Phase inverter - a circuit that generates two output signals, each 180 degrees out of phase with the other. This is a bit of a misnomer, since it does more than just invert the phase of a signal, it actually generates two out of phase signals.
Q: Do I need to replace my driver tube every time I replace my power tubes?
A: No. This is an "old wive's tale" propagated by the tube vendors who want to sell you more tubes. The "driver" tube, or phase inverter as it is more appropriately known, is under no more stress than any of the other preamp tubes. The output stage of a normal guitar amplifier is AC-coupled, class AB1 or class A1. The "1" suffix indicates that the tubes do not draw grid current during any portion of the input cycle. A "2" suffix indicates grid current during some portion of the cycle, as in class AB2. Since almost all guitar amps are class AB1 or A1 (it takes a DC-coupled cathode-follower or interstage transformer to be able to drive the power tubes into the positive grid region), the phase inverter does not have to supply any current to the grids (it can't really source current anyway - it only sinks current through the plate load resistor, which is quite large, and inherently limits the plate current to a few mA). The AC coupling (capacitor coupling) between the phase inverter and the output tubes precludes any flow of grid current anyway. The output tube grids will merely act as a grid clamp, shifting the bias downward as the output is driven harder. This in no way stresses the phase inverter tube. In addition, the plate load resistors and the bias current in a typical phase inverter are identical to the preamp stage values, indicating that they are operating in the same area, dissipation-wise, so they cannot be "wearing out" any faster. In fact, the reverb driver tube on a typical amplifier is dissipating more power than the phase inverter, and should be replaced more often, if anything. Don't be misled by the higher plate voltage on the phase inverter either, because the cathode is usually sitting somewhere around 30V - 100V above ground, which lowers the plate-to-cathode voltage by that amount. The plate-to-cathode voltage is what determines the power dissipation of the tube, not the absolute plate voltage.
The Marshall/Fender phase inverter is commonly known as a "long-tail pair", or "Schmitt" type phase inverter, or phase splitter (actually, the original Schmitt inverter was a differential pair with a large "tail" resistor; the "standard" guitar amplifier phase inverter is a self-biased version of this circuit that works better with positive-only power supplies and ground-referenced inputs).
Following is a schematic diagram of a typical phase inverter found in some guitar amplifiers:
http://www.aikenamps.com/" onclick="window.open(this.href);return false;
The basic circuit is commonly known as a "differential amplifier", which means that it amplifies the voltage difference between the two grid inputs. Technically, it is a differential in, differential out amplifier, because it has differential inputs on the two grids as well as differential outputs on the two plates (the two plate signals produce the same voltage signal, but one is inverted, or 180 degrees out of phase, with respect to the other).
It should be noted that there are actually three inputs used in this type of phase splitter. The first input is the obvious one, the left side of C1. The second input (the lower end of C2) is useful as a feedback input, a reverb or effects return input, or as a second channel input. In the circuit shown above, the second input is used as a feedback return input, taking the signal off the junction of the feedback divider.
The third input is not so obvious; it is the lower end of R6. If a signal is input at this point, the phase splitter will produce an output signal on each output that is in phase with the other, rather than 180 degrees out of phase, and also in phase with the signal input at the lower end of R6. This means that if a signal of equal phase is applied to the first input (C1) and the third input (R6), it will subtract from the out of phase output (R2) and add to the in phase output (R1). Likewise, if an equal phase signal is applied to the second input (C2), and the third input (R6), it will subtract from the in phase output and add to the out of phase output (this is because the out of phase output is actually in phase for the signal applied to the second input, C2, and the in phase output is out of phase). This third input is useful for balancing the feedback signal by subtracting from the in phase output and adding to the out of phase output in order to compensate the unequal gains to each output from the feedback input. The gain is much less than the gain into the first and second inputs.
The two outputs provide (nearly) identical signals, except for a 180 degree phase difference between them. This is exactly the type of signal needed to drive a push-pull amplifier, so this circuit is commonly seen in higher-power guitar amplifiers.
http://www.aikenamps.com/" onclick="window.open(this.href);return false;
