On Mon, 18 Jun 2012 10:26:44 -0500, John Byrns
wrote:
In article ,
flipper wrote:
Okay, so I like to make up cutesy names 
I wanted to do 'something' with a 6ME8 so I tried using it for an AM
transmitter and the first prototype works rather well. Bandwidth is
'too much', less than 1 dB down at 18 KHz, but we'll worry about that
later.
This is a 'dollar days' special, 2 whole bucks worth of tubes, and L2
is a UHF converter coil scramble rewound, so that was 'free'. Power
supply is from the same converter and if the remaining issues get
worked out it'll probably end up in that cabinet as well (but where to
put the air variable?).
The schematic is rough, but should be rather self explanatory, and the
ganged tuning hasn't been fleshed out yet. I'm still using separate
caps while jiggling things around.
Orphaned web page, not yet ready for prime time, has a recording of it
playing through a table radio.
http://flipperhome.dyndns.org/Beamus.htm
Hi Flipper,
This transmitter has been bouncing around in the back of my head for the past
week because something didn't seem quite right but I couldn't put my finger on
it until this morning when I jumped out of bed.
The problem is that there is nothing wrong with the transmitter concept, except
that it doesn't take advantage of the 6ME8 in the way one might have hoped based
on your previous comments on the tube.
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.
A DC pentode might work similar but I'm not sure the plate/screen
current transfer is as balanced as the 6ME8 deflection plates. I
wondered about that, though, but the final deciding factor was 'non
technical': I wanted to use a 6ME8
Might be interesting to try because I'd think it would take less RF
amplitude but, ironically, it looks like a DC pentode might pull more
B+, even without 'wasting' the 6ME8 plate two power, because it's
screen pulls so much current. It also looks like the 6ME8 might have
better plate impedance but it's hard to tell because there are no
plate curves down at the relatively low voltages it's being run on.
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'.
It could also be done 'single ended' but then you have large audio
swing required and deflector 'non-linearity at the extremes with no
'simple' means of NFB.
So, this topology was picked because it seemed 'simpler', reduced the
number of "never done this before" things that could go Murphy's Law
wrong, and required no 'special' parts. Doubly so because I already
had them.
I forgot where you are buying your 6ME8s for a dollar apiece, could you refresh
my memory?
ABC vacuum tubes and ESRC1 vacuum tubes (which seem like maybe they're
the same people). Both sites have a perennial !dollar days! section,
which says qty 10 minimum but I've also been able to get a few tossed
in when buying others at 'normal' price. However, since those are
'normally' $3 (occasionally $4, as with the 6ME8) you really only have
to want '4' (maybe 3), and get 'extras' to make up 10, to come out
ahead.
vacuumtubes.net has no !dollar days! section but pretty much the same
tubes simply listed for 1 buck in their normal price lists, but I've
never tried ordering from them. I normally use ABC, mainly out of
habit.