04-11-2016, 05:58 PM
(04-11-2016, 03:11 PM)Diabolical Artificer Wrote: I thought to design a voltage amplifier stage we need an OP whatever the following valves g1 bias is + a bit extra for losses due to following g1 resistor and valve capacitance etc.
Sometimes. If the output stage is a triode, probably yes. But not in this case.
What you need to do is to provide enough drive to the OP valve to swing it from double the standing Ia, to zero (so you get the same swing each side of the standing Ia). If it's a pentode, the anode current should be reasonably independent of anode voltage, so you don't need to take much notice of the load.
If the quiescent Ia is 40mA, say, you just need to check what the grid voltage would need to reduce to, to give 80mA, and make sure it's still negative. The EL84 might, for example, be biased at -7V for 40mA, and swing -2V to -12V (5V either side). There's no point in trying to take the grid 'up' above -2V because on the other half cycle you're already touching zero current, so about to hit serious distortion. But if your grid voltage for the double-quiescent, 80mA, happens to need to be positive, you're going to get distortion due to grid current. You'd then not be able to take advantage of the full swing in the other direction towards zero Ia, and so you need to rethink your operating point (less Ia, more bias). And then, you'd need to recalculate your optimum load (approx. 0.9 Va/Ia, as earlier posts).
If you use an ECC83, yes, you would have more gain than you need. You can 'lose' this gain with a bit of NFB, gaining a reduction in distortion, or by adding tone controls, or just by turning the volume knob down.
Mark Hennessey is dead right, the stage which goes into overdrive first should be the output stage. Otherwise you just aren't using all its capability. And of course you want to make sure you don't 'waste' its capability bu having it go into overdrive on frequencies you can't hear - so the frequency response curtailment should be done in earlier stages.
