Valve Amplifier Push-Pull Output Stage Operation Explained

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Valve Amplifier Push-Pull Output Stage Operation Explained: The Push-Pull Output Stage: When large amounts of output power are required (more than about 15W) the push-pull output stage becomes the order of the day. This arrangement allows the valves to be operated in Class AB, which is considerably more efficient than Class A (up to 80% compared with up to 50% respectively) reducing the amount of power wasted as heat in power valves.

Not only this, but Class AB also allows us to operate the valves at much higher anode voltages and the load line can actually go beyond the maximum power dissipation curve. In this way the peak power level can actually exceed the combined maximum anode dissipation power of the two valves!

Push-Pull Output Stage
Fender Deluxe Circuit Diagram

Balanced Push Pull Amplifier Circuit

A perfectly balanced push-pull stage will cancel all even harmonic distortion and sum odd harmonic distortion generated within the power stage. However, guitar amps rarely use matched valves and usually have a less-than-perfect phase inverter. (Even if the phase inverter is perfectly balanced it will inevitably go out of balance when the power stage is overdriven.) This is desirable for the retention of even order harmonics, providing distortion with both warmth and ‘growl’.

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An output transformer designed for Push-Pull Output Stage operation has a centre tap. The HT is applied to the centre tap and either end of the primary winding is connected to the anode of a power valve. In this way, current flows in opposite directions through the output transformer. If these currents are equal they will cancel out and this is why push-pull output transformers can be made much smaller than single ended ones with the same power rating- there is (almost) no standing current in the transformer so no air gap and less iron is required.

The Push-Pull Output Stage valves are driven with identical but out-of-phase grid signals. When one grid goes positive, the other goes negative. When one valves conducts more, the other conducts less: one pushes, the other pulls. The output signal developed in the transformer is the difference between the two AC currents.

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