Bias tremolo in a SE amp?

[martin manning]

Let's see if we can dig a little deeper... Tremolo is where the output signal is modulated in amplitude at a low frequency ca. 5Hz. In both the class A and class AB cases this effect can be achieved by shifting the bias point, but the effect on the signal (the way in which the output level is modulated) is different.

In a push-pull amp operating in class AB, modulating the bias reduces the signal from the middle by pushing the amp from class AB into class C, where the output tubes are conducting for less than a half cycle. Both phases are reduced simultaneously. If the bias modulating signal is large enough it will also cause clipping on the positive swings. At low guitar signal levels where a class AB amp is operating in class A, shifting the operating point causes the two phases to be clipped on opposite swings (with respect to the output), pushing the operation from class A towards class AB and eventually to class C to achieve a modulated output signal level.

In a purely class A amplifier, the LFO voltage shifts the center point of the signal such that the output will be clipped on one side and then the other, reducing the signal amplitude from one side only on each excursion. This method has been applied to preamp stages as well as to output stages as shown in the examples in this thread.

[Firestorm]

Martin,
I appreciate your continued interest in this arcane topic, but I don't think I've been able to well articulate the thought that initially interested me. Let me figure out how to restate the question and we'll revisit it.