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A term used to represent an equation of one aspect of a vacuum tube’s efficiency. It is the quotient of the amplification factor (mu) of a tube divided by the plate resistance, and is measured in micromhos. Many times the most accurate test of a tube’s useful life is a transconductance test; all tubes have a rated value at which they should be operating that will gradually decline throughout a tube’s life. Although a tube will still perform at transconductance levels below its suggested rating, the level of its performance (and audio quality) will begin to suffer.

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