Ther may be a few engineering or scientific reasons.
Perhaps the most important of all is that one cannot say in the middle of a recording session, "I'm off to the super market to buy a freshly charged 3.7 volt lithium ion battery". As a last resort in case a charged AA 1.2V battery which is very common is not available a 1.5V battery is readily available from everywhere.
Other reason is that the low anode potential produces a low anode current which is turn produces a high plate resistance which in turn produces a high cutoff frequency (interacting with L of output transformer primary) which in turns compensates various effects. Mostly objective bass increase by directional microphone proximity effect. And subjective bass increase by equal loudness curves and voice effort curves, [Lowe, Morgan].
In very low plate voltage domain there should be almost complete absence of secondary electron emmision effects, ionization and other source of noise. The too low anode current on the other hand may not be ideal. Nevertheless so far a loud ticking clock could be captured by the system with the MD441 feeding the V6 booster amp, mic in other room about 30 feet away from the loud ticking clock. By loud we mean that the bare ear could just hear it at 30ft. Same plus hiss plus plenty of ambience was achieved when wearing the HD580 headphones and rec level volume was turned up on the Sony TC-D5 Pro fed by Pleiades V6. The sound quality at singing voice is very big, natural and detailed.
Another good news of very low 1.2V plate potential is that the rail is small so the electron tube sound is possibly there with just one stage of amplification. How would the amp sound with a ribbon mic on drums?
More tests, experiments are needed.
Reference:
(Flat frequency response from producers brain to listener's brain), Sound Picture Recording and Reproducing Characteristics - D. P. Lowe, K. F. Morgan - Journal of the Society of Motion Picture Engineers
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