|[AMRadio] Home Brew|
Brett.gazdzinski at mci.com
Wed Sep 29 08:29:10 EDT 2004
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.
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.
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