Froumy wrote:The higher the Zener, the more crossover distortion you should get. If you clip it right at Bias voltage, I don't see any way that you could be experiencing crossover distortion.
If you clip it right at the bias voltage - today...
... tomorrow, or at another place you have 5 volts more from the wall, means 15...20V higher B+, and of course 2...3 volts more negative bias voltage - actually - but it's cut off by the zeners in this case.
Result: Your tubes are running too hot, because the higher B+ wouldt require more negative bias, but the bias now is *regulated* by the zeners, which don't allow more negative (as it wouldt be due to the higher wall voltage)
So better go at least a few volts higher with your zener's value in respect to your bias voltage!
Another reason for this - IMO still more important:
Keep in mind, that your output tubes still aren't in cut off at idle at the bias voltage the amp is adjusted to! They still are drawing current!
So allow the tubes - with the right choice of the zener - to go a step further to cut off while the negative signal cycle, riding on your pre-adjusted bias voltage. After my understanding, the zener shouldt only avoid an unneccessary low bias voltage below the tube's cut-off point - but the zener shouldt allow, that the tube still *reaches* its cutoff point.
The only reason of the zener is - as Randall already has pointed out in-depth - that when the signal swing on the bias voltage is so high, that the positive cycle reaches the zero volts point, where it can't 'grow' further through the roof, by still larger signal swing only the negative part of the cycle can go deeper and deeper into the still more negative direction.
The positive part is clamped on the roof at zero then (or just a few volts above the roof), but the negative part is steeping deeper and deeper into the negative area - with the result, that the DC/AC mixed bias voltage then is creating itself 'a new center line', another average bias voltage, which only can be more negative, then you'd adjusted before!
Hence the from itself created new bias voltage is more negative, the amp is running cooler with crossover distortion now.
And I still see another reason, why you shouldn't limit the bias voltage exactly at its adjusted value by the zener:
Keep in mind, that any class AB push-pull power stage at low input levels is operating in (true) class A - first when the input signal swing is increasing, it's sliding over into class AB mode. With a zener's value spot on the bias voltage level, the power stage will be forced, to operate in somewhat like a double SSB modulated SE mode and some signal cut out might appear.
Hope, that you all can understand, what I wanted to say with my weak German's English!
Still let me point out, that all what I wrote above I haven't verified by myself yet - it's just a theoretical based consideration!
Maybe Randall can chime in, to confirm or to deny it.
Larry