On Tue, 26 Jun 2012 15:36:48 -0500, John Byrns
wrote:
In article ,
flipper wrote:
snip.
I'm also going to try the DC pentode because, like you said, it seems
to me that ought to work about the same.
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.
You're quite right. You said the "6SA7/6BE6/6SC6 family of heptodes"
and it was I who 'converted' it to dual control heptodes because
'total cathode current appears to be largely independent of the
voltage on G3' on those too.
An interesting point about your 6ME8 transmitter is that it appears to be a
vacuum state analog to the solid state designs using a three transistor "cell",
which seem to be the designs of choice with many old radio fans. In this
circuit two of the transistors are connected as a long tailed differential pair
with RF drive applied to the base of one transistor and with the modulated RF
output taken from one of the collectors. The third transistor operates as a
current source in the emitter "tail" circuit of the first two transistors, with
audio applied to its base to modulate the RF output of the differential pair.
Yes, I know the topology and was 'that close' to building one but am
not sure which was the cart and which the horse here. I originally
started thinking of the 6ME8 back when Patrick was tinkering with
Gilbert Cells because I noticed a 'similarity' between it and the 3
transistor 'cell', of which the Gilbert has two on top a current
source.
Anyway, when working on this transmitter I tried 'emulating' something
akin to the beam deflection tube because I don't have a model for one
and constructed a 'long tail pair' using a pentode for the tail, which
is also akin to the 3 transistor 'cell'. That did point out one
difference, though. The deflection plates do not depend on cathodes
(emitters) driving the 'other side' to get the 'see saw' effect.
There are also a bunch of dual plate tubes that were used in electric organs,
that I have been meaning to lookup for a couple of years to see if there are any
that might be useful in these circuits, so far I haven't gotten around to it.
The one I'm aware of is the 12FQ8 but have never been able to figure
out how it was intended to function.
I found the page.
http://www.radiomuseum.org/forum/gra...ansmitter.html
Down at the bottom, "The sharp-cutoff heptode solution" by Jacob
Roschy. His second circuit is "The cathode oscillator attempt." "Since
this resulted simultaneously with the desired amplitude modulation
into a strong undesired frequency modulation, I abandoned this
experiment and scrapped this circuit very soon, I will never try this
any more !"
That's an interesting page, but back to that in a moment.
Re unintentional FMing of AM carriers, that seems to have been a traditional
part of early AM broadcasting, although the radios back then may have had a much
wider bandwidth than today's radios, so the FMing may not have mattered much.
Consider the FMing that was likely present in the following circuit taken from
an old notebook that belonged to a gentleman doing research on building a radio
station back in the day, which I bought on eBay.
http://fmamradios.com/Old_Notebook/K..._Schematic.jpg
And here is a photograph of what is probably a later but similar transmitter.
Unfortunately this photo doesn't include the power supply and modulation reactor.
http://www.hammondmuseumofradio.org/...y-kdka-exp.jpg
While this is obviously not a photograph of the exact transmitter described by
the schematic, I suspect that it is similar, possibly with improvements to
eliminate FMing, but probably not to improve modulation capability.
The Radiomuseum web page really took me back as I owned a Motorola FM converter
using the 12-volt tubes back in the 1962-66 time frame. The Motorola was
probably a little more upscale than the Granco discussed on the Radiomuseum
page, IIRC it had an extra IF stage and a nicer case than the Granco, however it
used a "phono oscillator" circuit similar to the Granco's to drive the existing
AM radio. At the time I was impressed with how good the "transmitter" circuit
in the Motorola FM converter sounded, and the audio level was not especially
lower than on an AM station.
I had been wondering how much of the FMing with the self-excited heptodes is due
to coupling within the tube, and how much was due to layout and shielding
issues. Joe seems to have gone into that to some extent, although a deeper
exploration of the subject would be more satisfying to me. Jacob's use of the
ECH84 is interesting in that it presumably eliminates any direct "electron
cloud" coupling issues, but the oscillator and modulated amplifier elements are
still in close proximity so that there could still be inter-electrode coupling
issues. None of Jacob's circuits include a buffer stage between the oscillator
and modulated amplifier stage to completely eliminate the last vestiges of
"electron cloud" coupling affecting the oscillator frequency.
Yeah, and he also doesn't provide any measurements of the 'FMing' in
either. Just that it's apparently 'bad' in one and the other circuit
works well, whatever that means.
Another issue that Jacob addressed that I have been wondering about is what he
calls "G3 Blurring", which he reduces by using a cathode follower to drive G3.
It occurs to me that a refinement of this approach would be to use a
direct-coupled cathode follower to allow driving G3 positive, although that
would probably require an additional power supply, increasing complexity.
I was not previously aware of the 6BY6 heptode mentioned on this page, it
appears to have a considerably more linear G3 voltage vs. plate current curve
than the 6CS6 most people on this side of the pond seem to talk about.
I've got both and they're both '1 buck' tubes. I think people kind of
like the 6CS6 because of the unusually high G3 gm, so it can
potentially be driven without a preamp.
Something else that I learned from this page, that should have been obvious to
me before, is that the two tuned circuit heptode circuit that I suggested
provides a degree of isolation between varying antenna capacity and the
oscillator frequency.
One thing that really strikes me about pages like this Radiomuseum web page is
that the authors rarely state what all their design goals are. If I were
building a small transmitter using four tube elements as Jacob did, I would
probably go with a plate modulation approach, eliminating several problems
presented by the grid modulation approach Jacob uses. The attractiveness of the
heptodes to me are the possibilities for building a single tube transmitter
Well, the 6CS6 mat be the choice, then, because of that high G3 gm, if
the FMing could be solved. Joe Sousa, who has another interesting
article on what he calls his 'hi-fi' transmitter, seems to think it
could be neutralized out.
Btw, his 'hi-fi' transmitter is the same idea I was proposing to
Patrick with the 6ME8 but he's using a dual control pentode to do it.
http://www.radiomuseum.org/forum/hi_...ansmitter.html
One tube job, like your goal. Btw, he speaks of neutralization in that
article too (3- Frequency Stability) but decides his circuit doesn't
really need it.