AC vs. DC drive for VFD cathode filament

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.

John

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?

P.S. I saw a lot of your stuff is being used over at NIF. Nice work.

All of the above, although open filaments were maybe the least of the problems. Low cathode emission was maybe the worst.

Thanks. They were (and still are) great people to work with.

John

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

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:

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

RL

Yes. ;-)

Cheers! Rich