no regulator, whether low or high voltage, is there yet. In fact, high voltage regulators tend to be inefficient, big, complex, expensive, fragile, and much poorer in performance than low voltage regulators. Fairly depressing. But like so much in this life, through much effort we gain what is worthwhile. In regard to a high voltage regulator, we want above all a low and flat output impedance. The lower the output impedance, the better the regulator is rejecting perturbations to its output and the better the circuit that is being fed this clean B+ voltage can function. If a super low output impedance is not possible, then let it at least be flat in frequency response. A regulator that only performed from 600 to 1200 Hz would probably be worse than no regulator at all, as it then would add a frequency selective skewing of the output from the amplifier.

In addition, just as we want clean windows, we want a clean output, free of noise, as some percentage of it will invariably find its way into the audio signal. Noise in electronic circuits is any signal that is not desired. This means that electronic noise's bandwidth of frequencies is much wider than human hearing. An oscillation at 1 Hz can be just as deleterious as one at 1 MHz. Therefore, an oscilloscope, not your ears, is the best instrument for identifying and tracking down noise in a regulator.

A regulator that exhibited a low and flat output impedance and was noise free from 1 Hz to 10 MHz still might prove unsuitable if its output voltage differed substantially from one day to another or if it slowly drifted with the slow changes in the wall voltage, which would occur well below 1 Hz in frequency.

The next goal is an efficient design, that is a design that does not require a great amount of overhead in voltage or current. A regulator whose idle current was three times that of the power amplifier it fed would make few other than the power company happy.  And a regulator that required a thousand volts of input voltage so that it might yield an output of one hundred volts would be equally undesirable. This example illustrates the need of regulator voltage headroom--to an excessive degree. The point at which the regulator can no longer provide adequate regulation because of too little of an input voltage is called its drop out voltage. With a solid state pass device this voltage might be as low as 5 volts; with a tube pass device, as great as 300 volts and as low as 30 volts.

In the previous comparison, the solid state pass device surpassed the vacuum handsomely. Now, we turn the tables. Vacuum tube are, in spite of low public esteem, surprisingly rugged devices. If the power line feeding your house is hit by lightning, you will need a new computer, microwave oven, and TV, but probably not a new Stereo 70 amplifier, as the tube can survive a few kilovolts across itself for a millisecond or two. The same cannot be said for the transistor or MOSFET. Furthermore, the tube will probably survive a short across its cathode to ground; whereas the transistor will certainly smoke.
In terms of overall reliability, careful design considerations--more so than which technology, SS or tube, is used--yield the best results. Imagine worst case scenarios and design accordingly. If a fuse is placed in series with only the pass device, what will happen to it if the fuse blows and the rest of the regulator is still charged up? At power up, are any of the regulator's parts overstressed by current or voltage? At turn off, how will the stored voltages bleed away? Reliability can be designed into a regulator.

As the intended use of this regulator is for feeding a piece of high-end audio gear, such as an MC phono preamplifier, the least important considerations are cost and size of the regulator. Within reason, that is. There is something obscene about a regulator that costs ten times as much as the circuit it feeds or requires two strong men to lift when all it powers is a phono preamp. But then for some what is ostentatious is what is desired.
     Review of desirable attributes for high voltage regulators:
          1) DC accuracy  and low voltage drift
          2) A low and flat output impedance