[Radio Wrangler]

A quick look at the anode characteristic of the EF86 in pentode mode and low current shows an anode curve slope of about 2.4 Megohms can be achieved. With a Gm of 1.8mA/v this would imply a gain of 4320 would be available with a perfect current source biasing the valve, and an infinite impedance load.

This is clearly silly.

At the low current for the flat anode characteristic, Gm will be appreciably reduced to maybe 0.5mA/V so that gain is down to 1200.

But this is still assuming there is no load on the thing and so it is doing no useful work. The Zin of the following stage will bring the gain crashing down 220k will give us about 100 gain.

If the impedance is left high enough to get more gain, the effect of stray capacitance will bring the -3dB frequency point crashing down. Heaven help us if it's driving a bit of screened wire to apower amp, or a Baxendall tone control feedback network.

So, yes, very high gains are possible from an EF86, but under such impractical conditions that you can't really use them.

There is also the issue of the control of the anode voltage with constant-current source anode bias. There will be contention with the large anode impedance of the valve alone, and very poor control of the quiescent anode voltage.

David