Subject: articles
    A couple of article ideas. I'd love to see a comprehensive treatment of interstage transformers, and this is why: I've been toying with the idea of creating a single-ended amp with the SV-572-3. I'd like to squeeze out 40-50W out of a single tube in class A-2. But according to the data sheets, this would require a voltage swing of 540v p-p, and in class A-2, the *grid* of the SV-572-3 can pull as much as 50 mA of current (!).
    There are many ways to skin a cat, but this problem suggests (to me, at least) the use of a single-ended interstage transformer with another SV-572-3 as a driver tube. Crazy? Perhaps. But the benefits, as I see them, would be that the driver can supply the necessary power to the output tube, and that it would offer some harmonic cancellation without a feedback loop. On the latter point, consider what kind of distortion you would want to inject (and thus cancel)--the distortion that is characteristic of a different tube, or the distortion that is characteristic of the same tube? Besides, it is generally agreed that transformer-coupled amps, with great care, can sound excellent.
   But there is a dearth of information on the subject of interstage transformers. It seems to be a real black science. Add a load on the secondaries or no? If so, how much? Is any special type of biasing for the output tube necessary through the secondary? Dennis Had has some designs. I wonder what he does?
    In addition, there appear to be some great possibilities for interstage transformers in pp amps, where there would be all-direct coupling and just one IS transformer.
   It would also be great to have a primer on slow turn-on warm-up circuits and relays.
    Thanks for everything. I've got SE Amp CAD and Tube CAD, and read the Journal  regularly. Can't wait for your new software.

-J.

   Not to dampen your ardor too greatly, but interstage transformers cannot solve all problems of Class A2 operation. The problem is that he grid and cathode define a diode, which when reversed biased, presents an open circuit, but when forward biased, conducts. A simple model for experimenting is shown below.

   Notice as long as the applied voltage is negative, no current flows. And as long as the voltage does not exceed the break voltage of the zener diode, no current flows. But once the breakdown is passed, the current will flow. In other words, this circuit is useful for testing a drive circuit meant to work into a conducting grid.

    In the first example, the resistor should match the rp of the SV-572-3's grid as a diode and the zener voltage should match the bias voltage of the output tube; in the second example, the bottom zener equals the voltage across the cathode resistor. Hookup an oscilloscope to the output and look at the waveforms. I am sure that you will find that cathode follower produces a much cleaner waveform. The interstage transformer works best in amplifiers that only occasionally run in grid current, as the grid current effectively loads the transformer and this loading is transferred to primary. The cathode follower, on the other hand, works supremely well in driving positive grids, as it controls the flow of current to the B+. As for slow start circuits, they are lifesavers and we'll see if we cannot come up with something soon.