Re: AC vs. DC drive for VFD cathode filament

On Fri, 19 Jun 2009 20:32:14 -0700, CRC
<crobc@xxxxxxxxxxxxxxxxxxxxxxxxxxx> 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?


Misapplication by design, misuse in service and incorrect failure
diagnosis in maintenance. Same as with most products. A tube or tube
vendor that didn't perform as expected had a very short sales history.

For formal data collected up until the late fifties, the dominant
factor in determining the actual life of miniature tubes was envelope
temperature, which affected the rate of glass electrolysis (ref01).

The electrolytic relation is exponential - doubling for every 20°C
rise (ref02). Reducing a typical envelope temperature rise by 30%
could increase lifetime by nearly an order of magnitude......and vice
versa. Operation below 100C, however, introduces inter-element leakage
phenomena (ref01).

The second most dominant factor was heater-cathode voltage stress.
Less than +50V was recommended.

ref:

01) M.W.Edwards, D.E.Lammers, J.A.Zoellner,
?Subminiature Electron Tube Life Factors?,
Engineering Publishers,
Elizabeth, New Jersey, 1961. p156 .

02) Reimann, Arnold L.,
?Vacuum Technique?,
Chapman & Hall Ltd. London, 1952.p243-4

RL
.

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