Don Black <donald_black@...>

If we're serious for a minute and take a flight into fancy (is that an oxymoron?), the only obvious circuit with just the tubes is a triode (or pentode perhaps) using the input pot (volume control) as grid return, zero bias and the headphones wired as the load in the plate circuit with its current flowing through it. For that a "normal" tube running 12 volts B+ could work. Since most of the power (7 watts) would be for the heater it suggests a power tube. A pair of 6AQ5s would be less than 7 watts while two EL84/6BQ5s would be more. They look like 7 pin tubes so 6AQ5s may be a good bet. From its triode curves it would draw about 2 mA with 12 volts and zero bias, though the voltage drop across the phones would reduce it slightly.
I don't think I'd like DC through my expensive headphones.
The frequency response would be interesting too, as it's claimed into MHz. At some point the phone inductance would resonate with tube and stray capacitance and the plate resistance is more than the headphone impedance. I'd guess the inductance would give a rising response too.

Don Black.

On 30-Dec-13 8:30 AM, Michael A. Terrell wrote:

snapdiode@... wrote:
> Ah, a 12V battery. "Starved tube" operation.

There was a series of tubes developed to operate normally with 12
volts on the plates. These were for car radios in the late '50 to early
'60s when transistors took over that field.

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