Out of interest, has anyone else had experience of a valve (tube) op amp module that used 5 sub miniature valves (pentodes I think) that had a covering of silvered paint? They were directly heated and made by Hivac. I came across them in the late 60's used in experimental missile systems.I remember that they apparently cost a small fortune and were extremely robust for the technology, we had to put them through severe vibration, temperature and humidity cycles before they were accepted for use.

 

You can look here for starters:

http://g3ynh.info/valves/old/Data-Books/hivac/index.html

 

You can look here for starters:

http://g3ynh.info/valves/old/Data-Books/hivac/index.html

I love the "Pocket Radio" ha ha.
The Valves I saw were little wire ended ones a little larger than hearing aid Valves.

 

Acorn valves? http://www.vintageradio.me.uk/radconnav/acorn.htm

 

Acorn valves? http://www.vintageradio.me.uk/radconnav/acorn.htm

Much smaller, just over 11/2 inch long an 3/8" x 1/4"