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Author Topic: Sourceing glowbug transmitter parts  (Read 2418 times)
BRIANBURNS
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Posts: 28




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« on: March 03, 2012, 04:30:08 PM »

Hello All,

I'm interested in building a 100 watt AM and CW transmitter that would be approximately the equivalent of a Johnson Viking. I'm wondering if it would be possible to find new parts for the project from the usual suppliers like Mouser and RFparts, or are there "unobtaniums" at this late date. I don't have a junk box worth mentioning, and I'm not eager to go on a scavenger hunt, or take my chances with finding something on ebay.

I know that new modulation transformers are hard to find, but I think that there is a possibility that they are available from AnTek, perhaps on special order. High volltage variable caps don't seem to grow on trees either. Any other "gotchas" that would stop the project in its tracks?

Thanks,

Brian Burns...soon to be K6UCD once again
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G3RZP
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Posts: 4589




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« Reply #1 on: March 04, 2012, 12:24:20 AM »

You don't want a toroidal modulation transformer unless you are going to feed its output via capacitor and use a choke in the PA feed. This is because toroids have a lower saturation level as a result of the high magnetic circuit efficiency. A modulation transformer has a DC component in the iron because of the PA current through the winding.
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K0IZ
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« Reply #2 on: March 04, 2012, 07:30:10 AM »

Brian, most vintage type parts are not available from current suppliers.  Some are, but prices are super high.  Local hams (vintage like me) might well have big junk boxes and would be willing to part with stuff.  Also hamfests are real good sources, especially the big ones like Dayton Hamvention.  Another possibility is to purchase a homebrew transmitter on ebay for parts.  Usually the homebrew stuff goes for very little.  Another possibility is a "for parts/repair" commercial rig that goes cheap.
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W5FYI
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« Reply #3 on: March 04, 2012, 01:16:28 PM »

Check out Antique Radio Supply, www.tubesandmore.com and good luck with the project. Let us know how it works out.
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BRIANBURNS
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« Reply #4 on: March 04, 2012, 04:45:53 PM »

Hello Peter,

Thanks for the warning about toroidal modulation transformers. Could you post a schematic of how you would go about using one?

Cheers,

Brian
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G3RZP
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« Reply #5 on: March 05, 2012, 12:58:13 AM »

Difficult right now - I have had to get a new computer and I am certainly not yet up to doing much with Windows 7 and Office 2010. Dreadful things - XP Professional and Office 2003 were far more intuitive.

So try this:

In the feed to the plate and screen of the PA, place a choke, rated for about 10 Henries - not too critical - at the rated DC plate current. Connect a suitably rated  capacitor from the plate end of the choke to the toroidal modulation transformer secondary: the otehr ened of teh secondary goes to ground. This technique is often used  to get rid of DC in the modulation transformer, and the added LF choke is referred to as a 'modulation reactor'.

Hope this helps.
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