Re: Modulated VTTC (plasma tweeter) question (fwd)

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

Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <cwillis-at-guilford.edu>


Hi Duncan,

Thanks for your most informative reply.  I have some numbers now (and still
the distortion problem!) which agree rather well with your guesses.  You
weren't far off on the 6CL6 estimation.

I changed some values from the last post and got some measurements:

The modulator (6CL6) has a plate resistor of 5600 OHM which drops about 175
volts, leaving about 200 at its plate (I use 375 V for the non-6146
circuits).  I have a 15 kOHM resistor between the modulator plate and 6146
screen to lose about 100 V for a DC screen voltage at the 6146 of 95 V
(measured.) I checked gridbias on the modulator and AF amp, and they're
good: -3 V for both.  Quiescent plate draw for the 6146 is roughly 70 mA
(depending on the coil used for the tweeter.)

I used basically the same method you suggested for circuit design.  But I
also took heed of the ARRL handbook's suggestion to take the Class-C
telegraphy plate current rating and operate the tube at half the value for
100% modulation.  But I am not using 100% modulation (I have no clue what
works best for modulating a tesla coil!) but rather my rough choice was
50%.  As per the ARRL handbook, at 100% modulation the plate current should
start to jump around with the modulation as the signal gets distorted.  I
can easily find this point as I crank up the volume on the tweeter input
(did my rough calculations that I would have 50% not matter?!), but the
sound output is so low at this point that it's hardly gonna be practical as
a speaker! (The sound is very, very good though.)

One issue I thought of was that the load (plasma arc) is not constant at
all- it may introduce plenty of distortion.  But on the other hand, at even
100% it should stay the same visible size whether idle or at full AF power.
This is gonna be a hard puzzle to solve, but thanks for your help!

Oh- by the way, I checked Hulrich's site and sent him an e-mail with the
same concerns I posted here to see if he could help.  My browser cannot
find your site, though, but I would like to see it if you could check the
address again for me.