"Jerry Stuckle" wrote in message
...
On 2/18/2014 4:29 PM, Percy Picacity wrote:
In article , Brian Reay wrote:
On 18/02/14 20:31, Jerry Stuckle wrote:
There was a time, back inthe 1920s and 1930s, that any active device
(valves in them thar days, tubes for the leftpondians) would cost
nearly
a week's wages for the average working man, and so it was good
economical
sense to try and use it as many ways as possible simultaneously.
Times have changes, and active devices with performance into the
tens
of MegaHertz are now ten-a-penny, so what is achieved by
competitions
such as the "Two Transistor Challenge" where it is the costs of
switching (manual, relays) which would be the major outlay?
Not carping, just curious.

There have always been "contests" like that, though sometimes they
were
about "build a whole receiver using the same transistor type", or
"build a receiver without any ICs" after ICs had come around.

What I find intriguing is the realisation that valves ("tubes" to
you?) can be operated with only 12V on the anode.

No unusual at all.

Not only were a number of valves for the car radio (and possibly other)
markets available, I recall designs which use 'ordinary' valves with
6.3V AC heaters and a voltage doubler and rectifier to provide the "HT".
I recall a one valve design in Radio Constructor, which I build. It was
the 'cover article'. The design called for an Eddystone Box, far too
expensive, so I used a tin box from some short bread.

I remember back in the 70's we had radar sets with ZERO volts on the
magnetron's anode (DC ground). Of course, there was -3KV or so on the
cathode

I haven't done anything with radar in well over 30 years, so I don't
know if they still do it or not. But I wouldn't be surprised. At
first
glance it sounds crazy - but it made for a very simple and efficient
way
to couple the output of the magnetron to the waveguide.

No reason why it shouldn't be done. I can imagine some of the more
"technically challenged" struggling with it but there are always those
who insist that all who have been designing kit for years have got it
wrong.

Indeed it is rather similar to what was done with TV and monitor CRTs,
to interface the screen with people's living rooms at an appropriate
potential!

Hmmm, they must have done it differently over there. I've been bitten a
number of times by not being careful around the anode lead on a CRT. The
cathode is nearer ground potential because it's exposed on the back of the
tube.

You'd think being bitten by 25KV the first time would teach me a lesson.
But some people never learn

I was a 1960s TV engineer in the UK. 15kV or so on the CRT anode was normal.
The viewer was protected by a thick layer of glass that also formed an
implosion guard. Colour CRTs had around 25kV on the anode, and in early
models with anode currents of 1mA or so the line output stage was lead
shielded to reduce X-radiation. Happy days.
--
;-)
..
73 de Frank Turner-Smith G3VKI - mine's a pint.
..
http://turner-smith.co.uk