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?
Thanks,
Ross NS7F

I'm an almost tube newbie, but here's what I know of this, in order of
importance:

* the tubes have to draw the same current, else one tube will get too
hot and the other will run cold.

* some tubes take longer to warm up than others. When they started
putting color TV's together with long strings of filaments that added up
to 110V, they also instituted "controlled warm-up" filaments so they'd
all warm up in about the same amount of time. I don't know if this is
such a big deal at 12V.

* If you're doing really low-level audio (or a VFO for that matter), the
filament can "talk" to the cathode a little bit. Running your 6V tubes
from a winding with a center tap will minimize this. Running the
filament slightly positive (I think) to the cathode will suppress issues
of the filament emitting electrons that bias the cathode. Building your
circuits so the cathode goes to a hard ground will significantly
minimize these effects, at the cost of quite a bit of circuit
convenience, although you see it done.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html