Greetings from Perth, Australia. Thanks for a very interesting video. I have been toying with the idea of building a simple AM transmitter and the idea of making it out of junk parts is very appealing. I might give it a try soon.
It’s very interesting that you wired the Hartley oscillation circuit as shown: both ends of the variable capacitors are hot (connected to high voltage). When the insulation on the control knob of your variable cap breaks down, you’ll be wired to HV and be shocked (and you’ll remember that). Typically, in a safe Hartley implementation, the input node is grounded (one end of the inductor and the shaft of the variable capacitor are DC and AC grounded, the grid is AC grounded.) The common node, the center tap of the inductor, is connected to the cathode of the tube. The output node is AC connected to the anode through a capacitor, and it is connected to HV through a RF choke. Topology-wise, it’s the same Hartley circuit: the inductor performs the phase reversal of the signals between the output node (the anode) and input node (the grid) while the common node (the cathode) is the reference. It’s just that the control knob is now at ground, lessening the chance of electrocution.
Its wired in the fashion that worked at the time. I experimented with some other configurations but couldn't achieve oscillation. I'm fully aware of the hazard it presents, which is why I don't really use it; it was more for fun and to experiment with tube electronic design which I'd never tried.
I really liked this video! Excellent topic and excellent presentation!. I acquired a huge old tube inventory some years ago, and one thing I’ve always wanted to do was build my own AM broadcast band transmitter. I finally did perty much what you’ve done here, to experiment with various ideas, to include copy what others have done. I finally settled on a design that included an oscillator followed by a buffer, followed by two stages of RF amplification. Then a matching tank circuit to a 50 ohm output. The audio stage has an AF input followed by two AF amplifier stages that feed a modulation transformer, the output joining the RF final stage. It works quite well. I don’t broadcast with it…oh I guess I have done a time or two just for fun. I just enjoy doing as you have done, to tinker to see what works, and what doesn’t! Thanks for your channel!
Having had a 1st class radio telephone operators license back in the day, I find your method of design abysmal at best. Modulating your oscillator is horrid, it will result in a great deal of FM component. With a VFO, unstable to start with, you should have at least a couple of stages of isolation. Your output, a capacitor right off the plate is going to result in horrid harmonic output, you should at least have a pi circuit to both resonate the output and suppress harmonics and also to match impedance. The antenna impedance is going to likely be much lower than the plate of the tube so it will result in much of the RF energy being reflected back into the tube heating it. Again a pi section is an ideal solution, the coil sees two capacitors in series and the series capacitance resonates with the coil to provide a tuned output. Depending upon your choice of capacitor / inductor you can control the 'Q' of the circuit. You want a fairly high Q but not so high that you destroy wanted modulation products, but higher Q means fewer harmonics which means less likely someone in the 80 meter ham band is going to call the FCC because your third harmonic just happens to land at 3.6 Mhz, etc. The ratio of the capacitors forms a voltage divider like circuit matching the lower load impedance to the higher plate impedance assuming values are chosen appropriately. Often these two capacitors are variable with the input side labeled tuning and the output side labeled loading though there is interplay between them. Then plate modulation is preferable to grid, it will provide more linear modulation and higher levels of modulation.
He says he should add a transformer with a winding for the B+. That's easy to do. Get another AC to 6.3VAC filament transformer. Connect the 6.3VAC secondary to the 6.3VAC secondary that's connected to the tubes. Now the second transformer's primary is putting out 120VAC, which can be rectified and is completely isolated from the AC line. No more risk of getting shocked from the AC line. 👍 Also, I've found that connecting a .01 uF, 500VDC ceramic capacitor across the rectifier diode will reduce the hum from the 60Hz line. Thanks for the video.
I've got a bunch of transformers now that would be better suited than my initial setup. The ceramic cap thing I've done before, definitely a good idea!
@@Eclec-Tronics Also another thing I would like to try to increase the % modulation. Try a Cascode circuit with the modulation in the cathode of the output tube. This might be done using a power transistor such as the TIP41C, or maybe a tube. Whatever the case, anything to increase the % modulation would be a big improvement. Thanks.
A 'scope trace of the modulation envelope with 80% mod tone will show how distorted the mod is, and a trapezoidal trace will show even more. But it won't show all the FMing such a circuit will also produce. It looks/sounds just like the Knight Home Broadcaster, which is pretty awful, even the modern (rather expensive) reproduction kit!
I actually have a Layfayette kit built transmitter similar to the Knight, and yeah, the output is "okay". I think the operating range is marginally better on it, maybe 10 feet instead of the 2 my crap design manages!
That probably wouldn't be too hard, it's essentially what I did here, but with the other tubes you could also get some audio amplification. A transformer powered setup would be safer, though.
@@kwacz Definitely. I have a Lafayette kit built AM transmitter that works well, but the audio input is tied to chassis ground; gotta be careful plugging in metal backed Ipods!
A few feet, optimistically. The audio quality is lackluster, owing to a lack of a preamp. If it's sitting within 5' or so, and the radio and trans have a short antenna, it seems to work ok. There are substantially better designs for AM transmitters out there.
Greetings from Perth, Australia. Thanks for a very interesting video. I have been toying with the idea of building a simple AM transmitter and the idea of making it out of junk parts is very appealing. I might give it a try soon.
It's really easy if you've got a scrap AM radio chassis, like an AC-DC set.
It’s very interesting that you wired the Hartley oscillation circuit as shown: both ends of the variable capacitors are hot (connected to high voltage). When the insulation on the control knob of your variable cap breaks down, you’ll be wired to HV and be shocked (and you’ll remember that). Typically, in a safe Hartley implementation, the input node is grounded (one end of the inductor and the shaft of the variable capacitor are DC and AC grounded, the grid is AC grounded.) The common node, the center tap of the inductor, is connected to the cathode of the tube. The output node is AC connected to the anode through a capacitor, and it is connected to HV through a RF choke.
Topology-wise, it’s the same Hartley circuit: the inductor performs the phase reversal of the signals between the output node (the anode) and input node (the grid) while the common node (the cathode) is the reference. It’s just that the control knob is now at ground, lessening the chance of electrocution.
Its wired in the fashion that worked at the time. I experimented with some other configurations but couldn't achieve oscillation. I'm fully aware of the hazard it presents, which is why I don't really use it; it was more for fun and to experiment with tube electronic design which I'd never tried.
A excellent demonstration of your Junk Box AM Transmitter Full Marks it operates well Ian from Australia.
I really liked this video! Excellent topic and excellent presentation!. I acquired a huge old tube inventory some years ago, and one thing I’ve always wanted to do was build my own AM broadcast band transmitter. I finally did perty much what you’ve done here, to experiment with various ideas, to include copy what others have done. I finally settled on a design that included an oscillator followed by a buffer, followed by two stages of RF amplification. Then a matching tank circuit to a 50 ohm output. The audio stage has an AF input followed by two AF amplifier stages that feed a modulation transformer, the output joining the RF final stage. It works quite well. I don’t broadcast with it…oh I guess I have done a time or two just for fun. I just enjoy doing as you have done, to tinker to see what works, and what doesn’t! Thanks for your channel!
I can definitely say you put much more thought into your design than I did!
These osciloscopio is beautifool!,
Thanks for this, I enjoyed this and learned a lot.
I like the transmitter. And your test set-up.
Don't give up, your videos are very good and make me very happy
Put an LPF on the RF out to the antenna. From the looks of the output waveform, it’s putting out spurious harmonics …
Thanks for the suggestion, I'll give that a try!
27:09 is pure serotonin 😮💨😮💨
Having had a 1st class radio telephone operators license back in the day, I find your method of design abysmal at best. Modulating your oscillator is horrid, it will result in a great deal of FM component. With a VFO, unstable to start with, you should have at least a couple of stages of isolation. Your output, a capacitor right off the plate is going to result in horrid harmonic output, you should at least have a pi circuit to both resonate the output and suppress harmonics and also to match impedance. The antenna impedance is going to likely be much lower than the plate of the tube so it will result in much of the RF energy being reflected back into the tube heating it. Again a pi section is an ideal solution, the coil sees two capacitors in series and the series capacitance resonates with the coil to provide a tuned output. Depending upon your choice of capacitor / inductor you can control the 'Q' of the circuit. You want a fairly high Q but not so high that you destroy wanted modulation products, but higher Q means fewer harmonics which means less likely someone in the 80 meter ham band is going to call the FCC because your third harmonic just happens to land at 3.6 Mhz, etc. The ratio of the capacitors forms a voltage divider like circuit matching the lower load impedance to the higher plate impedance assuming values are chosen appropriately. Often these two capacitors are variable with the input side labeled tuning and the output side labeled loading though there is interplay between them. Then plate modulation is preferable to grid, it will provide more linear modulation and higher levels of modulation.
My one tube AES AM transmitter kit uses a 12SA7GT tube.
Nice, very stable VFO
He says he should add a transformer with a winding for the B+. That's easy to do. Get another AC to 6.3VAC filament transformer. Connect the 6.3VAC secondary to the 6.3VAC secondary that's connected to the tubes. Now the second transformer's primary is putting out 120VAC, which can be rectified and is completely isolated from the AC line. No more risk of getting shocked from the AC line. 👍
Also, I've found that connecting a .01 uF, 500VDC ceramic capacitor across the rectifier diode will reduce the hum from the 60Hz line.
Thanks for the video.
I've got a bunch of transformers now that would be better suited than my initial setup. The ceramic cap thing I've done before, definitely a good idea!
@@Eclec-Tronics
Also another thing I would like to try to increase the % modulation. Try a Cascode circuit with the modulation in the cathode of the output tube. This might be done using a power transistor such as the TIP41C, or maybe a tube. Whatever the case, anything to increase the % modulation would be a big improvement. Thanks.
This is so awesome. U helped allot!
Hello
I d like to build your transmitter
I have the tubes
Tank you for Nice démonstration
Michel ON4KMC
Can you give me the power ?
A 'scope trace of the modulation envelope with 80% mod tone will show how distorted the mod is, and a trapezoidal trace will show even more. But it won't show all the FMing such a circuit will also produce. It looks/sounds just like the Knight Home Broadcaster, which is pretty awful, even the modern (rather expensive) reproduction kit!
I actually have a Layfayette kit built transmitter similar to the Knight, and yeah, the output is "okay". I think the operating range is marginally better on it, maybe 10 feet instead of the 2 my crap design manages!
عالی بود مهندس
With the ( mainstream ? ) AM broadcasters abandoning MW and LW this looks a good idea ( ? ) ...DAVE™🛑
I like tube,,
Have you thought of taking an old AA% radio and simply converting it into a transmitter?
That probably wouldn't be too hard, it's essentially what I did here, but with the other tubes you could also get some audio amplification. A transformer powered setup would be safer, though.
@@Eclec-Tronics or if hot chassis an audio transformer would need to be used on the input to provide isolation so you dont get shocked.
@@kwacz Definitely. I have a Lafayette kit built AM transmitter that works well, but the audio input is tied to chassis ground; gotta be careful plugging in metal backed Ipods!
what range does it get?
A few feet, optimistically. The audio quality is lackluster, owing to a lack of a preamp. If it's sitting within 5' or so, and the radio and trans have a short antenna, it seems to work ok. There are substantially better designs for AM transmitters out there.
Yay! junk parts, junk circuit, junk transmitter! This must be the biggest pile of crap on UA-cam! Nice going.