According to Tony Hwang :
You ought know then vacuum tube version of SCR, the Thyristor!

A thyristor is not a vacuum tube. Thyristors are semiconductors -
"four layers of alternating N and P-type material" according to
Wikipedia. Some sources consider thyristor to be synonymous with SCR,
others have thyristor include GTOs, Triacs, SJTs, SITH and MCTs too.

Didn't have much call to play with vacuum tube switching circuits, power
or otherwise - most of my playing around with tubes was RF, with RF
mid-power amp tubes and 0D3-type regulator tubes being the more exotic
stuff.

That said, we may be getting access to a 60' radio telescope dish
to do a little serious radio astronomy. The dish steering system
is a vacuum tube and electric motor implementation of an H-bridge,
similar to that used to steer things like shipboard radars. The
modern semiconductor version is used in the small scale for hobby
robots.

This has an example circuit for the small scale: http://tinyurl.com/39xw85
Here's an IC version: http://cache.national.com/ds/LM/LMD18200.pdf

The radio telescope has vacuum tubes controlling an AC motor which in
turn generates DC for the actuator DC motors. On the order of at least
5 (and probably 10 or 20HP) apiece... [Times two for the two axis]

[The unit was designed for tracking satellites back in the 60's,
so it was designed for rather faster traverse than is necessary
for radio astronomy.]

We don't believe the vacuum tube portion will still be working
(the dish been locked for ~15-20 years), so we're thinking that
we will have to replace everything but the movement motors with
modern solid-state very high power H-bridge modules. Eg: a hundred
A at 600V or something silly like that.

Also include Diac as well on solid state side.

I've played with those (and mentioned them backthread)
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.