[Boatanchors] Audio Quality Comparison

[Rob Atkinson]

My experience with vintage rx audio is limited, but I have looked at
schematics for a few---the S20R, NC300 and 75A-3.  S20R has some sort
of feedback setup to the control grid of the 6F6.  I'm working from
memory but I think the output transformer secondary feeds back.  6F6
is the audio output tube, single ended.  the other two don't have any
similar feedback at all, at least the NC300 certainly doesn't and the
stock 75A-3 is definitely not high fidelity.   I feed the 75A-3
detector audio out to an external push pull amp and to me it sounds
pretty good.  for the CR88, I got an old hamfest Jensen 12 inch full
range speaker, 8 ohms, and put it in an Electrovoice Aristocrat
mahogany cabinet.  this was paralleled with another 12 inch 8 ohm full
range speaker that for some reason seemed to have more high frequency
sound to it.  The two make a nice combination, but I am still blown
away by the audio quality coming from that single 6K6 output stage in
the CR88.  Everyone who has heard it, and these are hams who have been
around plenty of vintage receivers with p.p. outputs, can't believe it
is a single ended 2.5 watt stage.  the red Radiotron Designer's
Handbook has a lot in there about feedback pertaining to receiver
output and they seem to practice what they preach.

73

Rob
K5UJ