Post by BFoelschPost by John GilmerPost by Floyd L. DavidsonOtherwise there was rarely a better test for a vacuum tube than
to plug it into the circuit in question, and see if it worked.
If it didn't, it might (or might not) work in another circuit.
That seemed to be the folklore.
That was true enough. Due to availability issues, some tubes had way more
performance than the circuit needed and would work forever. Other designs
needed fresh tubes to work properly.
It wasn't so much "way more performance" as it was conservative
circuit design engineering. Using two stages, rather than one,
to get any given amount of gain and using feedback to stabilize
the entire circuit are examples. With good design most circuits
can operate without such great sensitivity the tube
characteristics. But that wasn't true of all circuits, and of
course it cost money to implement, so it didn't always happen.
Post by BFoelschBest example I ever saw was the OLD Western Electric Theatre amplifer, I
think it was the type 42. Used the 205 triode as the power amplifier and as
the rectifiers, with grid and plate strapped. WE told you to run the 205s in
the amplifier position until they were unsatisfactory, then put the spent
tubes in the rectifier sockets. Whn the no longer worked as rectifiers they
were COMPLETELY shot!
A great example. WECO was famous for such designs. Working on
the telephone carrier system they designed back in the 30's was
still great fun even into the 1970's because of the excellent
engineering.
Post by BFoelschPost by John GilmerPost by Floyd L. DavidsonThat was true of brand new tubes straight out of the box just as
much as for tubes in operating equipment.
I never experienced this, at least not in entertainment equipment. Some test
equipment used selected tubes, but by and large BRAND NAME new tubes were
pretty much interchangeable.
Test equipment was one area where they weren't interchangeable. RF use
was another. TV sets for example! And other uses for tubes meant for
TV sets, such as the many Ham Radio transmitters that were designed using
TV horizontal sweep output tubes. Many of those would work best with
one or two brands of tubes, and some would not work *at all* with some
brands.
Post by BFoelschThere was a big scandal in the late 1950's,
however, where businesses were buying up used tubes, washing them,
relabelling them and selling them as new. That may be the origin of that
story.
Nope. Experience in the 60's and 70's with tubes used in commercial
radio equipment.
Post by BFoelschPost by John GilmerWell, that partly explains how the Japs god ahead of us in consumer
electronics. With reasonable quality control one should be able to mix and
match.
As stated above, I never observed any real QC issues with name brand
product.
It doesn't have to do with QC, and Japanese made tubes were no
different. The problem is that a given tube type had a
relatively small set of target characteristics which defined it,
as compared to a much larger set of characteristics that
affected its actual operation. The same production line, never
mind two different production lines, generates significant
variations in the second larger set. Transistors are even
worse!
But manufacturer's did learn, and with transistors they came up
with the solution. Rather than hundreds or thousands of tube
types, there are tens of thousands of transistor types! The
same production line would be manufacturing several different
devices, and the difference was determined by testing them.
That continued to be done with IC's too, though to a lesser
degree. For example the 80386sx, the cpu without the math
co-processor, came off the same production as the version with a
working co-processor... :-) (Hmmm... I wonder if the external
math-coprocessor was just a cpu with a malfunctioning cpu and a
working coprocessor???) And usually there have been a least a
couple of different variations of clock rates for each cpu,
again all off the same production line.
(None of which detracts from the actual fact that the Japanese
had *much better* quality control for such manufacturing
processes, and that did indeed give them an advantage.)
Post by BFoelschPost by John GilmerPost by Floyd L. DavidsonWhich is to say, rather than a tube tester, the usual practice
was to have a large caddy of tubes for use as "swapping spares".
Aside from everything else, it's a lot quicker to just exchange tubes than
plug in a tester and test the tube.
Mostly because it is a definitive test, while a tube tester is a wild
guess in most cases.
Post by BFoelschPost by John GilmerIf quality control was really that bad switching tubes could lead to nothing
at all working.
You had to learn *how* to do it! People would randomly swap tubes
and lose track of which ones started where. Bad! Swap a tube, and
if it doesn't change, _put_ _the_ _old_ _one_ _back_.
That's hard to do perfectly every time though, and leads to
another problem, for which there is a definite solution.
Between swapping tubes and ending up with a mixture of used and
new, the *bad* ones get mixed into the pile! Our initial
solution was to toss the whole bunch the instant it was realized
that any one of them could be bad. That wasted a dozen or so good
tubes to avoid a bad one.
But a better idea came along... a bad tube should immediately
be "marked". Yeah! Just bend all the pins flat. That was so
easy to do that everyone in the crew I worked with found it an
easy habit to form. (We shared rolling test equipment bays and
tube stocks. Doing a full blown routine on a radio set might go
through a couple hundred tubes each for one or two types, and
another 50 for all others. So we're be grabbing spare tubes by
the handful at a time.)
--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) ***@barrow.com