On Jun 13, 4:49 pm, Ian Jackson
wrote:
In message , Scott Dorsey
writes

Ross, NS7F wrote:
In a circuit with two 6.3V tubes, can I just series the filaments so
that I can use a 12-13.8V supply? Or are there subtleties that I'm
missing?

You can, but the tube filaments won't last as long because of the turn-on
surge. If it's a circuit where the tubes fail with poor emission long before
the filament fails, that's fine.
--scott

I don't get it. Maybe I'm a bit thick, but why would the turn-on surge
higher? Provided the filament current is the same for both tubes, I
wouldn't have thought that it would have made any difference. In the
good old days, I'm sure that I have connected up 12AX7/12AU7/12AT7
heaters (12.6V centre-tap) to run off 12.6V (pinout =
anode-grid-cathode-heater-heater-anode-grid cathode-heater CT). Would
the switch-on surge be less if they were run connected for 6.3V?

If the tube filaments that are in series are perfectly matched, then
they both see the same turn-on surges. The two halfs of a 12AU7 are by
definition pretty well matched.

But if they aren't perfectly matched, then one heats up a little
faster than the other, there is an unequal stress applied to the tube
filaments causing extra abuse to the guy who heats up first. The guy
who heats up faster will have more resistance than the others, but the
current through all the filaments is by definition the same, and the
hot one will be dissipating more power than the other cooler guys.

If there is a filament transformer, the impedance of the transformer
will lessen the peak current at turn-on. So will other devices that
serve as inrush limiters.

In real life, while filament failures do occur, they are not the most
common cause of a tube's end-of-life. Just in my experience, low
emission, gassy tubes, and interelement shorts are all far more common
failures of the tube. Not to mention the #1 cause, a leaky coupling
capacitor causing a tube to overheat and fail.

Tim.