[Arny Krueger]

"Karl Uppiano" wrote in message
news:Ir85h.53$xD.38@trndny08...

In my first engineering job, I had to maintain lots of tube amps as well
as solid state amps, and I can guarantee that they *all* had feedback.
Degenerative cathode feedback, overall feedback, RIAA equalization using
feedback, you name it.

Agreed.

In the Army I maintained 3 Hawk CW radars that were pretty much entirely
tubed*. They had audio-like circuits for handling the Doppler and the
modulation of the transmitter, and they had DC-coupled circuits for antenna
servos, and numerous other things. The largest had about 400 tubes.

As you say, these tubed circuits were full of every kind of local and loop
feedback one could imagine. We had one feedback loop that essentially
traversed the entire radar from transmitter to receiver with numerous
subloops that stabilized segments of the overall loop. IOW this feedback
loop passed through about 350 tubes. The other 50 or so tubes were in
regulated power supplies and other service circuity. MTBF was about 1 day.

* there were some solid state tubed replacements that were packaged as tubes
and were socketed. Most were in the regualted power supplies. They were 300B
replacements. There was also some SS-based built-in test equipment.
Ironically the SS built-in test equipment was a bit marginal in places and
would not work when it was really cold, because the beta of some critical
transitors would fall off too much.