Jim,
From what I remember of AckAArgh88 they can be quite stable but require
that all c's are in good condxn and volts are stable but most of all the
ventilation must be "good". This means using an original enclosure because they
were (for their day) quite properly engineered to expect certain vent conditions
in quite a range of environments. As you have surmised, this probably means that
for all practical purposes (eg aural cw) this is adequate. However, I have
recently been playing around with "huff & puff" stabilisation of oscillators
including local osc in valve rx's and am beginning to conclude that if the
oscillator has good basic stability qualities there is life yet in quite a few
old dogs of receivers - with a little help from modern devices strategically
placed! When I get the opportunity to try out valve rx's in the kind of strong
signal environments which exist for instance on 137Khz, I am always impressed by
their ability to carry out "front-end" signal processing but, as you have
discovered, subsequent signal handling and processing is let down by selectivity
problems etc. I am still a strong believer in tuneable, selective preselector
stages in advance of rx mixers whether they are or are not upconverting to
higher IF or lower. This facility is present in many good traditional valve rx
front ends and can be added to with external bandpass filters and antenna
matching arrangements. Once you get down to a suitable IF there are then many
possibities for further processing which I do not need to tell you about.
A suggestion, therefore, is to either provide an enhanced stability regime
to the existing LO or to substitute an external source and to then remove the IF
as you have already done! Fortunately, rx's like the AR88 leave plenty of room
(by modern standards) to introduce additional circuitry. My guess is that by
utilising the virtues of a selective front end together with the strong signal
handling qualities of the AR88 front end (even though it is WW2 technology there
are folks who believe it was an answer to EMP) you would end up with a very
acceptable LF rx'ing device.
I still cannot understand why there is this "modern" fetish to try to
contain receiver tuning even in "narrow" amateur bands to one-knob operation.
So-called broad-branded "octave" filtered front ends seem to be the norm on so
many rx's but there is more than ample evidence that the switching arrangements
alone can give rise to major IMD components. As Alan 3nyk will confirm, I always
operate rx's through preselective matching devices which are absolutely no
trouble to "twiddle" as I tune across a band of interest and this includes my
trusty AOR7030 which, good as it is, improves with such help. Maybe it is all a
product of this now "easy" way into getting a licence which in my day meant
serving an apprenticeship as a SWL amongst other things. This entailed "tuning"
rx's with more than one knob! It didn't seem to deter me from either pleasure or
achivement with my listening!
Anyway let's put the hobby horse back in the stable
73 Pat G4gvw
