[AMRadio] Home Brew

Larry Roohr kz0e at comcast.net
Thu Sep 30 08:46:19 EDT 2004

Hi Brett,

Would your friend with the full garage be interested in selling a mod 
transformer? I've been slowly gathering parts for an 2-813 by 2-813 
transmitter and would be interested if he is.



Brett gazdzinski wrote:

>A very good choice!
>Build it right, and it works like a Swiss watch, and is quiet, no fans
>or blowers needed. 250 to 300 watts out easy, with plenty of audio
>to go with the carrier.
>The tubes are easy to find and cheap, there were many surplus transformers
>that would power the setup nicely, mod transformer size is reasonable.
>Fair radio used to sell a Collins solder sealed power transformer that
>would give about 1000 volts at 600 ma full wave center tapped choke
>input, small size, 110 or 220 volt primary, used to go for $30.00!
>Variac it and it does 0 to 1500 volts, and will do the rf and mod decks
>if you want to keep things really simple.
>I have tried 812a's, 811a's, 572b's in the rf deck, and they all work
>fine and act the same as far as modulation, drive, and power output go.
>I have 3 old RCA mod transformers that work with this combo very well,
>and also work well with the pair of 813's and 4x150a's.
> 10,500 to 4350 ohms I think they are, and large!
>They are around, a friend has a garage full of them.
>I retired the cvm5, as the freq response was quite poor.
>Building a homebrew rig is incredibly fun, and quite a source of pride
>if you take the time to build it RIGHT so it works trouble free for
>years and years, and looks nice.
>To get parts, you have to network hams, look on ebay, go to fests,
>buy old homebrew pieces of crap for the parts, and use some of the
>vendors that sell rf stuff.
>Even building receivers has not been a problem for me, parts are around,
>you just have to dig for them instead of going to one place
>like in the old days.
>Often, the prices are very reasonable, I picked up a bunch of nice
>oil filled caps for $10.00 each at Gaithersburg fest last year.
>I got about 6 of them.
>They still make new ones, but they wanted $120.00 each!
>The two big fun points are the chassis chess part, where you
>Figure out just where things are going, and how you are going to keep
>things symmetrical, rf and front panel, and the initial testing.
>Building something, and firing it up and finding it works great
>is quite fun, and the fun continues every time you use the rig.
>It also gives plenty to talk about, there is not a lot to say
>about a rice box into an amp.
>Old vintage ham gear like Johnson 500, Collins 30k-1, globe king 400/500,
>and military surplus like the T368, art13, and others can be fun to play
>with, and sometimes turn up at reasonable prices (but not on ebay).
>80 meter night time is hard to break into, and needs good power
>levels, but 40 in the daytime is very friendly and you can do
>nicely with 100 watts and up.
>Valliant's, the Collins 32v series, dx100's, and others in that power class
>do very well on 40 meters, are fun to play with, and can be got
>at reasonable prices.
>People think they are expensive because they used to get them at fests
>for $10 or $20, but if you pay $500.00 for a 32v3, I think you got a lot
>of rig (and fun) for the money. The same money gets you an old crappy
>plastic rice box.
>Pay $3000.00 for a 30k-1 or some other bigger rig, and you have
>a lot of fun for the money, people spend that on little plastic
>boxes that put out 25 or 50 watts!
>For a lot of guys, playing with the equipment is as much
>fun as yakking into it.
>-----Original Message-----
>From: amradio-bounces at mailman.qth.net
>[mailto:amradio-bounces at mailman.qth.net] On Behalf Of John Coleman
>Sent: Tuesday, September 28, 2004 7:08 PM
>To: 'Discussion of AM Radio'
>Subject: RE: [AMRadio] Home Brew
>	If I were going to build with tubes again and two hundred watts was
>the carrier output that I wanted.  I would use a pair of 812s in push pull
>class C and modulate with a pair of 811As push pull class B.  Run about 1500
>volts @ 200 ma for 300 watts plate input on the 812s.  Find a Modulation
>XFMR to match the output of 811A class B (about 12500 ohms as I remember) to
>the 7500 ohm class C final. 
>John, WA5BXO
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