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<DIV>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..</DIV>
<DIV> </DIV>
<DIV>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.</DIV>
<DIV> </DIV>
<DIV>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.</DIV>
<DIV> </DIV>
<DIV>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.</DIV>
<DIV> </DIV>
<DIV>Chris, KL7DM </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV>In a message dated 1/25/2016 10:02:26 A.M. Pacific Standard Time,
kgordon2006@frontier.com writes:</DIV>
<DIV>
<BLOCKQUOTE
style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: blue 2px solid"><FONT
style="BACKGROUND-COLOR: transparent" color=#000000 size=2 face=Arial>Although
the AR-3 manual states that all the selectivity AND sensitivity is
<BR>generated in the 12BA6 IF amp stage, reducing the noise from the 12BE6
<BR>mixer MAY enhance receiver sensitivity on the upper
bands.<BR><BR>Replacing the 12BE6 with a 12CS6, which has double the 12BE6's
<BR>conversion-transconductance (950 umho vs 450), may make a significant
<BR>improvement in receiver sensitivity on frequencies above 10
MHz.<BR><BR>The two tubes have identical base diagrams and filament
requirements, so <BR>all one has to do is remove the 12BE6 and plug in a
12CS6.<BR><BR>I have not yet tried this, but intend to as soon as I get an
AR-3 working <BR>properly here.<BR><BR>I HAVE replaced a 12SA7 with a 6SB7Y
(and resistor) in another receiver <BR>and it did make a significant
improvement in that receiver.<BR><BR>Ken
W7EKB<BR>_______________________________________________<BR>Heath mailing
list<BR>Heath@puck.nether.net<BR>https://puck.nether.net/mailman/listinfo/heath<BR></FONT></BLOCKQUOTE></DIV></FONT></BODY></HTML>