5 Dec
1999
5 Dec
'99
5:12 p.m.
Upon the date 06:07 PM 12/5/99 -0500, William Donzelli said something like:
Here are a few comments about tubes, as there seems to
be some
misinformation floating around here and elsewhere...
Grid Emission is another failure mode less talked about but found more
often than most repair folks realize. Takes a special tube tester or one of
the very few dynamic mutual conductance tube testers marketed which will
test for this (my Jackson 658A is an example). Caused by sputtering of
cathode material onto the control grid as a result of overdriving of the
tube or the heater voltage too high or an operating environment which
allows the envelope temp to rise very high. Exhibited by unexpectedly odd
or just plain poor performance of a circuit even though the tube checks
perfectly okay on any tube tester that doesn't check for grid emission --or
grid current as Jackson names it. Grid current increases spontaneously
driving the tube out of cut-off or even harder if it operates normally out
of cut-off curve and increases cathode current into a high range which
accelerates the sputtering problem.
I feel tubes in the early computers probably suffered from this as some
were driven hard-on under normal circumstances, say, in a flip-flop circuit
where the circuit was held idle in a particular state for long periods.
Design of the circuit was likely conservative but over time grid emission
problems developed. These machines normally ran 24x7. (Actually keeps this
thread leaning more on-topic ;-) Folks who went through and tried to
troublshoot a defective circuit and saw the tubes had checked "excellent"
and didn't simply swap for a new ones would probably send themselves off
on a tangent to find some other problem within the circuit.
Anyway, Mr. Donzelli is, like myself, one of our rather quiet tube
historians in the AWA and knows even more than I do on the subject. His
clarifications above are a few of the ones I was fixing to post. Circuit
designs of the tube-type Tek scopes are usually never such that tubes will
drastically degrade in periods less than tens of years. Pretty conservative
designs they made.
Only in the last years of tube production ('70's - '80's) has quality
been
observed to degrade in the TV and radio type tubes. Far less quantities
where made, because of drastically declining demand in the replacement
market, which meant that testing criteria was lowered slightly to maintain
a reasonable yield vs. cost to produce them.
Now, only a handful of the hundreds of types of tubes are made anymore
-mostly in Russia, Hungary, Yugoslavia and China and are aimed toward the
audio market or high power radio/TV/military transmitters.
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...
Nice to see you again ;-)
William Donzelli
aw288(a)osfn.org
Regards, Chris
-- --
Christian Fandt, Electronic/Electrical Historian
Jamestown, NY USA cfandt(a)netsync.net
Member of Antique Wireless Association
URL: http://www.antiquewireless.org/