In article ,
flipper wrote:
On Mon, 18 Jun 2012 10:26:44 -0500, John Byrns
wrote:
Why not connect the 6ME8 cathode-grid circuit as a Hartley oscillator
circuit,
as I think you have suggested in earlier posts, and connect the modulation
signal to the beam deflection plates? I suppose the downside of this scheme
is
that it would require a balanced push pull RF output transformer to achieve
full
modulation, and building such an RF transformer could be a non trivial
project
in itself.
That was my first thought too although you may be remembering the FM
stereo multiplex discussion, where we needed DSB-SC output, because I
think that's the only one I posted a (conceptual) schematic for.
However, unbalancing the deflectors and then 'subtracting' one plate
from the other via a PP transformer was, indeed, what I proposed to
Patrick for the '100% mod' solution back when he was tinkering with an
AM modulator for, I think, 'testing' AM radios. I did wonder, in that
discussion, what the effect of the 'extra' side bands might be. I
mean, you have, in essence, DSB with 'partially suppressed carrier'. I
may still try that some day but, yes, the PP RF transformer kind of
put me off and I'm glad I tried this one first because of what I
learned: the pot core I bought for that very idea, and would have
used, wouldn't have worked well at all.
The other 'advantage' would be self excite but everyone keeps telling
me "don't do it." Self excite 'FMs'.
Would "FMing" actually occur with a "Self excited" oscillator built with a beam
deflection tube, or would the grid-cathode oscillator circuit be isolated from
modulation effects that would cause "FMing", by the construction of the beam
deflection tube? Since the sum of the plate currents for both plates remains
constant with modulation applied to the beam deflection electrodes, I would
think that the cathode-grid oscillator circuit wouldn't even see the modulation
applied to the deflection electrodes. G3, the accelerating electrode probably
provides further isolation between the oscillator and modulation effects.
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.
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/