[Heath] A possible AR-3 "upgrade"...

[ChrisIwata at aol.com]

The overwhelming reason for lower sensitivity and S/N at higher freqs  on 
the those early general coverage radios is they used the same 365pF  (AMBC 
radio) tuning capacitor on all the bands..
 
Those pentagrids all used Hartley oscillators.  At the higher freqs,  the 
L-C ratio was unfavorable.  On my AR-2, I measured 4Vrms on the  oscillator 
grid for the AMBC band.  This dropped to 0.5Vrms on the 10-30Mhz  band.  So 
both the sensitivity and S/N ratio went downhill. Plus the  pentagrid ran 
hot, drew more juice on the higher freqs.
 
If they had added caps in series with the 365pF tuning cap (to effectively  
create a smaller tuning cap) and increased the coil inductances on the 
higher  bands, things would have been better. But then you'd have a nighmare 
with  tracking, trying to get the antenna coils to jibe.
 
The Franklin (not Vackar as I previously stated) oscillator overcomes this  
problem.  It has to do with how the feedback to create oscillation  is 
derived: For Franklins, the tap on the coil is unused. My Franklin  produced 
4Vrms on the AMBC band and still 3Vrms on the 10-30Mhz band with that  365pF 
tuning capacitor. This is why Franklins are used often for signal  generators, 
good well above 100Mhz.
 
Chris, KL7DM   
 
 
 
In a message dated 1/25/2016 10:02:26 A.M. Pacific Standard Time,  
kgordon2006 at frontier.com writes:
 
Although  the AR-3 manual states that all the selectivity AND sensitivity 
is  
generated in the 12BA6 IF amp stage, reducing the noise from the 12BE6  
mixer MAY enhance receiver sensitivity on the upper  bands.

Replacing the 12BE6 with a 12CS6, which has double the 12BE6's  
conversion-transconductance (950 umho vs 450), may make a significant  
improvement in receiver sensitivity on frequencies above 10  MHz.

The two tubes have identical base diagrams and filament  requirements, so 
all one has to do is remove the 12BE6 and plug in a  12CS6.

I have not yet tried this, but intend to as soon as I get an  AR-3 working 
properly here.

I HAVE replaced a 12SA7 with a 6SB7Y  (and resistor) in another receiver 
and it did make a significant  improvement in that receiver.

Ken  W7EKB
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