Ian Jackson wrote:
I don't get it. Maybe I'm a bit thick, but why would the turn-on surge
higher?

When you turn it on, the filament resistance is very low, BUT even worse,
the tube is in series with another tube whose resistance is ALSO very low.

Watch a series string device like an AA5 radio or a TV set turn on... the
filaments get VERY bright, then they drop down to normal brightness after
a second or so. There are tubes designed to deal with that, but not all
of them are.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."