Michael Black wrote:

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

That's not new. There were some articles in the various hobby electronic
magazines in the fifties and sixties about running them at low voltage, "starved circuits".

Of course, that was about the time when new tubes came along that could
be run off low plate voltage, a sort of last gasp before transistors took
over. You'd see such tubes in car radios in a very specific time span,
tubes for the RF stages, maybe in the IF but those might have been
transistors, and then the audio stage. That period when tubes weren't
yet really good at radio frequencies.

Or the COllins R392, that used 28v plate voltage.

The best "starved circuit" was an article by John W. Campbell (of science
fiction fame) in CQ in the late fifties. It was about running a CRT at
relatively low voltage in an oscilliscope. It drove up sensitivity so
you could do away with amplification for many purposes (and thus the
scope was broadband) but you lost deflection and I think brightness. The
scheme came out of Bell Labs, he mentioned some specially built CRTs for
the purpose that included magnifiers.


Indeed Michael.

Anyone active on the home brew scene must have seen projects using valves
with low HT supplies, they were quite common and popular in the UK. I
expect someone with access to back issues of Radio Constructor could turn
up a couple per volume. RC was a popular, if slightly old style, magazine
which seemed to include more valve projects than the other magazines of the
day. Sadly, it is no more.

I think some valves were developed for use in 'Walkie Talkies' which were
baseless (ie were wire into circuit via flying leads not plugged into
sockets) and were designed to operate with a low HT.

There were also valves for hearing aids which were, I suspect, similar.