Thermionic tubes
Thermionic tubes are very tolerant if their specified operating conditions are temporarily exceeded; unlike most semiconductors which tend to suffer from ‘catastrophic’ failure (defined as a failure mode which causes instantaneous and irreversible damage).Unlike semiconductors, tubes do however, ‘wear out’, which means that they have a finite operating life.
Operating them under unfavourable conditions or beyond the manufacturers working specification, usually results in a reduced lifespan, although this may not be audibly apparent in their operation. In audio applications, the majority of seriously damaging problems lie in the power output stages.
Introduction to Tube Amplifier Operation
The majority of amplifiers use at least two output tubes in a circuit configuration known as ‘Push-Pull’. Basically, this means that the audio signal is split into two halves by preceding circuits. Each of the two ‘Split’ signals now drives one output tube.
The signals then undergo power amplification by the output tubes and are re-combined by a special audio grade transformer which also drives the loudspeaker. Please note that, although there are many different types of tube for various applications, the vast majority operate on the same basic principles.
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