I would like to design and build some balanced, 600 ohm capable equipment to experiment with this audio flavor.
   Driving 600 ohm loads is tougher than I had first thought; the 600 ohms refers to 600 ohms from output to output, positive polarity to negative polarity. When tubes ruled, output transformers used to be an industry standardized way of handling this low load impedance. By the way, 600 ohms from + output to - output means 150 ohms from any one polarity to ground; remember, it is winding ratio squared that give you the impedance ratio. And if you use only half of the output winding, you have just doubled the winding ratio. Now, 150 ohms is a brutal load for most tube to drive without the aid of an output transformer. 
   The problem of course is finding high quality output transformers. Thus, the attractiveness of a direct-drive or OTL output stage. Once again, 150 ohms is a tough load for most tubes, even in a Cathode Follower configuration. 6BX7s and 5687s are much better suited to this task than 12AU7s or even 6SN7s. Small power tubes, such as the EL84, EL86, 6V6, and 12B4 might be the best choices for the output stage. DC coupling at the output would save the cost of the 50 to 100 µF coupling capacitors, which if they are of the highest quality (SETI, Hovland, MIT) will cost a small fortune (the output transformer is looking better than we at first thought).
   In short, expect to see some articles on balanced preamps soon.

Subject: Ask for designing a bipolar shunt regulator

   I really appreciate your webzine and wonderful articles. Now I have a request for tube regulator.

   I'd read your article about bipolar shunt regulator on volume 1 number 5. I'm building Mr. Rozenblit's grounded grid preamp and +/- 200V is required. Would you please give me an example of how to design a tube shunt regulator for use with a bipolar power supply? I want to use 9-pin tube to accomplish this mission. Thank you very much.
                                                                                                                                          Pasteur

    The first step to designing a high quality bipolar power supply is to make sure that each rail is burdened with an equal current draw. Symmetry helps us null the noise.

The circuit to the left fails to provide an equal current draw on each power supply rail, as the positive rail must supply 7.5 mA more current than the negative rail.

   The circuit to the right does pass the test, as each rail sees a 15 mA  load.  Furthermore, this circuit offers an equal impedance to each power supply rail, which ensures that the noise on each rail will match that on the other rail.

   The second step is to endeavor to use the ground solely for referencing the audio signal. In other words, all the power supply charging currents and noise currents should be terminated at a single point

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