Vacuum Tube Mixers
Page Two (2) of Three (3)

This is a short section to illustrate the use of a 12AT7 Dual Triode Vacuum Tube in a Mixer Circuit. We start off with a simple mixer circuit utilizing this "Dual Triode" Vacuum Tube. The 12AT7 has the equivalent of "two" Triodes in "one" tube package. The plate voltage requirement is still 90 Volts D.C. The filament however requires 12 Volts.


The Simple tube mixer
Using the 12AT7 Dual Triode Tube

       
Shown here is a common alternative to the simplest tube mixer circuit and then its improvement. Each input source finds its own triode and all of the plates are tied together. This variation provides the needed isolation, but fails to provide an unity gain output. The obvious fix, reducing the plate resistor value until the gain drops to unity, adds too much distortion and too much power supply noise at the output. Fixing these problem compels a negative power supply. A negative rail allows for a much larger valued cathode resistor. With this new cathode resistor value, the gain drops to unity, the distortion disappears, and the power supply noise drops out of equation (with a small added twist that is).

       If plate and cathode resistor equal each other, then the anti-phase noise on the negative power supply rail is summed with the positive rail's noise at the plate and they cancel. But if these resistor match in value, the necessary voltage relationships within the circuit cannot exist. Adding one bypassed resistor completes the trick. This resistor's value is the same as all the cathode resistors placed in parallel. Thus, if eight inputs are needed, eight triodes and eight cathode resistors (for example, 80k) are also needed along with one 10k resistor.


A Simple 4-Input 12AT7 Tube Mixer

       
If you are thinking that what we actually have eight split-load phase splitters working into one common plate resistor, you are right. The functioning is the same and final use differs. Finding what is common in what is apparently different is the key to understanding tube circuits (and life).

One disadvantage to this circuit is the need for so many triodes. How do we retain the desired functioning of the mixer and use fewer tubes and no feedback? The following circuit uses only one triode and no feedback.



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