Several types of triodes tubes that are popular in audio preamplifiers were tested under typical operating conditions to determine the relationships between noise and operating conditions. In most cases the total noise in the audio band was dominated by the flicker effect. Most triodes exhibit an optimum anode current at which the equivalent input-noise voltage reaches a minimum, and this current value is relatively consistent between samples of the same type. In general, high-gm triode types are likely to exhibit superior noise figures compared to low-gm types for a given mean anode current. Reduced heater voltage tends to reduce equivalent input noise at small anode currents, 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.
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https://aes2.org/publications/elibrary-page/?id=17071
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