On Mon, 11 Jun 2012 14:16:59 +1000, "Alex Pogossov"
wrote:
"flipper" wrote in message
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.
Yep, NFB is the key to getting g1 linear. I did the same thing on my
reactance tube FM transmitter. And the Twin Triode Transmitter, come
to think of it.
For not being an 'RF guy' I sure have done a lot of RF lately.
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.
That sounds a lot like the old joke "Doc, it hurts when I poke a stick
in my eye, What should I do about that?
Don't poke a stick in your eye."
Don't touch the antenna.
I do intend to shield the LO and, for cost reasons, dropping the gang
tuning idea. (Maybe I should blame it on a 'bean counter', eh?)
There's been a revision. The 70 uH was low and the scramble wind on a
UHF core not so hot, which was causing low plate swing. So I've now
gone to a 250 uH ferrite rod antenna for the plate load and am getting
80 Vpp at idle. And, good as it was, modulation depth also improved.
It's now so close to 100% you can't tell the difference unless you
blow the scale up.
Looks to me like this one is a winner but, of course, I'm a bit biased
Revised schematic is posted
http://flipperhome.dyndns.org/Beamus.htm