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Old 16th November 2012, 09:18 PM   #1
sco1t is offline sco1t  United Kingdom
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Unhappy Short wave Valve transmittor :) what valve is best?

Hello all, I'm trying to make a 8Mhz shortwave transmitter but I can't seem to get a valve to amplify 8Mhz,,, I can get them to amplify audio easy though so i'm guessing the frequency is to high for the valves? I'm using ecc81 valves I've also tried a big pl504 but it was not happy too
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Old 16th November 2012, 09:35 PM   #2
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you need a license!

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Old 16th November 2012, 09:41 PM   #3
sco1t is offline sco1t  United Kingdom
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I know, but I only want to transmit about 1 meter to an old valve radio. you can buy FM ones for in cars with no license for playing cd's or ipods over a car radio. same thing almost
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Old 16th November 2012, 10:08 PM   #4
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Quote:
same thing almost
Almost but not quite.
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Old 16th November 2012, 10:08 PM   #5
asmith is offline asmith  United States
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Get an old Heathkit signal genarator and feed the audio in to it. Lots of them had an input so you could do just what your wanting to do.
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Old 16th November 2012, 10:21 PM   #6
sco1t is offline sco1t  United Kingdom
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hi Asmith, that looks very handy machine but they cost a lot.
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Old 16th November 2012, 10:59 PM   #7
DF96 is offline DF96  England
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An ECC81 will happily amplify 8MHz or 80Mhz. I don't encourage unlicensed valve transmitters. A few low power transistors will be better, as a few mW won't do any harm. A valve transmitter will naturally produce nearly 1W without even trying.
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Old 16th November 2012, 11:20 PM   #8
sco1t is offline sco1t  United Kingdom
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Hi DF96, Wow 80Mhz is a lot. So I must be doing something wrong with the valve? I have thought about using transistors but I don't have any... oh 1 watt is a lot but I guess I could turn it's gain down to a few mw's .

Should I up load a diagram to see where I've gone wrong with the valve?

Thanks every one for the help.
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Old 17th November 2012, 12:03 AM   #9
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1. Some simple SE amp
2. Some simple 1-tube oscillator

For the oscillator you can use some small cheap tube like 6BA6, with 1 MOhm or higher value of grid leak resistor, for grid auto-bias, between control grid and cathode. Wind some coil with about 100 turns on a pill cap, tap from 1/3. The tap goes to cathode, the lower end to the ground, from the upper end to the control grid through the cap like 51-100 pF. Add variable capacitor in parallel with the coil. Put some short antenna to the same upper end of the coil. Connect together screen grid an anode, and connect them to the anode of the SE amp. Turn it on, put some music, and wait for FCC and police to come and arrest you.

Bear is right, you need a license to do that.
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Last edited by Wavebourn; 17th November 2012 at 12:05 AM.
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Old 17th November 2012, 12:06 AM   #10
Enzo is offline Enzo  United States
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80MHz is a lot? The entire FM radio band is higher in frequency than that, and they had no trouble making FM radios with tubes.

The entire amateur radio hobby used to run on tubes, and 8MHz was not at all high frequency. 2 meters was 144MHz if I recall, and tubes handles that.

And in any circuit, it is about the whole circuit, not the parts. It may not be the tube at all, you also have to have proper circuits around the tube.
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