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Author Topic: seeking modulator transformer or command set modulator sources at decent prices  (Read 505 times)
AK0B
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Posts: 351




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« on: April 30, 2019, 09:22:14 PM »

I ran an ad in eham classifieds with no response.  I want to use a command transmitter on both 40 and 80 meters AM.   I have everything i need for the RF and power supplies,   but need a modulation transformer or an old command set modulator  --  perhaps you got one in the garage or under the work bench.

I have a 60 year collection of old parts.  But always operated CW or SSB and never bother to thank about running AM.  Perhaps we could trade something.  Or I can put your old modulator on the air again.  It could be reborn

Stan AK0B,   address is in QRZ listing.   or email  at   W9IFZ  at   yahoo  com
73 and thanks for reading
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G3RZP
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« Reply #1 on: May 01, 2019, 03:15:22 AM »

The BC456A modulator used screen grid modulation, while the BC456B and later used plate and screen. Screen grid modulation can be done without a modulation transformer..

The problem with plate and screen modulation is the DC for RF stage tends to saturate the transformer core, so you need a bigger air gap in the transformer core and more turns. You can get around this by using a suitable iron cored LF choke in the feed to the PA and feeding from the modulation transformer through a high voltage capacitor of say 2 or 4mFd. Then you could probably use a power transformer as the audio transformer in the modulator. You would ideally want a 230 volt primary power transformer with say a 300 - 0 - 300 secondary to which you connect the modulator tube plates.

BC456s fetch quite a lot of money these days....
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AK0B
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« Reply #2 on: May 01, 2019, 09:53:13 AM »

Thanks G3RZP,  you cause my memory to function.  Smiley   I think I will go the way of a  non-transformer modulation solution.  Back in late 50's i had a DX-35 and it did a reasonable job. 

Found  several good design aids for Controlled Carrier on the web.  Would be cheap and quick and fit easily with my Command transmitters.  Not sure if the 1625's will work as well as the 6146s, but worth the effort and will not have to pay the ridiculous prices some think a modulator from 75 years ago is worth.

Between the R. H. Baer patent and AC0OB SGM/CC notes should be able to get a reasonable sounding signal on the air.

Thanks, 73  Stan AK0b
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G3RZP
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Posts: 1103




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« Reply #3 on: May 01, 2019, 11:19:09 AM »

I don't know about controlled carrier, but screen modulation is perfectly practicable. An improvement in controlled carrier using NFB was in the UK 'Short Wave Magazine' for September 1952, pp309 -402, under the title 'Clamp Modulation using NFB'. Look at:

https://www.americanradiohistory.com/Archive-Short-Wave-UK/50s/SWM-1952-09.pdf

I think there are few more articles on screen grid modulation about that time. Don't weep too much at the surplus prices in adverts in the magazine - it was around $3.50 to £ then and wages were much lower!
« Last Edit: May 01, 2019, 11:21:42 AM by G3RZP » Logged
N2DTS
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Posts: 949




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« Reply #4 on: May 01, 2019, 01:06:44 PM »

Screen mod will get you much lower power output.
Google  dx60 mods, Steve (wa1qix) came up with a fantastic circuit with a neg limiter and adjustable carrier level control that works fantastic.
Broadcast audio if you want.

But plate modulation will get you much more power output.
A 25 watt mod trans should be easy to find.
A nice pair of 6L6 type tubes would be nice.
You could drive the grids of those or 807's with a 8 ohm to 5000 ohm hifi transformer driven by a solid state amp (even an IC chip) or tubes.
its all fun.
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G3RZP
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« Reply #5 on: May 01, 2019, 01:32:56 PM »

Over here, modulation transformers are about as common as rocking horse manure these days. Maybe because there is too much interest in AM!
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N2DTS
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« Reply #6 on: May 01, 2019, 07:38:14 PM »

What happened to all those cool Geloso rigs?
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G3RZP
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« Reply #7 on: May 03, 2019, 05:19:32 AM »

You mean the ones with Geloso VFOs, like the KW Vanguard? Still in demand despite the fact that they drift and chirp and have incidental FM....The actual Geloso tx wasn't any better than the KW one, either.
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