On 03/01/2010 14:47, David Looser wrote:

Big power valves with directly heated filaments have heavy filaments and
were intended to be a.c. heated. If you run them on d.c. then the
characteristics change as one end of the filament is more negative (w.r.t.
the grid) than the other, and so carries most of the current. With a.c. the
most negative point changes over the mains cycle from one end to the other
distributing the current flow much more evenly over the filament. At the
sort of signal level these valves run at induced hum from the filament
wiring isn't a problem.

Of course d-h valves intended for battery operation *must* have d.c.
filament supplies.

David.

Thanks, David. If I was to pursue such a design so, I'd be well advised
to only use it for stages with low input signal.

--
All the best for 2010 de Walter.