An amplifier with a push-pull circuit consists of a stage with two power tubes–each of which amplifies one half-wave, either positive or negative, of the audio signal.
The special design of the output transformer in the P-40 then enables the correct reconstruction of the amplified signal at the output terminals. The advantage of the push-pull system lies in its ability to produce greater output power than is obtainable from a single-ended design using the same type of valve. This advantage is determined by the greater efficiency of the push-pull system, which translates to lower heat dissipation for any given output power.
The disadvantage of push-pull stages is that since they rely on the amplification of two separate half-wave signals by two separate amplifying devices, they must be particularly well-engineered in order not to encounter significant distortion. The P-40 boasts an impressive specification: 40 watts into a 6 ohm load with full-bandwidth distortion less than 0.2%. This performance is similar to that of a well-engineered amplifier using a single-ended topology.
Note: The glass of P40 and P70 is made in one piece, and not all colors are always available.