A wrote:


If we extend the universe of tubes observed to the TV and radio types, I
would have to say that it is very rare for me to find a burned out
filament
in the 6.3 and 12V tubes. And very common in the very delicate 35-75V
tubes
used in series string sets.

-Chuck


Well, my comparable experience is the ac/dc AM/FM radios (12 v and 35 v
tubes) and in another post I mentioned buying up a number of them at a
couple of hamfests (and a thrift store) and finding that they all still
worked AND had the original tubes (marked with same brand as brand of
radio) in them.

It can happen, but probably your good luck was the result of the radio
not seeing much use. Does it show signs of obvious use?

The 35 - 117V tubes have a large hank of zig-zagged very thin wire
as their heater. The wires come to a sharp V which isn't a good
idea. I have almost never found a 50V tube that was good.

But, the series string strategy was obviously the "cheap" way for
manufacturers to save on the price of a power transformer AND any
irregularities in the filament mechanical tollerance contributed to
early failure, which, of course, contributed to pressure on the consumer
to buy a whole new TV or radio, thus contributing to the nation's
economy (if you get my drift).

The AA5 tube lineup was definitely an economizing move that could only
happen in the US. At one point, GE put everything in two tubes: one compactron,
and one rectifier/audio output tube...if I recall correctly.

-Chuck