Phono Oscillator uses 1.2 volt Russian "pencil" tubes Following on from an earlier valve design phono-oscillator circuit, i.e. Low-powered (0.5-1.5MHz) AM radio transmitter this is a construct using, (1.2 volt filament) minature russian valves. These valves are of 'rod type' construction, which were utilized in the sputnik radio beacon in October 1957. Sputnik off-air beep c1957 (10 secs) Instead of fine wire mesh type control grids of conventional radio valves, electron sheet beams are controlled by "rod" type electrodes. Link: excellent Rod Tube technical article (radiomuseum.org) (1.2 volt filament) Russian minature wire ended valves are "direct-heating" type (i.e. no cathode as such). A thin wire filament is needed in order to be a "point source" of the electron beams. Heater connection (f-) is normally connected directly to circuit ground Because of this 'limitation': with an oscillator circuit, no cathode to grid-circuit feedback would be feasible (needed to maintain oscillation). After trying a few different ideas, I found A.C. (capacitor) coupled anode to grid coil feedback worked the best. HT supply line is 45 volt DC (can be a battery) Filament supply is a 1.2V rechargeable ni-cad cell. Oscillator grid current should be ~ 200 µA Self excited oscillator (plus audio mod') is possible using 1J37 tube as it has two control grids! An online forum suggests that the 1J37 is just a 1J17 without a connection between the 2 opposed grid rods inside the tube? Oscillator coil is wound on small salvaged ferrite bobbin. A calculation gave 125uH as the required inductance 75 turns gave this inductance, which I then measured on my LC meter. Anode (feedback) winding is 33% (25 turns) of main winding Winding polarity must be observed to guarantee oscillation (red dots) i.e. if it doesn't work 'first-time' try reversing one windings connections. A small 320pF variable capacitor gives tuning over the frequency band (0.5 ~ 1.5MHz) The modulation pot' is advanced so the volume is similar as other AM bcst stations. Phone-oscillator development circuit is based on an example from cool 386 found at: Low Power AM Transmitter. I have earlier successfully used russian minature valves (6.3v fils non-rod type) in minature oscilloscope project (scroll down) Bench prototype construction using double sided veroboard. An ON-OFF-ON DPDT toggle switch powers the circuit and will recharge the filament NiCad at 20mA in the other position. LED's indicate function. Most componentry was recycled, except the (cheap) tubes sourced from Russian Ebay valve sellers � 3Q 2018