3d printed vacuum tube transmitter

thingiverse

3d print based simple vacuum tube transmitter. Transmits on the 80m band (3.5-3.8Mhz). Also works on 40m with either less turns on the tank, or less capacitance. Designed to work with many dual octal triodes. Use schematic as reference and make appropriate connections to grids, anodes and cathodes. Schematic also shows a very simple form of modulation, using a carbon mic and an "absorption loop". The trimmer capacitor is made from 2 pieces of pc board laminates, approx. 50x30mm. The wires are soldered to the plates, then the plates were taped up to avoid shorts. A small pc board strip is used as spacer. Capacity is around 45pf. Make them longer to get more capacity. The tank coil is 24t centertapped. Make a small loop at the centertap to connect the high voltage. Grid coils are 10t centertapped. Hint: if the transmitter does not oscillate, try swapping grid connections. Antenna link turns is critical. one loose turn gives very good frequency stability, but only about 500mW, 4 turns gives close to 5W with a 4k7 grid leak resistor, but signal tends to be very unstable, I settled for 2 turns, centertapped to ground, which gives me around 2.5W into 50ohm load with decent stability (see video) I printed the screw 1% smaller to get a good fit of the thread. https://www.youtube.com/watch?v=x0XmR1_32DI&t=12s Materials needed: A twin triode octal vacuum tube (6sn7,12sn7, 6sl7,12sl7, 6n7, 6n6 etc) Pc board strips, 30mm wide 50mm long. Mica caps to buffer the tuning capacitor, about 200pf. a bypass cap across the grid leak resistor, I used a 100pf mica. 1mm copper wire. used bare copper wire to "thread" the socket. Insulated wire for the grid and antenna coils. I used a 300v power supply and 6.3v for the filaments. suitable HV switching modules are available on ebay. Another approach is to connect 2 small transformers back-to-back and rectify to get a high voltage source. To modulate the transmitter, use a carbon mic element (an old telephone handset for example) and connect that to a one turn loop over the tank coil.