In which case my reference to AN211A may not have been so helpful.
Replacement in classic RF/IF/AF stages where exact gain isn't picky and
the class-A bias is mostly self-adjusting (the examples in AN211A) is
the easy thing.
IN the old days, the replacements seemed to take three forms.
Creating general replacements that plugged in without any fuss.
The Fetrons that someone mentioned were this type, but of course
only a handful of types were made, to replace a handful of extremely
common tubes. And I recall a detailed letter in QST about them, how
they worked well in some applications (I seem to recall limiters
in two-way FM receivers) but less well in more linear applications.
The exact details may be garbled, but there did seem to be limitations
of even such a general replacement.
More common were trying to build to the specific stage. So you look
at the circuitry around it, and then try to create somethng that fit
in that socket that worked with the surrounding circuitry. This required
more fussing, but seemed to be a better choice. I seem to recall that
the articles I saw often did adjust external resistors to some extent.
I once solid-stated a surplus Collins PTO by soldering an FET in place
across the tube socket, and running it off low voltage, and it was fine
(though it was a 1 to 1.5MHz PTO so was pretty simple). I don't think
I had to change anything else, but note I wasn't using the PTO in
the original piece of equipment.
Then there were the conversions where the chassis and tuned circuits
and variable capacitors (or whatever) were basically just the foundation
of a new project. It got the hard work out of the way, but of course
didn't leave it intact. On the other hand, it was a way of improving
equipment that had limitations. In that sort of conversion, the audio
amplifier would be tossed out, and replaced with an audio IC amplifier.
THe IF strip might be seen more like a function, and then something suitable
could replace it. The thing about this was you could tamper with external
components, so if that plate resistor was too high for whatever you were
doing, you'd just replace it.
But of course, most of this took place in the late sixties and by the
end of the seventies it had faded. At the time, the equipment was
still seen as functional, but the hope of solid state made people want
to modify them, or maybe it was just for the sake of doing it. The
equipment was often getting pretty old, as the seventies progressed
it was all getting replaced with equipment that was not only solid
state (even using those new-fangled ICs) but that switch to solid
state allowed for a leap forward in functionality. A tube shortwave
receiver with single conversion to 455KHz would often still suffer
from image problems, whether the tubes were left in there or replaced
with transistors, but a more modern receiver might have moved to
a single conversion to an HF IF, or tossed in lots of features that
would require extensive work to add to the old receiver.
Michael
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