The battery vacuum tube has been invented in the ex UdSSR in 1950. I recently bought 20 tubes 1П24Б at an ebay auction. They consume only about 200mA at 1.2V for heating and were used in the Sputnik satellite and in the MIG-29 military jet, as well as in various Russian military equipment of that time. I wondered how difficult it would be to create a small QRP transceiver with one such tube.
There is a very nice web site by DL3JIN about building a small HF oscillator with the 1P24B (see here). I tried the fourth circuit, with G2 connected directly to the positive supply voltage. Unfortunately, the highest DC voltage that I can easily create is about 38 volts. Probably one of my next steps is to organize a power supply with a higher voltage for my tube experiments...
The circuit is very simple:
At the left, you see the 1P24B tube. The anode is located at the top of the tube and is connected to a small variable capacitor in parallel with an inductor (9 turns, about 35µH). The secondary side of the inductor (3 turns) is fed into a low-pass filter. At the bottom, you see the 3.579545 MHz crystal, the only one I found which falls into an amateur radio band. The frequency is the upper limit of the CW area of the 80m band.
At 38 volts, the circuit generates a nice 2Vpp sine wave into a 50Ω resistor:
So, the current I is U / R = 0.6Veff / 50Ω = 0.12A and the power is approximately U * I = 0.6V * 0.12A = 0.072W. According to DL3JIN’s web site, it should be possible to get about 200mW at 60V and between 500mW and 1W at 120V. I will try again when I have a better power supply.
It’s easy to receive the signal in a transceiver at the other end of the table, disconnected from the antenna:
I wondered how the oscillation would start when connecting the anode voltage. Here is the measurement:
It seems to take about 20ms for the signal to stabilize. May be I will try to make a QSO with this circuit later this week...