tiny 0.2w speaker

[firemedic]

Does anyone know which tube I could use to drive an itty bitty 0.2W 16 ohm speaker? Maybe from paralleled 12AU7 cathodes or something?

[sluckey]

You could use almost any amp, even a twin reverb. Just put a 1K (or higher) resistor in series with the speaker and pretend it's a headphone speaker.

[HotBluePlates]

On a side-note, driving the OT from 12AU7 cathodes would deliver the same power as driving from 12AU7 plates. That's because cathode current and plate current are equal.

However, because there's no amplification from grid to cathode, it would take a larger driving signal to the 12AU7 grid to get the power output. There are some gimmicks that can be done, within limits, like reducing the load impedance presented by the OT. That said, in general a good plate load is the same as a good cathode load, as far as output power goes.

[firemedic]

I had envisioned a very small tabletop amp, perhaps without an OT. I figured cathodes would be a closer impedance match. 

[sluckey]

Impedance matching is useful for maximum power transfer. I wouldn't think impedance matching would be much of a consideration when driving a 0.2W speaker.

[HotBluePlates]

I had envisioned a very small tabletop amp, perhaps without an OT. I figured cathodes would be a closer impedance match.

Output-transformer-less (OTL) amps are a special problem.

Speakers measured in 10's of ohms need relatively low voltage, but high current, compared to typical tube parameters. OTL usually takes cathode loading of multiple parallel sets of high-power triodes to deliver enough current into the speaker.

0.2w seems small enough to be insignificant, but you ought to run through the numbers to see if that's true. In one form of the equation, Power = Voltage²/Resistance. If power and resistance (impedance) are known, that's Voltage = √(Power * Resistance)

Voltage = √(0.2w * 16Ω) = ~1.8v RMS
1.8v / 16Ω = ~112mA RMS

I'm pretty confident no 12A_7 can sustain 112mA (or even half that), so OTL operation is a no-go.