[ARC5] Way OT -- SX-28

[Richard Knoppow]

     The lowered filament voltage was common on both diode 
detectors and especially diodes used as series noise clippers. 
The idea was to reduce hum from the electron cloud of the cathode 
(I am drawing blanks today, there is a proper name for this 
effect). I have never been able to find the origin of this change 
but it was adopted by most receiver manufacturers beginning about 
the late 1940s. Some manufacturers issued modification procedures 
to add a dropping resistor to the noise limiter.
    I wrote before about the noise _blanker_ in the SX-28. This 
was an invention of James Lamb, who also thought up the first 
practical crystal filter. The blanker was a very advanced design 
feature. Unfortunately, the blanker in the SX-28 was applied to a 
point where the bandwidth was somewhat limited so it didn't chop 
the noise as sharply as it should have. I think also a lot of 
users did not understand what it was supposed to do so, at some 
point Hallicrafters issued a modification to eliminate it and 
replace it with a conventional series limiter. It was some years 
before another attempt to use the blanker was made. I am not sure 
but think Collins may have been the next with a blanker for the 
KWM-2 and 75A-4. These also had some problems, partly from pilot 
error, but are still around. A plain blanker will NOT take the 
typical grinding noise from power lines off. What it does well is 
to limit impulse noise, such as ignition noise. When the blanker 
is used ahead of the AVC sensing point it prevents the high 
amplitude impulse noise from driving the AVC and desensitizing 
the receiver. This much more evident when used with slow-release 
AVC such as is used in SSB receivers, since they will hold the 
noise peak and drop the receiver sensitivity. Fast release AVC 
tends to recover so the effect of the noise is not so noticeable 
(but is still there).
    Boy, I seem to be wordy today.

On 8/6/2021 12:51 PM, Hubert Miller wrote:
> Doesn't the SX-28 diode detector have a lowered voltage on its heater?
> I am thinking of the right set?
> If so - someone please remind me of why ( some ) radios had the diode
> detector tube use a lower than rated heater voltage.
> -Hue Miller
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL