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RE: Schematic?

Driver topology is an important consideration. The original ST-70 used a 7199 in each channel, essentially a pentode driving a cathodyne phase inverter. I'm not especially fond of this arrangement in amplifiers above 25W or so. It's workable, but the cathodyne has no gain (actually a slight loss). This means the first stage has to provide all the gain, and I think it's better to share the gain across all the active stages in the driver. Rather than requiring the first stage to provide a gain of 160 or more, the same overall driver gain can be achieved with (just as an example) a stage with a gain of 10 followed by a stage with a gain of 16. This can be easily accomplished using a triode (or a triode-wired pentode) followed by a long tailed pair. Another option would be a tube with two triodes in cascade driving a cathodyne. Despite the slight loss in the cathodyne, the two triodes share the total gain, and that's helpful. Either of these arrangements can be accomplished with three 9-pin bottles. One advantage of the former (triode to long tailed pair) is that the two stages can be DC coupled. This results in an amplifier with only two coupling capacitors in each channel, just like the original ST-70. In addition, the long tailed pair lends itself to incorporation of an AC balance control, and that can aid in nulling moderate imbalance in the output stage.

I do know that someone is making driver boards for the ST-70 using a pentode or triode driving a long tailed pair. Whether it's Bob Latino or someone else, I'm not sure. I'll just add that my own current amplifier projects use this general topology, as have many highly regarded amplifiers of the past.

Incidentally, I used to perform factory warranty service for Dynaco in the early '70s. I never put them under the microscope like we do know, but I'm pretty familiar with how everything works. :)
















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