Who remembers the little AM transmitter featured in the May 1989 issue of Electronics Australia on the Vintage Radio pages ?
The same article was reprinted in the Electronics Australia booklet, Discovering Vintage Radio by Peter Lankshear.
The January 2006 issue of Silicon Chip Magazine featured a solid state AM Transmitter project called "Little Jim".
A Web search will find a host of similar projects, kits and articles proving there is a need for such a device.
With broadcast band AM station going off the air to be replaced by FM stations and many of the stations that remain being special interest stations, us Vintage Radio Enthusiasts do not have much to listen to now.
One way to solve that problem is to build your own "flee power" broadcast band transmitter and feed it with program material you like to listen to. You could for example re transmit your favorite FM station or use an MP3 player to broadcast your taste in music inside the confines of your house or radio room. In the above photo you can see the cheap MP3 player providing my program material. This one has an SD card slot and I have several SD cards, each with a different flavour of program.
In addition to old time music I have old time radio programs from Radio Lovers a wonderful source of a vast range of programs from the 1930's and 1940's. Save these as MP3 files, load them into a cheap MP3 player and you will have an almost unlimited source of authentic program material to listen to on you vintage radios. This is a fantastic site and all the files are available free. There are also quite a few sites that will sell you Old Time Radio programs and if you want some Australian programs, these are available from the Kurrajong Radio Museum.
My version of Mate uses a 6AV6 as the audio preamp and a 12AT7 as the RF oscillator and the modulator. I found this tube gave me the cleanest modulation envelope.
The power supply uses the little power transformer and filter choke salvaged from a 1950's AC/Battery portable receiver that had had a bad accident. These transformers usually have a 6.3 Volt winding at 600mA and an HT winding of about 125 Volt. At first I built my mate using a bridge rectifier on the HT winding but the 150 Volt DC available was not enough to provide good clean modulation to about 90%. When I used a voltage doubler for the HT I had about 320 Volt and so I reduced and stabilised this with 0A2 and 0B2 gas discharge tubes in series to get 255 Volt HT.
I found my program sources, MP3 Players and CD players, had a wide variation of output level and I decided to add a Vu meter and peek indicator to keep an eye on the modulation level. Here I had to go modern and I have used an Op Amp (LF351), a comparator (LM311) and powered this circuit off the 6.3 Volt winding, rectified with a bridge rectifier and regulated with a modern low drop out regulator device (LM2940CT-5). The Vu meter was rescued from a written off cassette deck. I have also used a little switch mode inverter to supply a negative rail to the Op Amp. I could have used a single rail supply but I had the NME0505 left over from another project.
I found the original circuit required a few little tweeks, for example with the 6AV6, the anode load resistor should be 220K and a 100 pF capacitor to ground on the anode filters some of the RF getting into this preamp. The Modulation monitor circuit requires a low impedance take off point so it does not load down the Audio Pre Amp and the un-bypassed cathode resistor of this stage provided an ideal audio source for this purpose. I coupled the LF351 stage through a 10mH choke, with a 1000pF to ground to make sure very little RF gets into the meter circuit.
Now that the shortwave bands are becoming unusable with all the noise generated by modern electronic devices I may just have to build a short wave version of Mate. My thoughts are I will crystal lock that version because frequency drift will be a problem with a simple tuned circuit. I might just call that one DUNCAN, with apologies to Slim Dusty.
A print of the Kicad file of the circuit diagram in JPG format
If you look at my Kicad page you will find the original of this diagram and the library files that are required to display it.