[AMRadio] Home Brew


Larry Roohr kz0e at comcast.net
Thu Sep 30 08:58:08 EDT 2004


Ooops, sorry, didnt mean to send this to the list, but since I did, 
havnt heard from Brett yet of course but I'm in the market for a good 
mod xformer for a 4x 813 transmitter if anyone has one available they'ed 
like to sell.

thanks,

Larry


Larry Roohr wrote:

> 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.
>
> Thanks,
>
> Larry
>
> 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.
>>
>>
>> Brett
>> N2DTS
>>
>>
>> -----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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>>  
>>
>
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