How a Push-Pull Works

The Push-Pull topology is special arrangement with tubes often found hi high-power output stages due to his efficiency and characteristics.

 
A typical Push-Pull circuit

The Push-Pull topology is special arrangement with tubes and transformers which permits power and precision.Each channel is composed of

  • A loudspeaker Connected to a transformer and which transforms electrical current to sound, .

  • An output transformer Comprising three bobbins (one for each tube and one for the speaker). The transformer transforms the high-voltage low-current of tubes to the low-voltage high-current needed by the speaker.

  • A high voltage power supply Supplying tubes.

  • Two tubes One for the positive part of the signal, the other for the negative one.

Tech PP 3D
 
  1. A Left tubeTo handle the positive part of the signal

  2. A Right tubeTo handle the negative part of the signal

  3. Right tube's current

  4. Left tube's current

  5. Supply current

  6. Output transformerTo adapt the high-voltage and low current of tubes to the low voltage high current of loudspeaker

 The output transformer is special in that it has 3 bobbins : one for the loudspeaker and two for the two tubes :

Transfo PP
The working of a Push-Pull

Now that we know from what a Push-Pull is made of and how every elements are connected, we can analyze the working in three differents states, depending on the incoming signals:

When no signal (idle)

HowPPWork State IDLE

Thes currents drawn from the tubes are equal and steady. They are in opposite direction, thus producing opposite magnetic fields which cancels each other. As a result, no current in the speaker bobbin is produced. There is no sound.

Normal signal

HowPPWork State SIGNORM

Depending on the signal, one current in a tube is superior to that of the other tube, producing a positive or negative magnetic field in the transformer. That field produces current in the speaker bobbins, and the speaker produces sound.

Strong signal

HowPPWork State SIGSTRONG

One tube does not draw current anymore. The tube driving circuit compensate for this lost and makes the other tube to draw much more current.

Below is an illustration showing output current and each tube's current :

HowPPWork Signaux Legend
  1. Output currentOutput current from the transformer to the loudspeaker

  2. Right tube's currentThis curent is of same shape as the output

  3. Left tube's currentThis curent is of same shape as the output but inverted

What is the bias ?

The bias is the amount of current flowing through each tubes, when idle. When tubes produces sound, it is still present, because the current produced by the signal representing the sound adds itself to the bias. The bias is a critical parameter of the amplifier, affecting quality, tone, longevity and many other aspects. You will see the importance of bias in the next pages.