A Hearing Aid Radio??
Some years ago George purchased a giger counter from a local army surplus stores. Having never done anything with it George decided to dismantle it for the parts, these included 3 CV2288 battery hearing aid valves - what to do with them?? - well build another radio of course!
Thanks to Allan Wyatt at www.virtual-museums.org, George obtained a data sheet for these devices, it turnes out that they are 'subminature output pentodes' with a 1.25v heater (suitable to run from a 1.5v battery) requiring an HT of +22v.
In keeping with the age of the valves George decided to breadboard the new radio in the old style - on an off cut bit of a friends' decking (no their deck isn't missing a bit - it was a bit they didn't need!). The junk box furnished a very old tuning capacitor, a huge radio frequency choke and a small 240v 9-0-9 v mains transformer.
After a little look round the 'net at old valve regen radio circuits, and a bit more research on the manner in which these devices are typically used to 'get a feel of the circuit' George devised a simple 2 valve regen receiever using 2 of the CV2288's.
With a simple tuning coil and tickler wound on a discarded bit of ferrite rod, and the small mains tranformer in reverse as a speaker transformer and 4 'Tesco value' PP3's in series to give 36v rather than 22v and a big 'Tesco value' D cell for the heaters, it more or less worked first time (well after reversing the lead to the tickler coil). Though to get good results the value of the capacitor coupling g1 of the first valve from the tuned circuit turned out need to be rather larger than George was expecting at 470pf.
It is prone to burst into oscillation with too strong an input signal (regardless of the setting of the reaction control) and odd hand capacity effects and the output is... well somewhat low - the loudspeaker is more a 'hold-it-to-your-ear' speaker. However output to a pair of 'walkman' headphones is acceptable - but it's better than letting these interesting little devices gather dust in a drawer somwhere.
The images below show firstly one of the CV2288's compared to one of Jenny's modern day NHS hearing aids, then a circuit diagram and then the project itself.