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Old 15th Apr 2021, 9:03 am   #2
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: Single ended Class a Op stages.

Class A output stages run with a standing current.

When you apply a signal, the valve swings up and down in current, and the anode voltage goes up and down, too.

As you drive the valve harder, you run into clipping. The valve can't pull its anode down any further than the 'bottoming' voltage, which is generally about 25V for pentodes and beam tetrodes, so you get clipping on this half-cycle at this point. And on the other half-cycle the current swings to zero, and can't reduce any further, so you get clipping on that half-cycle too.

For symmetrical clipping, you want the clipping on each half-cycle to kick in at the same input level, so nothing is 'wasted'. This is the condition for getting maximum undistorted output power out of the valve.

So if you have 290V HT, and a standing current of 0.1A, then the downward swing is going to be (approx) 250V, because 25V is lost across the valve, and 15V is lost across cathode bias resistor. And the upward voltage swing is, symmetrically, 250V too, as the current reduces from 0.1A to zero.

This means your optimum load is 250V / 0.1A = 2,500Ω.

And maximum output power is Vrms x Irms = Vpk x Ipk / 2 = 250 x 0.1 / 2 = 12.5W.

The input power to the stage is 290V x 0.1A = 29W so efficiency is 43%. Of course, there is also the screen-grid current (and heater power) so the total power draw is probably nearer 40W. And the 12.5W output is also going to be a bit reduced because the output transformer isn't ideal - it will have a bit of winding resistance, so when the output transformer has instantaneous 250V across its primary, about 7% of this in a typical transformer will be absorbed by winding resistance so the load thinks it's only 233V. But, hopefully, you get the idea.
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