The Floating Paraphase Circuit

Author: N.H. Crowhurst

The next circuit aims at overcoming this problem. It is called floating para-phase. Instead of using a fixed voltage divider, the voltage divider is arranged to have a self-adjusting action that overcomes, to some extent, possible differences between individual tubes or changes in operating conditions. Instead of using a large resistor and a small resistor, as employed in the paraphase circuit, the tapping point used for connecting to the grid of the second paraphase tube is a junction point of large resistors from the output-tube grids and a third resistor from the bias point.

Floating paraphase circuit

Assume that the three resistors have the same value, 100,000 ohms, and that the tubes have a gain of 10, as before. For the second tube to get its 2-volt grid iuctuation, the current through the common resistor must be 2/100,000 or 20 microamperes. The voltage at the grid of the second output tube will fluctuate 20 volts in the opposite direction, hence the resistor connecting this grid to the common point will have 22 volts across it and pass 220 microamperes. Because the resistor from the first output-tube grid joins this same point, it must have 220 + 20 or 240 microamperes flowing in it, which will produce a fluctuation voltage across it of 24 volts. As the common point is already fluctuating 2 volts in this direction, the total fluctuation at the first output tube grid must be 26 volts.

Floating paraphase circuit with corrected resistance values

The unbalance between output tube grids can be corrected by using a smaller resistance value from the first output tube, so that 240 microamps only drop 18 volts. This requires a resistor of 18/0.00024 = 75,000 ohms, in place of 100,000 ohms.

Now what happens if the second tube changes gain to either 9 or 11? If it still gets 2 volts on its grid, there will be either 18 or 22 volts fluctuation at its plate, so the current in the resistor from the second output tube grid to the common point will be either 200 or 240 microamperes. The current in the resistor from the other output tube grid will need to be 220 or 260 micro-amps, instead of the original 240 microamps. This means that the potential at that grid will be (75,000 X -00022) or 16,5 volts, or (75,000 X .00026) or 19.5 volts, from the grid to the common point. This represents 18.5 or 21.5 volts total fluctuation at this grid, to compare with 18 or 22 volts at the other grid.

With the ordinary paraphase, the grid of the first output tube would remain at a potential of 20 volts, while the second output tube would get 18 or 22 volts. Here, when the second tube gets 18 volts, the first has 18.5 volts; when the second gets 22 volts, the first gets 21.5 volts, which reduces the imbalance. If the first grid continued to get 20 volts, the second grid would get within 19.5 to 20.5 volts with this much change in gain, instead of 18 or 22 volts with ordinary paraphase.