3 Jan
2010
3 Jan
'10
10:33 p.m.
On Sun, 3 Jan 2010, Tony Duell wrote:
I certainly wouldn't do it as my first valved project...
Doing it with vavles that are still made is going
to be hard, since the
vlaves currently made are mostly audio types. If you will allow 'valves
that aare still trivial to find in large quatities' it becomes a lot easier.
Doing some more reading, I think I've settled on not doing FM with tubes. I found some material on doing it with discrete
transistors. But I still
want to play with tubes. I found a page on enhanced AA5 designs that,
among other things, get rid of the hot chassis problem. AA5 tubes seem
very cheap and easy to find on ebay.
If it's based on the AA5 design (series-string heaters, HT by
half-wave-rectifying the mains), it'll still have a hot common rail. It
may not be the chassis, but you can't clip your 'scope and sig-gen onto
it.
I would strongly recmend using parallel heaters and a fully isolated HT
supply . I am sure valves like the 6K8 or 6A7 (I think those are the
numbers for frequecny changers), 6K7 (RF or IF amplifer pentode), 6Q7
(detector diodes and audio triode) and 6V6 (output beam tetrode) are not
hard to find. 6.3V transformers much still be available so you can get
the ehater supply. The HT is more of a problem. A 1:1 isolating
trasnformer + halfwave rectifier is the obvious solution I guess.
For the little B7G-based valves (OK, '7 pin minatures') used in battery
portables over here, I had great success gettign the 90V HT from a 30V
trasnformer (easy to get -- a 15-9-15 one, ignore the centre tap) and a
voltage doubler circuit. It gives about 84V
-tony