INART 55

History of Electroacoustic Music

The Audion


In 1906 Lee de Forest (1873-1961), introduced the audion (from the words audio and ionize). He expanded the Fleming valve by putting a grid into the tube, a third electrode. The electrical oscillations resulting from the radio signal were not directed to the filament, but rather to the grid. The addition of the grid constituted a third electrode in the tube, so the audion also came to be called a triode. Diodes and triodes form the basis of vacuum tube technology.

In a basic demonstration audion circuit, the small battery (A) heats the filament, causing electrons to boil off and create a space charge, as with the Fleming valve. The large battery (B) gives the anode (plate) a positive voltage. The medium-sized battery (C) charges the grid. Battery C charges the grid. Depending on the orientation of the battery, the grid can be given a positive charge (solid line) or a negative charge (doted line). With a negative charge, the current flows into the grid. Electrons from the cathode (filament) are repulsed by the charged grid, and are not as likely to go to the positively charged plate. Most simply return to the filament. With a positive charge in the grid circuit, current flows away from the grid. With a positive charge, the grid attracts cathode electrons, many of which pass through the grid to the anode plate.

Thus the grid acts as a variable resistor. The changing voltage levels on the grid are reflected in the current passing through the plate battery (B). Thus, the triode acts as an amplifier. In a radio circuit, there is no grid battery (C), but rather the AC coming in from the antenna gives the grid alternating positive and negative charges. The triode is extremely important because radio signals are weak, but the triode allows them to be amplified to the point of audibility. The circuit of battery B includes a diode to rectify the current to positive cycles only, and filters to remove the carrier frequency, leaving only the oscillations created by the program material to be sent on to an amplifier. If the charge on the filament exceeds that on the grid, the signal is amplified.

Click here for a Flash animation of an audion.


A spectral plot showing the
frequency response of a bandpass filter.
The desired carrier frequency could be tuned in by using highpass and lowpass filters in combination to create a bandpass filter. A bandpass filter may be described by its center frequency and its bandwidth. The audion represented the beginning of the field of electronics. "Wireless" technology was soon to be superceded by "radiotelephony," the radiation of high frequency signals, called radio for short. A variety of tools and testing equipment were developed for the maintenance of electronic systems. Among these tools were standard oscillator types.

As innovative as it was, the audion required a few more enhancements before radio would change the world to the extent that De Forest described. The audion remained a little-understood curiosity for another six years.