Re: Success with Audio modulation of a VTTC Or: how about a plasma tweeter ;o)
> Is there a good circuit I could use for protecting the amp from RF?
> I know that the so called "audio" transformer I used is not ideal
> grabbed it off my shelf) is there something I need to look for in a
>audio "coupling" transformer, regarding this project?
You just about built yourself a plasma tweeter. I jerry-rigged
something (just a maze of alligator clips, leads and parts) very
similar. If I remember correctly, the 811 is a triode. My setup
consisted of a PL-519, which were used in old TVs. You donīt
need lots of power to get a "clean" tone. The PL-519 is good for
about 60-70W of power, which is plenty for these experiments.
What is real important, the PL-519 is a pentode. My mini TC runs
at a few Mhz and makes about 3/4" sparks. I use an old tea
strainer as a counter ("arc-to") electrode. The TC itself is only a
few turns of heavy silver wire and the "primary" is more of a loop
than a coil. Anyway, the pentode has a BIG advantage over
the 811 setup. You (as I) can simply modulate the tube with the
middle control screen. I have a high voltage power transistor
(a BU 208) that controls the grid. To keep the BU-208 within
operating specs and to eliminate the need for a power amplifier,
I control this transistor with a small signal transistor (a BF 869),
which sits directly behind a high frequency cross-over. This type
of setup eliminates the need for a coupling xformer. I scoped the
input of this setup (really quick as it isnīt a "finished" project), but
I couldnīt see any RF coming back. Of course, my setup isnīt all
that powerful, but you donīt need it either, to hear the high
frequencies of Pink Floyd, etc ;o). As you canīt hear frequencies
above 20khz, you could just block them off. With 30 years on my
back, (;o}) I can still hear till about 17.5khz. Most of my friends
(same age) canīt even hear the whine of a TV flyback
(15625Hz), so I consider my hearing to still be in pretty good
shape. Anyway, you could simply add a high frequency "spill"
(I mean a HF drain to ground) to filter out anything above
30-35khz. A few caps and resistors should do the trick. That
should save your amp from any returning RF.
Sounds like you had some real fun!
Coiler greets from germany,
P.S. If you are interested in a schematic, I could send you
a hand drawn specimin (not before the weekend, tho).