Short wave Valve transmittor :) what valve is best?

[sco1t]

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

 

[bear]

you need a license!

_-_-bear

 

[sco1t]

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

 

[Joe Roberts]

same thing almost

Almost but not quite.

 

[asmith]

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.

 

[sco1t]

hi Asmith, that looks very handy machine but they cost a lot.

 

[DF96]

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.

 

[sco1t]

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.

 

[Wavebourn]

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.

 

[Enzo]

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.

 

[sco1t]

thanks for the tips. I thought LPAM (low power AM) as in super low power did not need a license in the USA as long as it meets part 15 of the fcc??????????
Look here Part15.us | THE FREE Resource For Part15 Community Radio! or google it any way I was only going to transmit 1 or 2 meter so how would any one ever find out..... unless I lived next door to the police station lol

I'll open up an old fm valve radio to see how they have there I.F valves connected for tips

Thanks

 

[Wavebourn]

For such power you need 1 transistor and 1.5V battery.

 

[Wavebourn]

I'll open up an old fm valve radio to see how they have there I.F valves connected for tips

Look for heterodyne, not IF. It should have appropriate for the frequency coils.
If you want to transmit FM on 88 mHz it is a different story. The easiest way is to modulate it by a varactor diode,

 

[sco1t]

Hi wavebourn, thanks for the tips
So vales are to powerful?? That's normally a good thing but not in this case.
I have a 8Mhz quartz crystals oscillator which I was trying to get a valve to amplify but it would not do it. (mean valve)
I'll stick to AM, FM,s is a bit harder and i'm not as smart as you lol :'(

 

[Wavebourn]

Here is an example: use your crystal from base to ground instead of a resonant tank.

 

[kevinkr]

thanks for the tips. I thought LPAM (low power AM) as in super low power did not need a license in the USA as long as it meets part 15 of the fcc??????????
Look here Part15.us | THE FREE Resource For Part15 Community Radio! or google it any way I was only going to transmit 1 or 2 meter so how would any one ever find out..... unless I lived next door to the police station lol

I'll open up an old fm valve radio to see how they have there I.F valves connected for tips

Thanks

Well the other issue is that you don't live in the United States and the laws here don't apply in the UK.. You need to check your local laws, and having a ham radio license may be necessary.. There are some unlicensed experimenter bands here in the US but the power levels are extremely low.

 

[DigitalJunkie]

You might want to look around this site for some tips,circuits,etc. Antique Radio Forums • View forum - Homebrew Radios

Just do be careful of the power output.
In fact,for a short distance,you could probably use a dummy-load,and still have enough signal leakage to pick it up across the room.
I've done a similar thing for attenuating the 1W output of an FM transmitter. A long,~50ft piece of 'cheap' coax,with a dummy load at the far end. The cheap coax had poor shielding,so it would leak some signal out -just enough to cover about 1 square block.

 

[sco1t]

hi, that's similar to how I connected my valve . I'll get a transistor like that one and make that circuit. Thanks for helping

 

[sco1t]

I know, the laws are even tighter in the UK but a few meters is still ok . I guess I will ask my friend. He's a manager at a (rubbish) radio station lol

 

[Joe Roberts]

I thought LPAM (low power AM) as in super low power did not need a license in the USA as long as it meets part 15 of the fcc?

We can't fire up on any frequency we choose. Part 15 is restricted to allocated broadcast bands.

8mc is fixed and mobile radio service in your ITU region. Your crystal might be on a government or military frequency. I doubt you are going to be loud with a simple rig with a lousy or no antenna but you might be hearable. I worked hams in Asia with 3 watts.

That being said, I assume you want to modulate the signal with audio rather than just producing an empty carrier. I'd recommend getting a triode/pentode and screen modulating the pentode with the triode section as modulator amp.

Have a look at this, for example: A One-Tube Controlled Carrier AM Transmitter

That is quite a simple circuit for what it does.

Maybe pick a frequency with some forethought. There might be some legal frequencies for this sort of project in the UK, but I wouldn't know about that.