02-06-2019, 06:22 PM
Hello Everyone,
I just remembered that I had promised to post some more of my AM TX projects here so here goes:
both transmitters use the principle of a balanced modulator with the screen grid and anode as differential outputs. When I first came across this principle I was worried that the output transformer might be difficult to make but this turned out to be relatively uncritical as long as the carrier frequency and the modulating frequency are far apart. I tried using this principle in my stereo encoder project as well but here the carrier is only 38kHz and I could not get a clean modulated output from this circuit as the higher audio frequencies would also appear at the output of the transformer. For AM MW and LW this principle works quite well though and delivers a nice almost completely linear modulation trapezoid.
The output transformer is quite simple to make. I have used a type of litz that can be found on many SMPS power transformers. This litz consists of around 5-7 strands of wire which makes it ideal for winding this transformer. I start out with an empty T50-43 toroid core and put around 40 turns of this litz on it. Then I divide up the individual strands of wire to form the required windings e.g. if the litz consists of 7 strands I divide them up as 2+2 for the primary and 3 for the secondary windings respectively. You can also just use three wires and wind them on the T50 core in one go and then create the primary and secondary windings as needed of course...
One project is a transmitter that uses Russian tubes that can be found on Ebay for reasonable prices.
This project uses one part of a 6N3P double triode as an oscillator and the other part as an AF amplifier. A 6J2P pentode is used as the actual modulator. The 6N3P is a little like the ECC81 but has the filament connections on pins 1 and 9 instead of the usual 4 and 5. The 6J2P-EV is the Russian version of the 6AS6 and more or less a drop in replacement for it, so if you have a 6AS6 somewhere this will work just fine.
The other project I have built is using P-Valves. I had the opportunity to buy a bunch of those for a very fair price so I had to do something useful with them. The circuit posted here is similar to the one I had presented in Radiomuseum.org.
This version, however, has an additional output stage with a PCF82 and can deliver up to 250mW -so beware if you do not have a license.... I also have a version where I have used a PCL200 instead of the PCF82 that will output up to 1W. This design also has a power output and modulation level indicator using a PM84 which gives it a neat touch. Here is an image of the version I have built:
Apart from these two valve projects I also have built an AM TX sing a TBA520 PAL decoder chip as a balanced modulator and oscillator. Again this was being offered her from a surplus supplier at a cost of a mere 0.20€ so I thought this would be a worthwhile project to build. I still need to get teh data for that from my other PC and change the descriptions to English, so I will post it here in a few weeks when I get around to it.
Please let me know if you have any questions.
I would like to point out that I am posting these circuit diagrams for educational purposes only. By no means is my intention to motivate anybody here to build and run an unauthorized transmitter.
I just remembered that I had promised to post some more of my AM TX projects here so here goes:
both transmitters use the principle of a balanced modulator with the screen grid and anode as differential outputs. When I first came across this principle I was worried that the output transformer might be difficult to make but this turned out to be relatively uncritical as long as the carrier frequency and the modulating frequency are far apart. I tried using this principle in my stereo encoder project as well but here the carrier is only 38kHz and I could not get a clean modulated output from this circuit as the higher audio frequencies would also appear at the output of the transformer. For AM MW and LW this principle works quite well though and delivers a nice almost completely linear modulation trapezoid.
The output transformer is quite simple to make. I have used a type of litz that can be found on many SMPS power transformers. This litz consists of around 5-7 strands of wire which makes it ideal for winding this transformer. I start out with an empty T50-43 toroid core and put around 40 turns of this litz on it. Then I divide up the individual strands of wire to form the required windings e.g. if the litz consists of 7 strands I divide them up as 2+2 for the primary and 3 for the secondary windings respectively. You can also just use three wires and wind them on the T50 core in one go and then create the primary and secondary windings as needed of course...
One project is a transmitter that uses Russian tubes that can be found on Ebay for reasonable prices.
This project uses one part of a 6N3P double triode as an oscillator and the other part as an AF amplifier. A 6J2P pentode is used as the actual modulator. The 6N3P is a little like the ECC81 but has the filament connections on pins 1 and 9 instead of the usual 4 and 5. The 6J2P-EV is the Russian version of the 6AS6 and more or less a drop in replacement for it, so if you have a 6AS6 somewhere this will work just fine.
The other project I have built is using P-Valves. I had the opportunity to buy a bunch of those for a very fair price so I had to do something useful with them. The circuit posted here is similar to the one I had presented in Radiomuseum.org.
This version, however, has an additional output stage with a PCF82 and can deliver up to 250mW -so beware if you do not have a license.... I also have a version where I have used a PCL200 instead of the PCF82 that will output up to 1W. This design also has a power output and modulation level indicator using a PM84 which gives it a neat touch. Here is an image of the version I have built:
Apart from these two valve projects I also have built an AM TX sing a TBA520 PAL decoder chip as a balanced modulator and oscillator. Again this was being offered her from a surplus supplier at a cost of a mere 0.20€ so I thought this would be a worthwhile project to build. I still need to get teh data for that from my other PC and change the descriptions to English, so I will post it here in a few weeks when I get around to it.
Please let me know if you have any questions.
I would like to point out that I am posting these circuit diagrams for educational purposes only. By no means is my intention to motivate anybody here to build and run an unauthorized transmitter.
