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I Made a Mistake Once...

Dan, I ran the simulation at 5 kHz. There is sufficient drive at the 211 to create grid current peaks of more than 30 mA. I examined AC (short-term) and DC (long-term) voltages both with and without grid current being drawn. I also ran the sim with various values for transformer inductance, bypass capacitance and bias supply resistance. This was by no means a static test. :)

OK, with that out of the way, you know how people say "I made a mistake once - it was when I thought I had made a mistake"? Well, I haven't been paying enough attention to the polarity of currents in this sim. Now I think my first statement - that A2 grid current doesn't increase bias supply current - was correct. After studying the sim at length this evening, I now know what happened to make me think otherwise.

The phenomenon I missed is that grid current causes the bypass cap under L2 to gradually charge to a more negative DC value. I hadn't noticed that before, because I was so focused on the current flow. As the cap becomes more negative, it creates a DC current through the bias supply that I misinterpreted as current draw. What I was actually seeing was the negative "overvoltage" at the cap draining off into the bias supply through the -48V low-impedance voltage source. I have remedied this by installing a diode between the voltage source and resistor R2 (see D1 below). With the diode in place, the sim now demonstrates conclusively that there is NO ADDITIONAL CURRENT drawn from the bias supply when the 211 is in A2 mode. That holds true regardless of how long the amplifier remains in that condition. So, now I can say "I made a mistake once, but it was only because I thought I had made a mistake." :)

Ironicaly, all this additional simulation and analysis has pointed out a bias problem you didn't ask about. As I said above, the bypass cap becomes more negative when the 211 draws grid current. Over time, it becomes so negative in fact that the amplifier is no longer drawing grid current. Instead of -48V, the cap will rise to -60V or -70V (depends on drive level). This causes the negative peaks of the signal at the grid to extend beyond cutoff, so the output distorts. Worse, when signal levels subside, the cap holds that charge. It can't discharge into either the bias supply or the grid.

I'll give this situation more thought, but my initial reaction is that maybe a regulated bias supply is needed after all. It doesn't need to provide much current (uA only), but clearly, some form of shunt regulation is needed to maintain the correct voltage at the bypass cap.






Edits: 09/08/14 09/08/14 09/08/14 09/08/14
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