Catastrophic Failures. These are failures that occur suddenly, without warning, making the equipment unusable.

Examples of catastrophic failures are: -

• Glass failure, loss of vacuum due to mechanical damage or thermal stress.

• Heater failures open circuit or partial short circuit due to excessive heater voltage or high initial surge current and normal on off cycling over many thousands of hours.

• Arcing, due to low cathode temperature, causing damage to the cathode and grids.

• Bias failure due to component leakage or valve characteristic spreads using fixed or auto bias.

Degenerative Failures. The slow but eventual, deterioration of all tubes, which can contribute to the end of life of the tube.

Examples of degenerative failures: -

• Gas is present in all tubes and if the tube is used within its characteristics should not be a problem, however excessive dissipation can liberate gas from the tube structure and lead to eventual premature failure.

• Getters are patches of evaporated metal, which are deposited on the inside of the glass. The purpose of the getter is to absorb any gas that may be evolved during the life of the tube and work best at normal glass bulb temperatures.

• Spurious emissions are uncontrolled unwanted emissions usually caused by gas released due to excessive dissipation and elevated temperatures.

• Inter electrode leakage. This is current, which flows between the electrodes of the tube, which are not connected in any way. The cause is metallic vapours released by the hot metallic structure of the valve being deposited on the insulating micas and eventually leading to a conductive path.

• Cathode depletion can occur due to arcing, which can strip the cathode coating reducing the active area of the cathode. Arcing will vaporise the cathode material and generate gas, which can poison the cathode material. This can also be caused, by passing excessive cathode current before the heater has reached its normal operating temperature.

Subjective Failures. These are tubes, which will normally pass tests but do not perform satisfactorily due to for example: – hum level, microphonics and noise.

Examples of subjective failures: -

• Hum is an unwanted mains frequency signal, which is superimposed on the wanted signal.  It can be caused by heater to cathode leakage or due to electrostatic or electromagnetic fields within the equipment.

• Microphonics is defined as a signal originating inside the valve caused by mechanical vibrations being amplified by the tube.

• Noise is a signal originating from inside the tube, but not due to hum or microphonics. It can be due to intermittent short circuits, open circuits or arcing due to leakage paths between the electrodes.

Characteristic Variables. These are variations or spreads in the tube characteristics, due to manufacturing tolerances and follow a normal distribution curve.

Examples of characteristic variables: -

• The Standard is usually a tube that complies with the manufactures published data. The Upper and Lower limits are values that any tube within these limits can be considered acceptable for normal use.

• Characteristic Spreads is the degree of deviation from the standard tube.

• Design Tolerances is the normal variations of standard parts, which a well-designed piece of equipment will operate correctly.

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