[AMPS] AL1500

[Peter Chadwick]

Tom says:

>you can expect it to last dozens of years.

So you can with good tubes. There are many evidences of tubes such as klystrons
in TV and radar service which have still met full spec after 120,000 operating
hours. My 6L6 regulator tube still meets full spec on gm and gas, and is older
than Tom or I.

One problem, and this applies to all components, is that when you go down the
road of 'cost reduction', it's very rare that something doesn't have to give. So
tube quality now is likely to be worse than 50 years ago, especially as the
market is much smaller and we expect to pay much less in real terms for
equipment.  For example, the HRO as the top line receiver cost $279 in 1936
without tubes, power supply or speaker. How many $k is that in today's money? 

An example of cost reduction is the plastic IC. Plastic IC's are fine - if
you're careful of the environmental conditions. Large temperature ranges and
humidity are killers for long term reliability. Such doesn't affect 3-500s,
though! I did once have an interesting application problem where a guaranteed
life of 30 minutes was needed at a chip temperature of 225 deg C...that was in
the drill bit of an oil well drill.

One area that I am convinced is responsible for a lot of unreliability is the
soldered joint. Interestingly enough, I'm currently engaged on an argument with
a big military manufacturer on this - if he accepts the argument, it's his
problem, so he wants to blame an integrated circuit which he's taken off the
board for testing and then put back, with the result that the equipment is
working again.  WW2 solder had a lower tin content than pre war, because of
essential material supply, and I read that even with the construction techniques
of those days, that had an effect on reliability. Of course, when you're
building a piece of equipment that is likely to be shot down within 100 hours of
operation, long term reliability isn't the aim. Similarly with glass tubes for
hams - how many 'operating' hours does a PA tube get? Even in a contest, I doubt
the tube is actually operating for 30 hours - you have to receive part of the
time, at least!

The major overload that many tubes won't stand is being dropped! OTOH, vibration
on IC packages can have curious effects on bond wires in non-plastic packages. I
wonder what 'g' forces a 6AG7 with its metal package can withstand? 

There was an interesting article in the IEEE Spectrum a few years ago about
tubes, and how the reliability wasn't that bad once you accepted the shorter end
of life. There had been this outcry about tubes being the cause of the FAA's
computer failures, when in fact, the major problem was very old connectors on
very old equipment, but it was politically advantageous for the Vice president,
no less, to hold up a miniature tube and tell the public that the FAA were still
using them.

The advent of microelectronics has led to major increases in complexity, and
hence made increased reliability a sine qua non. Nevertheless, ICs do fail, and
not always because of overloads. With the increasing move to custom ASICs, I
predict that a number of todays expensive transceivers will become 
non-maintainable items when the manufacturer's stock of ASICs runs out. The
chances of keeping a forty or fifty year old piece of equipment working will go
down enormously, especially if the ASIC happens to have a long term reliability
hazard built in - and sometimes, it does happen with an IC that that is the case
- and it shows up years later.

Finally, I don't think it's accurate to say that parasitics can't or won't kill
tubes. There's differing opinions on how severe and how long they have to exist
for to do it, and I'm much more in your camp on this one.


73


Peter G3RZP


--
FAQ on WWW:               http://www.contesting.com/FAQ/amps
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com