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 Post subject: AM Transmitter
PostPosted: Feb Sun 02, 2020 4:44 am 
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Has anyone been able to find an authentic 1920s AM single 01A tube transmitter schematic?


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 Post subject: Re: AM Transmitter
PostPosted: Feb Sun 02, 2020 6:16 am 
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Location: Mountains of Mourne. Ireland.
Two 01As for modulation...
viewtopic.php?p=2374878#p2374878

Greg.


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 Post subject: Re: AM Transmitter
PostPosted: Feb Thu 13, 2020 9:52 pm 
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Interesting, but would it be possible with one tube? The following photo is from:
Project Gutenberg's The Radio Amateur's Hand Book, by A. Frederick Collins

I've built this one before, but have had no luck. Possibly I made a minor mistake...Has anyone else tried something similar?


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 Post subject: Re: AM Transmitter
PostPosted: Feb Thu 13, 2020 10:58 pm 
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That circuit requires a carbon microphone. It won't work with any other kind.


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 Post subject: Re: AM Transmitter
PostPosted: Feb Thu 13, 2020 10:59 pm 
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G. Brook, you are probably aware that that "lamp socket" connection is for the old DC house wiring.

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 Post subject: Re: AM Transmitter
PostPosted: Feb Thu 13, 2020 11:49 pm 
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Yes I’m already aware of the needed power supply. I’m just wondering if a schematic as such would perform properly?


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 Post subject: Re: AM Transmitter
PostPosted: Feb Fri 14, 2020 2:15 am 
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It depends on what you mean by properly. It can probably be made to oscillate. If you can find a carbon mike and shout into it, then you can probably transmit some kind of speech to a nearby radio.

If your goal is to transmit music of reasonable fidelity, then no, it won't do that. Not without more tubes.


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 Post subject: Re: AM Transmitter
PostPosted: Feb Fri 14, 2020 4:00 am 
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Connecting the microphone in series with the antenna requires a mike that can dissipate the needed amount of power. Back when that method was the only way to go, they had water cooled microphones. That wouldn't be necessary with a 01A oscillator but it is still something to consider. A 01A might be capable of a watt or two of RF.

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 Post subject: Re: AM Transmitter
PostPosted: Feb Fri 14, 2020 12:44 pm 
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Jim Mueller wrote:
A 01A might be capable of a watt or two of RF.


Hmmm.... Just guessing, I would have to look up the data sheet, but I think the anode resistance of the 01A is about 10k, and as I recall in a class A audio amp you could only squeeze 30 to 80mW out of it if that. I'm not sure how an 01A could produce an RF output power of anything like 1 watt, let alone 2 watts. It might be possible to get a few hundred mW , with two of them, in class B push pull, I would have to look that up.


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 Post subject: Re: AM Transmitter
PostPosted: Feb Sat 15, 2020 7:39 am 
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You would operate it in class C which is a lot more efficient than class A. The 1 or 2 watts was a guess.

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 Post subject: Re: AM Transmitter
PostPosted: Feb Tue 18, 2020 2:54 am 
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Jim Mueller wrote:
You would operate it in class C which is a lot more efficient than class A. The 1 or 2 watts was a guess.


In practice though its not hugely more even in class C because to get any reasonable power output is a compromise in impedance matching, so in general the efficiency, in practice in class A might be 30%, class B perhaps 50% and class C about 70% (even though theoretically 98% might be possible). So I still cannot see how anywhere near 1W could be attained from an 01A even in class C, maybe a few hundred mW. Still, that would be ok for a short range AM transmitter, but it would require a good modulator.

For my own short range AM MW band transmitters I have moved to linear class AB push pull output amplifiers (really just class B biased out of cross over distortion) and modulate the carrier before the output stage with a precision amplitude modulator, to avoid the distortion that occurs modulating at the output stage.


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