thanks for your help. I found the documentation the previous owner
did, and he used half of a 6sn7 in replacement of the 6j5 bfo tube and
labeled it an "inf. imp. detector. what would be the advantage of
doing this?
Also, Johns sx-28 restoration (and others) are the most incredible
restorations i have ever seen documented. Thanks for the links!
I've got a working sx-28 on loan to help in comparing/restoring mine.

wrote:
thanks for your help. I found the documentation the previous owner
did, and he used half of a 6sn7 in replacement of the 6j5 bfo tube and
labeled it an "inf. imp. detector. what would be the advantage of
doing this?

The original detector in the SX-28 was what is known as a "plate-detector".
They have an advantage in short-wave radios in that they provide a lower
load to the final IF transformer, and only minimally affect the Q, and
hence the bandwidth. The stage stays sharper, improving adjacent signal
rejection.

Plate detectors are nasty things because they introduce high distortion
both on low amplitude signals, and on high modulation signals. They are
suitable for shortwave voice quality signals, but terrible for music.

The so-called "infinite-impedance" detector, also known as a "reflex-detector"
attempts to correct the problems of the plate-detector by adding some gain,
and negative feedback. The added gain even further reduces the loading of
the detector on the final IF stage, and the negative feedback improves the
linearity of the rectification function that does the demodulation of the
AM signal. In other words, it improves the distortion on both the low and
high volume signals.

The original owner must have been offended by the poor audio quality of
the SX-28, and wanted to listen to his short-wave music broadcasts in
high-fidelity.

-Chuck

Amazing! Thanks for the information. I am going to restore the section
back to its original condition.
Ben

I have managed to keep the infinite impedance detector, and make it
switchable between it and the plate detector.
The 6sn7 was configured to use 1 section for the bfo and the other
section for the new detector, which plucks the signal from the if
transformer. After that odyssey and replacing some wrong-value
resistors, also found the oscillator fixed pads were monkeyed with,
and the band six pad had a broken solder joint. Looking forward to the
next re-alignment, also with all NOS tubes. Have made a lot of
progress, thanks for your help.
Ben