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It is the most asked question, and for this we refer to the one and only genuine 300B, as made until 1988 by Western Electric in the Hawthorne plant in Cicero, Illinois, USA. We do NOT refer to new made, USA replicas under WE brand, from 1997 onwards.
The 300B tube has less interaction with the speaker system as smaller tubes like the 45 will have. This makes the sound character less dependant on the speakers, and for that reason quite dependant on the tubes itself. We see 300B often used with medium efficiency loudspeakers. Meaning, the tube often needs to work relatively hard, which is indeed what it is designed for.
However, a Single Ended, no-feedback amplifier can not combine highest output power with lowest distortion. The user often comes into a situation where relatively high output power, still requires relatively low distortion. Moreover, distortion should increase as little as possible, during the aging process, and lifetime should be high by itself, even though this tube is loaded often quite hard by the amplifier. All in all, these are not the easiest specifications for a tube in general.
At EML we have contributed to this, by extremely low hum and low distortion of our 300B tubes, over the major part of the lifetime. This is done by our unique filament and grid construction, featuring a real, physical filament center tap, positioned outside of the grid, but inside the glass bulb. We use not only gold grid wire, but we completely gold Plate the grid, after it is made. The clean, gold Plated surface increases bias stability, and good matching between two tubes stays constant like this over the full lifetime.
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