Figure 4.13 shows a simple 300V regulator from a Tektronix 575 Transistor Curve Tracer. It is the same topology as figure 4.8, except that a pentode is used. R607 and R609 are "grid-stop" resistors to protect against parasitic oscillation. R611 is a 10-watt power resistor that shunts part of the current around the pass tube. If the load current is stable and predictable, this technique can help reduce the power dissipation in the pass device.

The +225V supply from a Tektronix 536 oscilloscope is shown in fig. 4.15. The comparison circuit is a differential amplifier using a 12AX7, followed by a 6AU6 pentode. This gives both a stable differential input and high loop gain. R640 is a 20-watt resistor that takes some of the load off of the 6080 pass tube. R642 and R643 is a voltage divider to allow the high plate voltage from V646A to match the grid of V634. Some loop gain is lost, but is more than made up by the additional gain stage.

Series Regulator Frequency Response
   Notice that there is an electrolytic capacitor on the output of each Tektronix regulator described above. These are present because tube regulators with feedback have a finite high-frequency response. Beyond a frequency determined by the amplifying stages in the feedback loop, the regulation performance declines and the output will start to vary under high frequency loads. Adding a capacitor is a simple technique, since the capacitor can be chosen so that its reactance starts to become low (shorting output variations to ground) at about the frequency the regulator starts to poop out.
    Another benefit of an output capacitor is that it can serve as the "dominant-pole" in the circuit and damp out tendencies for oscillations. Since the series regulator is a feedback circuit, it is prone to the same oscillations and phase-shift problems that feedback audio amps have. If a careful analysis of the regulator stability has not been done, putting on a big output capacitor is a brute-force way of solving oscillation problems. Since one of the motivations for using regulators for audio circuits is to eliminate the evils of electrolytic capacitors, it is important that the regulator have enough high-frequency response so that not too high a value output capacitor is needed.
    Calculating the regulator response can be fairly complicated, especially since it can depend on parasitic capacitances that are hard to predict. The easiest way to test a regulator is to either sweep its output with a network analyzer or to pulse the output with a load and look at the voltage response. In a future installment, these test methods will be further described.

The +100V supply from a Tektronix 502 Dual-Beam Oscilloscope is shown in figure 4.14. It is similar to the previous regulator, except that a triode-pentode tube is used. This makes the comparison device a Differential amplifier, which lowers the offset voltage and makes it much more stable, since variations common to the two sections of the tube (such as aging or heater voltage) are essentially canceled. A Differential amplifier driven with a single ended signal, as in this case, cuts the amplifier gain in half. However, since the pentode has a very high load resistor, the overall gain is still reasonable. The inclusion of R669, near the grid of V666B is a technique for allowing C671 to stay small in value . However, when repairing these oscilloscopes, I've found that sometimes the 6AN8 develops grid current, and the high impedance in the grid circuit causes the output voltage to drift. A lower resistor and higher capacitor value solves this problem.

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