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Re: VTTC - audio modulation

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

Hi Herwig, David, All!

>The music, I have modulated onto the rf,  was much more distorted
>despite I varied the amplitude of the base notes using an equalizer.
>Maybe the oscillator didn't work in class A-mode really. I need to


The oscillator, if it's a power oscillator, will operate in class C
surely?  It needs to in order to get the efficiency (70%+ say) which
in class A would only be maybe 25-30%.

What you want is linear modulation of that class C stage, either plate
modulation (which I'm sure we discussed and largely eliminated on the
basis of needing a hefty plate modulation transformer plus a lot of
modulator power) or some kind of grid modulation, which is what you're
using (grid or screen or whatever).

Someone else was asking about sorting distortion from a modulated VTTC
and I said I'd do some experimenting and get back to him - well with
one thing and another I still haven't got that round tuit, so my
apologies - I haven't actually forgotten, and I do still have that
email of yours (buried under around 1000 others!) but other things
have got in the way.

I still have no definitive answer to this one, but it must come down
to a non-linearity either in the modulator stage or in the application
of the (undistorted) modulation to the power oscillator; I can see no
other options (though that of course doesn't mean there aren't any!).

It could be that the mere act of connecting your modulator to the VTTC
is causing the modulator itself to distort because of some kind of
load mismatch, so a monitor of the modulator output when connected to
the VTTC will help you discover if this is the case.  If it is, you
may still not know how to fix it but at least you'll know where the
problem lies!  If the modulator output remains undistorted you know
the problem is in the VTTC stage.  I would lean towards the modulator
distorting when connected to the VTTC, because with the VTTC operating
in class C, it will offer a widely-differing load impedance to the
modulator stage depending on whether it is hard conducting or biased
hard off (class C stages spend most of their time at one or other
extreme) and this may be upsetting your modulator, which probably
likes to operate into a fairly constant load.

>> Possibly a interesting idea would be to take Ulrich Haumann's
>(sp?) design to a more powerful tube
>- Do you happen to know the URL?


Careful with this one.  The plasma is live to HT positive, and the
main problem with a big bottle is going to be coupling the top of the
TC to the grid feedback loop without getting strikes to it and blowing
the grid coupling cap !!!  Full plate HT on the grid is not
recommended :-)  This, and this alone, has so far dissuaded me from
trying it with a pair of 813s.

It is, however, possible that a minor modification of the circuit
might solve the problem (more experiments I've just not had time for!)
The work I did with the Q&D VTTC (derived directly from Ulrich's
circuit) solving the minor irritations of the criticality of the plate
choke suggests that if the plate choke was made resonant at a lower
frequency than the TC fundamental (if the TC resonates at 28Mc/s then
the choke resonating at say 7Mc/s - which given the harmonic
relationship will still function as a plate choke) that there would be
a point on that plate choke at high rf potential and at a suitable
phase displacement which could be tapped via a suitably-rated coupling
cap of maybe a few hundred pF to give the right phase and sufficient
volts of grid feedback without that pesky coupling loop at the top of
the TC and thus without incurring the possibility of grid coupling
loop strikes.  You can locate high rf potential on such a choke with a
small neon bulb held on an insulating rod, so it ought not to be too
difficult to try this out, just haven't had the chance what with
trying to get the micro-magnifier ready for the Corby Teslathon one
week today - looks like I'll make it in time but it's going to be
close :-)

It might even be possible to connect the grid to a piece of copper
pipe (longitudinally slit so it doesn't form a shorted turn) cover the
(lower frequency) plate choke with some plastic tape or preferably
plastic pipe and simply slide the copper pipe up and down the choke to
find the right position and couple capacitively directly, though you
may not have enough capacitance this way.  If you have a working Q&D
VTTC this should be fairly simple to try, I'd do it myself but . . . .
You'll probably get the idea if you look at that long choke I used
originally at:
http://home.freeuk-dot-net/dunckx/wireless/qdvttc/qdvttc.html which is in
the first photo of the setup, lying horizontally.

I hope there's something here which helps identify the problems you're

Geek#1113 (G-1)