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RE: No worries

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Noise is still a tough issue. The literature is dominated by radio frequency noise, and most low-noise tubes are designed for radio frequency service. But 1/f noise, a.k.a. flicker noise, usually dominates in the audio realm. Hardly any tubes were actually designed to minimize this, and the physics were never fully understood before the advent of transistors - at which point the best and the brightest lost interest in tubes.

The EF86 is one of very few tubes originally developed for small audio signals - low microphonics, coiled heater to allow AC heating, and other features. These tubes do not feature high transconductance, which is the main factor in low noise at radio frequencies. I'm not sure why, perhaps microphonics are a problem with close cathode-grid spacing(?). Anyhow, shot noise and partition noise would not be too bad with the EF86 for phono use if they are the dominant audio frequency noise problem. As proof, we have used EF86s in the Tape Project repro amps quite successfully and tape head output runs about 2mV.

However, we have had to be picky about the particular EF86 used. They have been made by many factories and the best NOS is heavily picked over. We're still trying to understand the state of the art for current production. I have reasonably good hopes that we can find some reliably-quiet current production, but if that proves impossibly I'll just keep looking for a better tube. I already have three candidates in mind... :^)

Of course, what would be REALLY fun would be to commission a new EF86 design, rather than just refurbishing or copying the old tooling. I think it can be done for less than a quarter-million dollars. Doc, how's that R&D budget coming along?

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