Section
6
About Tubes
6.0 A Brief History of Tubes
The history of vacuum tubes began in 1883 when Thomas Edison discovered that the hot
filament in his light bulb was boiling off electrons. This Edison effect (known today as
thermionic emission) was to become the basis of the operation of the vacuum tubes that would
lead to radio, television, radar, computers, and, oh yes . . . guitar amplifiers.
A short time later (1904), in England, John A. Fleming created the Fleming valve by inserting an
extra electrode plate into a light bulb. Fleming discovered that electrons would flow in only one
direction in his valve, from filament to plate. The filament boiled off electrons which then
would readily flow towards the positively charged plate but resisted any flow back to the hot
filament. Today we call such devices diodes or rectifiers. The Fleming valve was the first tube
rectifier.
Two years later (1906), back in America, an inventor named Lee De Forest tried inserting a third
electrode between the filament and the plate of a Fleming valve. This new electrode consisted of
a fine wire mesh. De Forest found that a small voltage applied to the new grid electrode could
control the flow of a relatively large current between the filament and the plate. The device had
the effect of amplifying the signal applied to the grid and made possible the first radio
broadcasts of speech and music. Lee De Forest called his invention an "audion"; today we call
his device a tube triode.
De Forest's audion is the same basic tube amplifier stage used in the Quad X-Amp today. The
12AX7A tubes used in the Quad X-Amp are actually dual triodes. That is, there are two triode
amplifiers in each tube. The nine pins on each tube are for the following connections: two
plates, two grids, two cathodes, and three connections to the heater (filament).
6.1 Tubes, Amps, and Overdrive
The first guitar amplifiers used vacuum tubes as their sole means of amplification, simply
because that was the only technology available at the time (1950's). Players first started
overdriving their amps in the 1960's by turning the volume up until the amp distorted. The early
amps were never designed to be played this way and the fact that they had an interesting tone
Quad X-Amp About Tubes Section 6
6-1
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