Ian Jackson wrote:
In message , Scott Dorsey
writes
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

Nope. I still don't see the difference between a single 6.3V filament
fed from a 6.3V source, and 2 x 6.3 V filaments in series fed from a
12.6V source.

What you say is true. When you turn it on, the (cold) filament
resistance will be low, and there will be a current surge. What you see
is a heart-stopping bright-up of the filament, before it dims to normal
brightness.

However, you could argue that, with a single filament, it is in series
with nothing, other than the resistance of the transformer. As this will
probably be lower than that of a 12.6V transformer, the surge could be
higher. But this doesn't really hold water either, as you only have 6.3V
driving it.

Ian.

The only case where series is a problem is if one of the filaments heats
up more quickly than the rest. Because filament resistance is very
nonlinear, and changes about 30-to-1 in value from cold to hot, the
filament that heats up first will be drastically overloaded for a brief
period.

Most modern (if I can use that term) filaments are of the controlled heating
variety and should be ok in a series configuration.

-Chuck