Depends on the tube. I have vacuum tubes that pass 1000 amps to the
plate. About the size of your for-arm. Mil grade.

Then the real nasty ones for plating and welding - Ignatrons (sp) that
are mercury filled in a pool to splatter 10's of thousands of amps.

The big tubes I have are gas filled and were motor controls. Took two
on each motor and there were 3 motors to spin a very large triangle antenna.

Martin

On 4/3/2017 9:12 PM, DoN. Nichols wrote:
On 2017-04-03, wrote:
On Sun, 2 Apr 2017 19:27:38 -0700, MOP CAP wrote:

Many of such motors with wound fields required 90 volts for the field.
Why 90 volts and how was it derived?
thanks,
CP
Some had field rheostats for speed control. As for the 90 volts,
likely the output voltage of a rectified line voltage using the
rectifier technology and line voltage of the time.

With modern rectifiers (silicon ones) that would work out to
about 64 VRMS. But vacuum tube rectifiers would have a lot more series
resistance, so that might work out with 110 VAC (the old line voltage,
which then jumped to 115 VAC, then 117 VAC, and now 120 VAC.) (And the
frequency used to be specified as Cycles-Per_Second (CPS) or '~' instead
of Hz. :-)

Also -- the rectifier was likely not used with filter
capacitors, so the RMS value of a half-wave rectified 110 VAC might be
pretty close to that.

Enjoy,
DoN.