Plasma Tweeter construction and repair

Some history

Yesterday's dreams make tomorrows inventions

It was 1954 when the story began. Siegfried Klein, a French  physicist, was the first inventor of a practical plasma tweeter. He experimented since the mid 40's on his dream. Mr. William Torn, DuKane's Chief Enginieer for Commercial Sound, became involved when Mr. Klein contacted him and made a proposal to have DuKane to manufacture his invention. Klein was invited and Torn was impressed with the demonstration. He convinced the DuKane management to create a new division to develop and manufacture the Ionophone renaming it to Ionovac. It took three years development. In 1957 DuKane announced the first Ionovac model T-3500. About 500 of this speakers were made. Maket price $147.00 This was a lot of money in these day's.


DuKane Ionovac Model T-3500

The inventor Siegfried Klein


The first production had the power supply and oscillator in one metal chasis. The speaker horn and quarz cell were separate connected to the power unit with a cable. The design had some problems. The tune of the cell shifted with heat, the oscillator frequency become mismatched to cell resonance and this result in distortion. Furthermore the warmup caused the speaker to be unreliable or fail to start at all when it was cold.  The press called it "a brilliant failure"

Circuit of the T-3500

The model used a 6146 beam power tube as oscillator. 12AU7 as audio amplifier and modulator. The first triode was an amplifier and DC level shifter. The second triode was a series pass voltage amplifier with the cathode direct coupled to the screen grid of the 6146 oscillator tube.


Because of the bad reliability of the T-3500 DuKane continued with an intense effort to eliminate the problems and make manufacturing easier. They do a lot of research on electrode and cell. It was found that segmented wall of two conical sections with break point in the middle of the cell chamber was acceptable for good frequncy response. They found also that quarz glass was the best material for the cell. This design is used up to today.

The "new" Ionovac

The new production used a 6DQ6 beam power tube as oscillator. The modulation was made with a transformer connected directly to the screen grid of the 6DQ6. In this way the audio is either added or subtracted from the screen voltage causing the power output of the 6DQ6 vary in direct proportion. The new Ionovac were built in late 1960. They were indroduced as model 14A430 and 14A435 and were identical in the circuitry. 

The 430 came with power suppy in a metal box were the 435 had it mounted on a plate.

The last production of the new Ionovac was made in August 1961. Quantity is unknown but there were seen serial numbers up to 1600 on old tweeters. 

Here I want to thank Daniel Schoo for his excellent article dated 2004 with the authentic information of the Ionovac story. Unfortunately his site is offline.





I call it "Plasma Speaker Family"

I call it family because all the following plasma tweeters running more or less with the same principle. They have a power tube - in europe mainly a PL519 - in the oscillator and are using a Horn.

So far I know was Mr. Otto Braun with his company the first in Germany who has produced a tweeter like this. It was called CORONA. His company CORONA ACOUSTIG GMBH is not existent any more. Today the CORONA is manufactured by Lansche Audio GbR  here the link .

Following pictures I took when repairing some of these tweeters. They are reliable but older ones now suffer on bad elektolytic cap's, electrode is worn out, or owners have caused failures.

By the way this is the closest design to the Ionovac. Modulation is also done with transformer, power come directly from the main, new is that capacitance from the plasma is neutralized with cap's switched by a relay. The plasma cell is now larger than the old ones to go more down in the frequency spectrum.


This is a picture of the quarz cell and electrode, the electrode is worn out, no tip anymore.

Older Plasma Speakers

Otto Braun has manufactured also OEM Versions sold to other company's. He worked together with Mr. Hans Unbehend who was the developer. I have handwritten circuit plans from him dated july 1975. Here are some pictures of the old ones.

even the cell is identical
Modulation and power supply. Tube was a PFL 200.
In a Phonogen speaker I found this. There are two tubes in the modulation/power circuit.
RF part absolutely the same as the new Ionovac

Plasma Speakers from ATR - Audio Trade - the former company name of Acapella

Also a plasma speaker which follows the "root's". RF part is made with a beam power tube. The differnce is in the power supply and active crossover.

Tecnac a german Ionophone clone from the 1970's

About twenty years ago I had the chance to buy them. I have never seen these kind of speakers any more. No pictures in the Internet - nothing.

The speakers are dated on the back 1974. Manufacturer was "Thorolf Hartung Technische Akustik". The speakers are terribly assembled. The tube is a 807 and the power supply with modulator is covered with resin and tar. No electronic repair possible.