"flipper" wrote in message
...
On Mon, 11 Jun 2012 12:50:03 +1000, "Alex Pogossov"
wrote:
"flipper" wrote in message
. ..
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
But the smartest arses (aussies of course, he-he!) put their AM
transmitters
in the vintage radio cabinets.
http://www.ebay.com.au/itm/140770446...84.m1555.l2649
Interesting you bring that up because it was my original 'cost saver'
idea before the UHF converters and I even bought a supposedly non
working Philips radio to use. Turned out the radio only needed one
tube, which I had. It could also use a recap but it's in good enough
shape I didn't have the heart to scavenge it.
I might resurrect the idea for the air variable and dial indicator but
the biggest problem is I want power transformer isolation and most
don't have one (at least not the AA5s), nor 'extra room' to fit one. I
notice that Little Nipper does but it's 240 VAC.
I think my 2 bucks 2 tube Beamus scope shot looks as good as his 5
tube job 
Apparently, the more tubes -- the more "decent" and "professional" it might
look... But your 2-tube solution looks like gives clean modulation due to
the feedback.
However, the biggest (potential) problem of your design is frequency
instability. If you have a parasitic coupling between the aerial and the
oscillator coil, you will get the "frequency pull". If you touch the antenna
(changing the voltage on it) -- you will get a frequency shift! This pull
will also be modulated, so you will get a spurious FM. Probably acceptable
for an AM radio, but will sound crap on an SSB receiver. So your major goal
is to shield your oscillator circuit (L1, C1b and other components). And
even if you make a perfect shielding, you will not be able to completely
avoid parasitic coupling through the shaft impedance of the dual gang
variable capacitor.