On Sun, 24 Jun 2012 15:59:48 -0500, John Byrns
wrote:
In article ,
flipper wrote:
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.
Well, your logic is pretty much what mine was, plus the same 'constant
current' theory applying to dual control pentodes as well, but over at
radiomuseum (plus others) I read an exhaustive thread where the poster
said "will NEVER try THAT again." I can't find the link off hand.
He did try to theorize some kind of miller interaction but was vague
enough that I couldn't quite follow the logic. He also dealt with your
idea 'that can't be' or else a typical radio wouldn't work right. His
argument was the only reason it did work was the small RF levels and
LO carrier being suppressed.
Anyhow, everyone tells me don't do it but I might try anyway
As for the 6ME8, I thought, like you, cathode current should be
'constant' since the 'beam' is just being 'deflected' to the two
plates but was surprised to find gobs of RF on the cathode. I guess
the modest deflector bias, which is changing with RF, does affect it.
Maybe it wouldn't if I drove both.
Btw, my instinct was to 'clean it up' but that made things worse so I
finally decided to not try 'fixing' what ain't broke.
I'm also going to try the DC pentode because, like you said, it seems
to me that ought to work about the same.