Output tubes, using unusual ones.

[Shrapnel]

What would it take to use an EF86 or some other preamp pentode, as a low power (SE, pp) output tube. i.e. < 1W (probably <.5W)?

Any other candidates for such a design idea? Franken's situation has got me thinking of having something relatively small and low power for occasional use when I don't want or need even close to 18+ watts of power.

[PRR]

> What would it take to use an EF86 ..., as a low power (SE, pp) output tube

To get "full power": a high impedance load.

EF86 ratings are 1W 300V and 6mA max. So to tap 1W you have a zone between 300V 3.3mA and 166V 6mA. The optimum SE impedance will be about these ratios .... 90K to 27K. High-impedance transformers are rare and have poor high-frequency response, often poor bass too.

And a EF86 will barely pull 6mA at 160V on screen (and you don't want an additional supply to jack G2 above plate supply). As a quick sort-out: find the triode-connected plate resistance and double it, that's about the minimum load it will pull. Something like 40K for EF86.

Being 1W plate limit, the ideal SE sine output is 0.5W, a "good" power pentode will do 0.4W, and at 300Vp 150Vg2 90K(!) load the EL86 may come close to this.

You do get great sensitivity: 2V peak grid drive. It would play guitar with a single triode ahead of it.

6AU6 is a significantly fatter pentode, and cheaper. Sensitivity is less and it wants a 2-triode or a pentode driver to play guitar.

Since these tubes are not scaled for high peak current, the P-P output is maybe double SE, and the load impedance is higher yet (though you don't have unbalabced DC in the OT which helps).

There are scores of one-watt power tubes from AM table radios and they sell very cheap. They were scaled for low voltage and low impedance (2K) loading. If you don't need the whole Watt, drop the voltage or raise the impedance.