In article ,
flipper wrote:
On Tue, 26 Jun 2012 22:29:35 -0500, John Byrns
wrote:
In article ,
flipper wrote:
On Tue, 26 Jun 2012 15:38:20 -0500, John Byrns
wrote:
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.
Sorry. I'm the one who 'converted' it to dual control pentode.
Are you trying to play with my mind? I was the first to use the term "dual
control pentode in this thread when I made the statement "Applying the
modulation to G1 doesn't take advantage of the beam deflection capabilities
of
the tube and instead uses it in a way that a more ordinary tube, like a dual
control pentode, could serve." when I meant to say heptode. I'm the one
that
converted it!
LOL. Okay, sorry again 
For this application I tend to think of them as almost the same thing
but am beginning to realize that the 'extra' G4 screen is probably
what gives a heptode the higher plate impedance. Is that why, despite
the 'accidental' mentioning of dual control pentode, you seem to be
adamant about heptodes, or is there another reason?
The reason I am "adamant about heptodes", if that is what you want to call it,
is not because of the plate resistance, it is simply because I have no
familiarity at all with DC pentodes and their characteristics.
I don't even know any DC Pentode type numbers and am not sophisticated enough to
know that the plate resistance of a DC pentode is lower than a heptode.
Assuming the plate resistance of a DC Pentode is lower than a heptode, is that a
bad thing?
I am not "adamant about heptodes", in fact I don't particularly care for grid
modulation, preferring plate modulation instead, however it is hard to conceive
of how to build a plate modulated transmitter using only one single section
tube, leaving the heptode, or possibly the DC Pentode, as the only easy choice
for a single tube transmitter. I do have a design for a single tube transmitter
using your 6ME8 that meets my design brief, however it would require me to build
my own well balanced push-pull RF antenna coupling transformer.
--
Regards,
John Byrns
Surf my web pages at,
http://fmamradios.com/