Noise in Triodes with Particular Reference to Phono Preamplifiers

This paper presents noise data for a number of triode types popular in audio preamplifiers. Triodes were tested under typical operating conditions, and the relationships between noise and operating conditions are examined. It is shown that in most cases total noise in the audio band is dominated by the flicker effect. It is further shown that most triodes exhibit an ideal anode current at which the equivalent input noise voltage reaches a minimum, and this current value is relatively consistent between samples of the same valve type. It is also confirmed that, in general, high-gm triode types are likely to exhibit superior noise figures to low-gm types for a given mean anode current. It has also been found that reduced heater voltage tends to reduce equivalent input noise at small anode currents (<1 mA), but tends to increase it at higher anode currents. A general formula for estimating the equivalent input noise voltage for any conventional small-signal audio triode, at rated heater voltage, is presented.