[HBR] 1929-Style "QST SUPER 12" Receiver: UPDATE

[Diane Swynar]

Well, to anyone complating the construction of a receiver like this
utilizing ancient tubes---or ANY tubes, probably, for that matter---do not
always suspect the worst when trouble-shooting...!

Two cases in point: the CW & SSB signals on my newly-minted retro-receiver
sounded harsh, rough, & just plain AWFUL...I re-built the BFO with a new IF
transformer---same thing. I was considering yet ANOTHER re-build of the
BFO, when it dawned on me that the signals sounded an awful lot like those
CW/SSB sigs through my old Hallicrafters S-77A on 10-meters.

Well, the local oscillator on the S-77A is just terrible above anything
much higher that 14-MHz, or so---sure enough, I monitored my BFO signal on
a nearby Yaesu FT-980: sounded clean...perfect! Next, I monitored the local
oscillator frequency on the '980: shades of the old S-77A!

Now, what to do..? I added RF chokes to the plate lead of the 224 balloon
oscillator tube: no difference. Next, I make-shifted a shield for the same
tube: nil. Finally, I swapped in a newer shoulder-type envelope 224A and---
PRESTO!---the oscillator was as stable as the BFO.

Next problem: a very high, scratchy, ambient noise level coming out of
the 'phones. I started to pull out tubes from the audio driver on up, to
localize it. PRESTO! ANOTHER 224A balloon tube in the detector was the
culprit! I replaced it with another ST 224A---PROBLEM GONE.

The moral of the story is simply this: even if your tubes pass muster on a
dynamic tube tester, it is NOT an accurate indication of how they'll
perform "...in the field". Both of these old 224A's were NOISEY in their
specific applications, for whatever reason(s)---and yet both had
transconductance levels just in excess of 800 umhos.

I don't particularly like using the more modern "ST" tubes in a 1929
design, but then again, I *DO* want to use the dog gone thing! Guess I'll
just kepp my eyes peeled for MORE globe/balloon 224A's in the future, & use
the process of elimination as to which will be "keepers"...

So far it's been a real blast with this behemoth on the bench...before
setting it up alonside my 3x227 MOPA, though, I want to run experiments
with regenerative feedback on both IF stages---looks EXTREMELY promising!

Stay tuned...

~73!~ Eddy VE3CUI - VE3XZ (that's "c3CUI" & "c3XZ" to anyone REALLY old
enough to remember! Hi Hi)