John Nicholson's Valve MW Transmitter
G'day again Dave,
I have attached images of the little AM Broadcast Band Transmitter.
The construction article and schematic was prepared by Peter Lankshear, and published in Electronics Australia in May 1989. This little unit is a very effective low powered AM transmitter, using a triode RF oscillator V2a, series modulated by V2b. Incoming audio is amplified by V1. Audio can be supplied from a CD player or FM tuner. This allows you to transmit old time music and tune it in on a vintage valve radio - producing a most realistic program source.
The coil is a standard broadcast band oscillator coil salvaged from an old valve radio. My transmitter uses an old valve radio power transformer and valve rectifier to produce the HT. V1 audio amplifier is one section of a 12AX7. (Equivalent 12AT7, ECC83, ECC81, 6SQ7, 6B6) V2 Oscillator/ Modulator 6CG7 (Equivalent 12AU7, 12BH7, ECC82, 6SN7) Use a telescopic aerial from a junked transistor radio. The transmitter can be tuned to a vacant spot in your local broadcast band. Effective range is less than 50 yards.
The original schematic had the tuning capacitor connected across the coil in the V2a anode circuit. This effectively put HT voltage across both sides of the tuning capacitor - creating mounting and safety problems. The schematic has been modified to show the tuning capacitor connected to the anode circuit via a 0.1uF capacitor. This works very well.
I have scanned the original article. Please advise if you would like a copy.