On 19 June, 01:26, 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.
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.
A balanced RF tranny that is tuned requires coil with more turns on
the same core to get twice the inductance, fairly easy if you start
with a ferrite rod.
I'd probably say the best wire for low capacitance of the coil is
solid telephone hook up wire, or strands taken from a cat5 cable.
You then need two tuning gangs, 20-365pF are OK. The cap frame and
moving plates are at 0V and if you want to avoid B+ across the tuning
caps, then cap frame is bolted to chassis and 0.1uF caps from coil to
fixed plates. Maybe put 2M2 from fised plates to 0V to bias them down.
This allows you to have a 3 gang cap and use one gang for the
oscillator coil.
The old HP606 I have has this sort of set up. It used 6B4 to cathode
modulate a pair of 6CL6. Oscillator is PP type using maybe 12AT7, I
forget, but its designed for many ranges from 300kHz to 65MHz, all
with well calibrated dial. The whole thing has very high electro-
mechanical integrity; must have cost a huge amount in 1955. Its got
NFB around the modulator to make the AM more linear. But my home brew
SE pentode modulator also works just as well for 2 bands up to 1.7MHz
so I can test the BC band or any IF channel. I don't need the more
complex HP PP circuit. I can't see any problem having NFB around ANY
form of AM modulator because whatever triode is used to as a modulator
can be one of a pair of triodes in an LTP with TWO input ports, and
one port is for the detected AF NFB signal, and the other is for AF
input, so the modulator amplifies the difference between input and FB
signal and applies a correction signal at the output.
Such an LTP for an SE modulator isn't really a pair, because you only
need one anode's output with the other anode grounded via electro cap.
But the two available output could be used for a PP modulator.
I forgot where you are buying your 6ME8s for a dollar apiece, could you refresh
my memory?
What other beam deflector tubes are usable?
BTW John, be careful jumping out of bed. Lotsa guys come to grief that
way :-).
Patrick Turner.