The point is in making a very simple electron tube amp that just needs 12V supplying both heaters and anodes.
A high Megohm resistor from anode to grid can be used to neutralize the grid for a healthy anode current at low anode voltage. The principle is described on the Pleiades V series preamp euroelectron posts.
The tubes may be triode connected and an output transformer connected between the plate circuit and speaker.
The cathode can be connected directly to ground.
A coupling capacitor is needed to connect the signal to grid so as not to upset it DC wise.
How would this amp sound like when overdriven by a guitar or bass guitar signal?
The 12K5 can of course be used too and there are guitar amp schematics on the net. The 12K5 is excellent for directly driving high impedance and not only headphones as is done on the Pleiades Elektra II.
The 12SK7 is vari mu which is a good thing for low voltage operation.
The EF183 vari mu pentode is used using the above mentioned grid neutralization on the Pleiades ultra low noise Pleiades microphone V series preamps operating with typically 3.7 volts.
The EF183 is deliberately underheated and this was found to further increase the signal to noise ratio.
The VF14 on the Neumann U47 microphone is underheated too.
The PL84 will be underheated too if operated with 12V at its heaters.
An advantage of the PCL86 is that it has a triode too inside the same glass envelope which may be used as a preamplifier.
If a stereo amplifier with PL84 is built, the power is increased as follows. Since we have 2 heaters to drive in series, the supply can be 24V. Therefore two 12V batteries can be used. Having 24V available for the anodes should be great and by the formula for power V squared over R the power output should be 4 times greater. And since we have 2 amps the power output should be 8 times greater.
For more difficult projects 2 UL84 could be used with 48V derived from more batteries in series. Li-ion batteries if used carefully should be great too.
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