March 21, 2003

Simplest Push-pull Tube Amplifiers

      The key word here is “simple,” not “best,” as simple amplifiers often involve complex problems. Simple amplifiers usually make extra demands of the rest of the system and their performance is usually compromised in a few key specifications, such as output impedance, input impedance, frequency response, and distortion. Still, aside from the quicker assembly and lower cost, there is something special about a simple amplifier’s sound, something right sounding. Maybe simplicity is what has all along helped tube gear sound so good.

      The simplest push-pull amplifier is one that only requires two active devices. The amplifier at the right uses an input transformer to split the phase and provide signal  gain. The output transformer provides impedance matching between the output tubes and the loudspeaker. With only five parts, this amplifier is as simple as it gets. Yet a great deal of complexity is hidden: the transformers are anything but simple, as no transformer lives up to its ideal. The wire used has resistance; the core, hysteresis; and the two windings, capacitance. Let’s add some leakage inductance to really spoil the party. In other words, we are going to have to pay, either for a quality transformer or in performance.

     Since we are not likely to get rid of the output transformer in a tube amplifier, we should focus our attention on eliminating the input transformer. The two amplifiers shown below do just that. The leftmost amplifier uses a center-tapped choke to split the signal’s phase. In other words, it takes the input signal and creates a complementary inverted signal to drive the second triode. The rightmost amplifier relies on a balance input signal to drive both output tubes. In both amplifiers, the input transformer’s signal gain has been shifted back to the line amplifier, as the line amplifier must now provide sufficient gain to drive the output tube to full output. Depending on the output tubes, this increased demand for gain can easily be excessive (150Vpp), thus a high-transconductance output tube such as the 2A3, EL34, EL84,  or 8417 should be used. (Actually, even the input transformer version also makes great demands on the line amplifier, as the output tubes’ input capacitance is effectively increased by winding ratio squared, which means that while the output voltage swing is small, the line stage’s output current at high frequencies is greatly increased over a conventional amplifier.)

B² Spice A/D Circuits

    Download circuits that accompany the article below.  

Text Box: The Journal of Tube Audio Circuit  Design
Text Box:  < Back
Text Box:     Next >           Copyright © 2003 GlassWare           All Rights Reserved

More š

     In the schematic at the right, we see

The 12AU7 in a totem pole arrangement that results in a final gain of half the mu of the tube used. In this case, about 8. We also see an optimal White cathode follower that offers half of the output impedance of a single cathode follower. In this case, about 60 ohms. The PSRR is much better than 6 dB figure that a casual inspection implies, as the optimal White cathode follower presents half of the power supply noise to the bottom 5687’s grid, which matches the half of the power supply noise that the top 6587 sees at its grid. In other words, the noise cancels at the output.