[Amps] Amplifier for 630 meter band

[Jim Thomson]

Date: Thu, 9 Nov 2017 18:01:17 +0000 (UTC)
From: Catherine James <catherine.james at att.net>
To: <Amps at 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