This variation (on the circlotron topology) is functionally equivalent to the cathode-follower totem pole output stage. Unlike the simple circlotron, this topology needs signal-level compensation for its top and bottom tubes, as both top and bottom tubes must see signals of differing amplitude. This is the result of switching the right tube's signal reference from ground to the output. Why bother? This topology, like the simple circlotron, offers two advantages over the cathode follower totem pole: all the output tubes can share the same heater power supply and the same negative bias power supply.

This topology is the classic transformer-coupled output stage that 99% of all tube amplifiers use. The output transformer works as an electrical lever, reducing the output voltage swing, while increasing the output current swing. This electrical lever greatly increases efficiency and lowers both distortion and output impedance, as it matches the tube's impedance to that of the loudspeaker (it also protects the loudspeaker from tube meltdown.)

This topology is fundamentally identical to the one above, differing in the means used to establish a negative bias voltage. Often referred to as "auto-bias," the cathode-biased amplifier usually results in a safer and softer clipping amplifier than its fixed bias brother, as the bias voltage self corrects excessive current conduction by forcing the cathodes positive, making the grid voltage effectively more negative.

This topology is seldom used, in spite of several real advantages, such as an amazingly low output impedance and distortion figure. The same impedance matching magic that obtains in the grounded-cathode transformer coupled amplifier applies here doubly, as the impedance is matched to cathode follower outputs. So why isn't this topology more common? High input signal levels:  brutally high, say +/- 400 volts in some amplifiers.

www.tubecad.com           Copyright © 2003 GlassWare           All Rights Reserved

More Details ð

In TCJ Push-Pull Calculator, selecting a topology only takes a mouse click. Then the variables are assigned new values. And, lastly, the "Calculate" button is clicked.

TCJ Push-Pull Calculator does the rest. It simulates the output stage and then gathers and calculates 31 data results, which it displays on the "Results" page, for example gain, output impedance, 3rd and 5th harmonic distortion, power output, plate dissipation and idle current. But there's more results to view: seven graphs.