RadioBanter - View Single Post - sx-28 restoration ongoing

Chuck Harris · June 10th 08, 11:16 AM posted to rec.radio.amateur.boatanchors

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