On Tue, 26 Jun 2012 22:09:21 +1000, "Alex Pogossov"
wrote:
"flipper" wrote in message
.. .
On Mon, 25 Jun 2012 19:36:07 +1000, "Alex Pogossov"
wrote:
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.
Likewise in a dual control pentode the "linear G3 region" shifts with the
plate voltage.
I wasn't clear. I meant on the horizontal axis.
Look at the Tung-Sol (because it has the combined curves) datasheet
for the 6AS6 and the G.E. datasheet for the 6CS6. On the 6AS6 all the
plate curves tend to converge in the same cutoff region but it's
dramatically different for the 6CS6. Now, the 'linear' region of the
6AS6 does shift some horizontally but it's nothing like the 6CS6.
Dual control pentodes have one undesirable disadvantage. Their plate
impedance is not high and more importantly, it is not constant. It is high
at G3 cutoff (obviously!) and high at G3=0...+5V (where it works as a
conventional pentode without much current returning to G2). At the very
middle of the "linear" G3 zone the plate impedance is low, as in a triode
where the cathode is the space charge rather than a red hot physical
cathode. If such modulator runs into a LC tank, its Q factor will be
modulated. Peaks and troughs will be emphasized, carrier level -- reduced.
Hence -- the second harmonic of the envelope.
Yeah, I noticed the plate impedance curves and the change is rather
dramatic.
Are you suggesting a heptode would be a better 'alternate choice', for
the 6ME8 audio to G1 topology, than the dual control pentode?
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.
I managed to get a dual control pentode (6HZ6) model working with
Circuitmaker, although I'm not sure how 'good' it is. At any rate, it
seems to not like going over 80%, or so, mod. Can't drive it to near
cutoff. Another thing, input impedance to G3 seems incredibly low, at
least at RF, and, if the simulation is even remotely accurate, there's
no way my LO can drive it.
Both those 'problem's may be the model, so I'll probably try building
it anyway, but if the simulation is valid then the 6ME8 works quite a
bit better.
Yes, 6ME8 seems a good choice, though this tube is "exotic" -- not found in
evereyone's junk box, unlike say 6BE6.
Well, they're only a buck online, which is cheaper than the 6BE6.
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.
I'm curious how space charge makes it across to a separate LO.
Modulating grid capacitance, which is coupled to the LO tank?
Exactly. Likewise, in 6ME8 density of the beam (cathode current) affects the
space charge between the deflection plates which modulated capacutance
between the deflection plates. I do not know the magnitude of this effect
compared to a heptode. In a heptode typically space chage "equals" 0.3pF.
Wow, my guess wasn't bad for 'not an RF guy'
As far as the parasitic FM is concerned, do two simple experiments.
1. On a quality comms receiver in SSB mode listen to the carrier (set beat
frequency to a few hundred hertz) of your transmitter.
Without applying any audio, halve the cathode resistor of 6ME8 (touch it
with another resistor in parallel), effectively simulating +100% modulation
excursion. Ideally, pitch of the beat tone shall not change. If it
changes... you have FM.
2. On a quality comms receiver in SSB mode listen to the carrier (set beat
frequency to a few hundred hertz) of your transmitter.
Without applying any audio, play with the antenna tuning capacitor of your
transmitter. Ideally, pitch shall not change, only the volume as you tune to
and detune from the resonance peak. If it changes...
If you do these tests, could you "report" the results here. Then we can
discuss what causes FM in each of these cases.
I don't have a SSB receiver.
Regards,
Alex
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/