"John Byrns" wrote in message
...
The 6SA7/6BE6/6SC6 family of heptodes would also seem to provide isolation
between a cathode-grid "self excited" oscillator circuit and modulation
applied
to G3 as the total cathode current appears to be largely independent of
the
voltage on G3, minimizing "FMing". If this weren't the case, the local
oscillator frequency, in an AM receiver using one of these tubes, would
vary
with changes in the AGC voltage due to fading.
Of these tubes only 6CS6 (not 6SC6) is suitable as it has sharp cut-off on
G3.
With 6BE6 it is almost impossible to cut it off completely, so 100%
modulation os impossible, and deep modulatio will be distorted, even with
feedback.
By the way, in this case NFB shall be applied from the plate of a heptode,
not from its cathode.
It is better to use a separate oscillator and geef it to G3 of a heptode,
while feeding audio to G1. In this case any heptode can be used. NFB can be
taken from the cathode since plate current is *sort of* proportional to
cathode current. Do not forget to decouple G2+G4 to cathode (!), not to GND
and use a large electrolytic for passing AF as well, not RF only. But still
it is better to take NFB from the plate (I mean AF component, not RF).
Even in a best heptode, even with a separate oscillator you will have
residual FM -- due to space charge and stray capacitances. A small FM of say
100...1000Hz is acceptable for listening on an AM radio, but will not be
listenable on a synchrodyne or an SSB receiver.
Both of these schemes differ from the common phono oscillator circuit in
that
they require two tuned circuits, one for the oscillator and a second for
the
plate circuit feeding the antenna, the plate current can be completely cut
off,
neglecting leakage and so forth, without affecting the oscillator circuit
to any
great extent. The single tuned circuit approach of the common phono
oscillator
circuit would have a greater sensitivity to "FMing", and the oscillator
dies
completely when the plate current is cut off.
--
Regards,
John Byrns
Surf my web pages at, http://fmamradios.com/