Simple micro transmitter

[Terry]

Explains why they weren't much used in hearing aids, though ... 

My Grandpa had a valve based hearing aid in the fifties and sixties Terry ...

I think you've misinterpreted my comment, Andy, which was in response to Mike Watterson in this post where he said "I don't think very many valves were specifically developed for hearing aids, it was simply a pubic market for them."

I well remember those hearing aids, Andy, using wire ended Mullard valves, I think - I but I doubt your Grandpa wore one in his pubic area, though