• This is now my second set of plasma tweeters that I have constructed. This set, called the Plasmatone-RG1's was a commissioned set that I started in June 2000. There have been many ups and downs, but they are finally near completion. The main difference from the old set is that they incorporate a vacuum tube preamplification stage in addition to the normal stage. They also use a high quality, quiet 120vac fan, and each tweeter has it's own 250VA power supply. If I had to do it again (please no! I already had to tear the preamp board apart and rebuild it once!), I would have separated the power supplies from the tweeter cases, because I had a difficult time with power supply noise. The coils (see below) are wound with 16-ga magnet wire this time instead of 18-ga. I bought some 3" tea sieves at Lechter's Housewares for ~$5 each with I used as mini Faraday cages around the flame. The tea sieves did not take to solder, so I had to "sew" them to the copper rods with copper wire, which I then soldered to the rods. The housing for these tweeters is a Hammond steel cage with the corresponding aluminum cover plate acting as the bottom. Normally, there is a 2" steel chassis between the plate and cage, but I went without that, which made the parts mounting more difficult, but saved some space.
  
• I know several people have emailed me to get the schematic for this device. I just finished the schematics for the RG1's as I have implemented them. This may not have been the best possible circuit, so I encourage the tube gurus out there to design a better preamp circuit, better grounding, better power supply -- but these work and they work well! I think the best recommendation I could give would be to use a full-wave rectification on the power supply, since my half-wave causes some 60-Hz noise. The schematics are essentially the same as Ulrich's except I removed the BF869 transistor and replaced it with a 12AX7. The gain, which wasn't high enough before, is now even lower! I have been running them out of the 16-ohm tap on the Dynaco st70 that powers my Voigt Pipes, but I would recommend using two 12AX7's and have put a schematic together for that too here, although it has not been tested. Tubes are very easy to use because there is a +300 volt power supply available. An alternitive would be to just use two BF869's together capacitively coupled. Just remember that the BF869's generate enough heat to want a small heat sink (or use the chassis as a heat sink, but make sure you isolate it electrically!).
• Here's a picture of Bernhard's Plasma Tweeter, which looks very professional indeed.

A view of the tweeters with the bottom covers removed. The power transformers are visible on the left in the case. The preamp board and 12AX7 preamp tube are visible in the front. The controls are visible on the rear of the case.	The two tweeters set up for demo. The non-directional stereo effect created is unbelievable. No matter where you stand, you get the same frequency response. Regular tweeters only give good frequency reponse on-axis, so the stereo image is distorted if you step off-axis.
A view of one of the tweeters complete with engraved name plate (under the coil).	This is the coil I used on these tweeters. I wound thread around each turn of the coil. 4 of the theadings are basically perpendicular to the direction of the coil winding and the other 4 are diagonal. The coil is 15 turns of 16-ga. magnet wire wound on a 1.2" diameter air core. The turns have a little bit of air space between each winding to keep capacitance down.

These schematics can be improved upon! Here is a recommended configuration for higher voltage gain. I have not done any load-line analysis on the dual tube version, but I think it'll work fine (not optimal).

• Double preamp tubes.
• Double BF869 transistor coming soon.