Re: AC vs. DC drive for VFD cathode filament

CRC wrote:
John Larkin wrote:
On Fri, 19 Jun 2009 09:21:23 -0700, CRC
<crobc@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

Greetings:

I have been collecting single digit vacuum fluorescent display tubes of
Russian vintage to make into clocks.

These have a cathode filament voltage of about 0.9-1.5V.[edit]
The remaining consideration then, for deciding whether to drive the
filaments with AC, is lifetime.

Does electro-migration occur and shorten the filament life if driven by
DC? Is this effect really significant for a filament that is only at a
"dull-orange" heat?

We use VFs on several of our products, all with DC filament drive.
Maybe 10,000 or so by now, some in the field for 15-20 years now,
usually running 24/7. I don't know of any filament failures. DC seems
to work fine.

The filaments run at much lower tempearture than light bulbs, and
light bulbs that run dull red have enormous expected lifetimes.

I think electromigration requires huge current densities.


What then was the dominant failure mode in vacuum tubes? What it
filament failure, deterioration of the rare-earth oxide emissive
coating, degradation of the vacuum, or some other mechanism?


Cathode wearout due to loss of thorium, leading to much reduced cathode emission. Once in awhile you'd find a gassy tube, but that was usually caused by putting it in the socket crooked, and cracking the glass around a pin. Even with mishandling, they usually survived--tube testers used to have pin straighteners as well as cathode emission testers.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
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