[ARC5] Some info on the S-38

[Leslie Smith]

Hello Ken,
I read the sensitivity figures you give with great interest.  We have
some figures for sensitivity of the command sets.
    Set                   Freq                 Sensitivity
    S-38                 5000kHz           12uV        for 50mW from
    spkr (supplied by KG)
   R-23                   550kHz             5uV         for 10mW into
   300 ohms.  1.7 volts  Source: table 6-3 in AN 16-30ARC5-2
   R-26                 6000                   6uV               ..
   R27                  9100                   6uV               ..
   Command set     ??                     1uV        (figure given by K.
   Gordon (that's you, Ken) in YT video, (I think))

The S-38 has a comparable sensitivity to the "command" sets - 20 x
log(12/6) = 6dB down, 1 S point down.
I'm more surprised by the given selectivity figure.  60dB amounts to
one-millionth reduction in signal.
So - interpreting your figure of 65kHz "wide" (or 30kHz from the I.F.
"nose") means a 1 volt signal 35kHz removed from "the" station of
interest will have the same output as a 1uV signal tuned to the center
of the 455 kHz I.F. strip.  This into strengthens my intent to add a "Q"
multiplier to the circuit.  Guessing a little - I figure a 1000mV signal
25 or 30kHz will obliterate the signal of interest.  This is consistent
with the experience mentioned by others here.  
   

73 de Les Smith
  vk2bcu at operamail.com


On Tue, Nov 17, 2015, at 16:07, Kenneth G. Gordon wrote:
> According to a service bulletin on the first model S-38 I just found, the 
> sensitivity was:
> 
> 12 microvolts at 600 kc
> 12 microvolts at 5 mc
> 11 microvolts at 14 mc
> 23 microvolts at 30 mc
> 
> for 50 milliwatts output.
> 
> Also, the BFO injection in the first S-38 was to the detector, while it
> was 
> injected into the IF stage in the S-41G.
> 
> Selectivity for the single IF stage for the S-38 was:
> 
> 7 kc wide at 6 dB down
> 65 kc wide at 60 dB down
> 
> From what I can see, the mechanical and electrical layout of the S-38 was 
> identical with that of the earlier S-41. The tube string was identical.
> 
> I like Bob Fuqua's idea of replacing the 12SQ7 with a product detector,
> but I 
> think I would use a 12SL7 dual-triode instead. Using 1/2 for the BFO and
> 1/2 
> for a triode-product detector, the reason being that with the 
> triode-product-detector, it acts like a plate detector with the BFO
> turned off 
> and thus does a good job of demodulating AM.
> 
> Ken W7EKB
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