a
4-400 A / B /C in there as well.
## Being a monoband amp simplifies things greatly. No bandswitch required
for tuned input, nor KW output. Then plate choke can be optimized
better..for that one band. use the biggest tune and laod caps u can get ur
hands on..no need to worry about min C values. Then pad both of em..and leave
the padding in at all times. It only has to work from 472-479 khz.
Jim VE7RF
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