Tube experts! - I T WORKS!

From: William Donzelli <aw288_at_osfn.org>
Date: Sun Dec 5 17:07:37 1999

Here are a few comments about tubes, as there seems to be some
misinformation floating around here and elsewhere...

> In TVs where the tubes are low end and not always run at their best
> operating points that may be true. In Qualtity test equipment that is
> rarely the case.

This is a rather common misunderstanding. In truth, the tubes from the
1960s - the ones that most people call junk - are amongst the best. The
quality of manufacturing (improved outgassing and getter/keepers) was far
better than those made in the 1940s, bulbs shrunk down to submini sizes
(Nuvistors, T3/4 bulbs, and the ceramic tubes), and the electrical
performance went thru the roof (transconductance). Many tubes started to
be made in Japan and elsewhere, and are very good performers, but with a
very bad reputation.

Color TV service is very demanding on tubes (basically the only market
left in 1960), so the tubes were made tough. One must remember that some
of them, namely in the sweep circuits, were essentially transmitting tubes
with new bulbs and receiving numbers.

> Mechanical shock and power cycling tends to ruin tubes faster. Generally
> tubes have good lifetimes and can be considered reliable (excluding the
> effects of heat on surrounding parts) .

While mechanical shock certainly kills tubes, power cycling generally
does not. The filaments do not instantly get heated like a household
lightbulb - it is far more controlled and the filaments have time (and
room) to expand. In some of the older tubes, this was done with springs,
and on modern minis and Compactrons, the filament is allowed to slide
inside the cylinder of the cathode. Even some of the tubes rated for
mobile service with quick heating filaments to not really care much about
power cycles.

> That is true, usually they get filiment failures and those are easy to
> spot. Power cycles tend to accelerate that. then again I had a tube
> organ (some 80+ tubes) and only had one failure in 8 years.

Most tube failures are due to loss of emmission, rather than a burnout.
The most common failure is that the filament loses the outer coating
(generally thorium based), and emission drops to half, maybe zero. There
are tricks to rejuvenating filaments, but it is mostly magic and luck -
trying to get trapped thoria to diffuse to the surface.
Soft vacuums (gas in the tube) also is a common failure mode, as the
glass-metal seals just are not perfect things.

Going back into hiding...

William Donzelli
aw288_at_osfn.org
Received on Sun Dec 05 1999 - 17:07:37 GMT