Secrets of Home Theater and High Fidelity

Push Pull Amp

To get more power out of a tube amplifier, tetrode and pentode tubes are used, and in a different configuration than single ended, a configuration called "push pull". In this type of amplifier, the output tubes (at least two are required) are utilized by connecting the anode plates to the two wires of the output transformer primary winding. One tube "pushes" the signal, and the other "pulls". Often, several tubes (in multiples of two) are connected in parallel to get more power. The push pull tube amplifier is capable of producing 1,000 watts RMS per channel, plenty for home theater, but this much power will cost in the range of $20,000 for a stereo pair because they are hand made in small quantities.

Push pull tube amplifiers with outputs of 125 watts per channel are available at more reasonable prices, and, if you decide that the tube sound is for you, such amplifiers could be worked into a home theater system. For example, a tube amplifier could be used to drive the front left and right stereo speakers, while a less expensive three channel solid state amplifier (see below) could be utilized for the center channel speaker and the two rear surround speakers. In this system, when playing music, the "bypass" mode could be selected on the surround sound receiver, which directs all the sound to the front left and right stereo speakers, just as in a standard two channel stereo system, and the center channel as well as the rear surround channel would be turned off (muted). Thus, you would have the tube sound when playing CDs in normal stereo, and a combination of tube and solid state for home theater. If this idea appeals to you, work carefully with your dealer to match the best combination of components, and listen to the complete setup before making the final purchase.