[Radio Wrangler]

OK, they're easy enough and make output stages a lot more understandable.

Take a graph of characteristic curves for you EL84 (for example) from one of he better data sheets.

Anode voltage is scaled horizontally, increasing to the right. Anode current is scaled upwards, more current is higher.

On this graph there is a nested family of curves, all starting at 0 volts and 0 mA, climbing as they go rightwards and then levelling-off at different amounts of current.

Let's forget transformers for a moment, and do a simple anode resistor load amplifier.

At maximum current (80mA = twice the quiescent current) we want the anode voltage down around 0v (in this simple example) with the valve at its quiescent current (idling) then we want it to have half the HT voltage on its anode. At zero current there is no voltage dropped across the load resistor, and so the anode must be at the HT voltage.

If we say the HT voltage is 300v, then the resistor must drop 150v at 40mA which means it must be 150/0.04 Ohms = 3750 Ohms.

when it's carrying 80mA the 3750 Ohm resistor drops 300v... exactly as required.

We can plot the anode voltage and currents which are forced by this load resistor on our characteristic curves. It intersects with the anode voltage axis at 300v and it intersects with the anode current axis at 80mA. It is a straight line, and you can now read off where each different curve (representing a different grid voltage) crosses the load line. You can see how the output circuit works.

Now with the transformer output stage, the inductance to the HT says that the average anode voltage is equal to the HT voltage. The anode swings to twice HT voltage at zero current, and to 0v at max (80mA current)

The load line is created by the impedance of your loudspeaker, increased by the square of the turns ratio of your transformer. You now have twice HT swing over 80mA swing

600v/80mA = 7500 ohms load. If you use 8 ohm loudspeakers then you need a turns ratio of Square root(7500/8) = 30.6 to one.

Of course, doing this for real, you'd not take the anode down to zero volts, but pick a limit where the valve is still reasonably linear.

Both my typing fingers now hurt! so it must be time to stop.

David