Radio tubes have largely disappeared from modern electronic devices. There are only a few very specialised applications left for them, such as high-power RF transmitters and high-fidelity audio amplifiers. Yet, they are beautiful components with a sophisticated assembly.
Recently, I discovered an experimentation kit when cleaning up my room. The kit contains two russian battery tubes, 2Ж27Л, or 2SH27L, an instruction manual and a couple of resistors, capacitors, two piezoelectric transducers and some more stuff. Don’t know if it’s still available, but here is a link to it.
The nice thing about the kit is that they use low plate voltages (9V, 18V) for most experiments in the booklet, so the risk to kill yourself is small. One experiment is a medium-wave radio which I built (see the photo above). I can receive many MW stations in the evening with this radio.
At the moment, these tubes are still very easy to get. I got 32 on ebay, for about an euro each. I also discovered some other tubes in my box of spare parts - a handful of Philips 5654 tubes which are military-grade 6AK5 / EF95, one E80L, a couple of E810F, two 6146B and several 7587 nuvistors.
A simple Meissner circuit with the 5654 started oscillating already at 18 volts:
I plan to learn some more about these nice devices and to build a transceiver some time in the future...