Re: VTTC - audio modulation

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi Herwig, All!

>Thank you very much for enlightening me. Now I understand, that
>the flywheel (a very good analogy indeed!) will restore the sine
>signal. But as the flywheel hasn't got any intelligence, it can't
>restore the modulation. Right? So, in order to keep distortion low,
it
>is necessary to operate the tube in the linear region of its
>characteristic?


Yes!  Well, even in a class B amp, the quiescent operating point (in
the absence of drive) is usually on the slightly curved foot of the
characteristic (this is being a bit pedantic!) but yes, the overall
response is linear and for exactly the same reason as you want
linearity in a hi-fi amp, low distortion.  Basically, the output
voltage in the tank circuit is directly (i.e. linearly) proportional
to the grid exciting voltage, and thus the power proportional to the
square of that voltage; the power gain of the stage is constant
regardless of the input signal level.  The fact that the quiescent
operating point is at the foot of the plate current characteristic
simply means that the circuit will respond to low level grid input
signals, which of course a class C amp won't (leastways not on the
grid) because it is biased well beyond plate current cut-off.

The flywheel effect simply restores the missing half cycle, and given
that the carrier rf frequency is so much higher than the modulating
frequency, the flywheel effect has no influence on the linearity of
the overall response.  I suppose it's stretching the analogy a bit to
try imagining a flywheel with a varying radius (!) but effectively
this is what you've got under modulated conditions.  The rate of
rotation doesn't vary, but the "radius" does, and it does so fast
enough not to affect the linearity of response in the af region.

>- The most important thing I've bought is the 2001 version of the
>ARRL Handbook on CD. It certainly will provide lot of useful
>information so that I don't need to bother you again and again.


The section on modulation is excellent stuff.  You may have to do a
bit of translation from solid state to hollow state, but I think
there's now a bit more on valve circuitry than there used to be a few
years ago, as valves have made something of a come-back.

>I acquired a sturdy looking stepless-adjustable-coil which I
>hopefully can use for experimentally tuning the primary of my
>VTTC. Using an off-axis-coil in series with the primary coil will
>lower the coupling between the primary and the secondary coil. But
>as the system has prooven relatively insensitive to variation of
>coupling this should not be a problem. Hopefully the coil will
>withstand the tank circuit voltage.


Sounds good to me, I recall doing some tuning experiments with my
500VA TC using a roller-coaster inductor and it worked fine.  This was
a spark-driven one, not a VTTC.

>Surprisingly a lot of Russian surplus was offered, especially power
>tubes (counterfeits of EIMAC types), for very reasonable prices. As
>in this part of the world it is very difficult to get service pulls,
these
>tubes are a cheap alternative.


>From what I've heard, these can be a real bargain.  I know of a number
of hams who have built high power rf amps using these Russian bottles
and they work very well.  They're not all copies of US tubes either,
there are some originals (Gb13? I'm not sure about the type number)
which have exceptional performance at UHF/microwave frequencies (1kW
output at 23cm wavelength) with no Western equivalents to touch them.
A fair few moonbounce and dx operators are using these.  Nothing
special for VTTC use though.

>> "Ohrtrompete meiner Großmutter ist angeschlagen worden durch Blitz"
>- Duncan, where did you get this? :-))))) You are really
multilingual!


One of those completely useless stock phrases from the sort of ghastly
"foreign phrase books" which we used to have years ago, and which you
could never imagine using for real in any situation . . . I confess to
having had to resort to BabelFish to stir my memory on this one!  But
it seemed somehow appropriate given the context . . .

>BTW: Thank you very much for your very intersting report on the
>Teslathon. I'd really like to partissipate, but the distance is to
large.


Maybe the time is right for one somewhere on the mainland?  There must
be enough mainland European coilers to justify it by now.  The Corby
show started up just three years ago.  I remember the first one - I
think we had maybe a dozen people and half a dozen coils, anyway there
were sufficiently few of us that Alan and Linda were able to feed us.
It has grown so rapidly - yesterday I think we must have been pretty
close to capacity, and several people who came didn't bring their
coils, which was probably just as well!  You might find the first one
or two Teslathons only draw a few people, but once the word gets
around you'll be surprised how many "closet coil-winders" are lurking
in the woodwork!  I bet if you started making noises about a German
Teslathon, you'd find people showing some interest, I'm sure there are
several German list members and I've seen several excellent German
Tesla websites.  Maybe you just need to go for it?!

Dunckx
Geek #1113 G-1