[John Byrns]

In article ,
flipper wrote:

On Mon, 25 Jun 2012 19:36:07 +1000, "Alex Pogossov"
wrote:


"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.

Yes, audio to G3 is the most common implementation with people looking
to the G3 curve for linearity, which is why dual control pentodes seem
to be the preferred choice. For one, as you mentioned, the 6CS6 is one
of the few sharp heptodes, the ECH84 being the only other one I can
think of off hand, and there's more DC pentode choices. Second, the
dual control pentode G3 'linear' region remains relatively constant
over bias while the 6CS6 cutoff region shifts with screen volts and
bias.. That makes the DC pentode easier to bias.

There's a ton of Dual Control Pentode AM broadcaster schematics online
and I've breadboarded a couple of them. In fact, my LO was originally
developed for a 6GY6 version using a 1 MHz brick osc.

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).

This, using a dual control pentode, is what John was suggesting as an
alternate to my 'Beamus' 6ME8 modulator. It does seem like it ought to
work similarly.

Actually that wasn't what I was trying to suggest, that was either a "typo" on
my part or I was having a "senior moment". What I was actually talking about
was a circuit using a heptode such as a 6BE6, 6CS6, or 6BY6, I'm not sure how
the term "dual control pentode" managed to escape my keyboard.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/