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feedback applied to grid

OK, so let's consider whether there might be other means to resolve the issue of cathode-node feedback distortions. You apparently work exclusively with balanced designs, so your techniques have been developed for that mode. I'm all about single ended signal chains, so the ultimate topology for this (if there is such a thing) will probably be different.

Since the cathode isn't available as a feedback node that has forced us to use the grids. We apply feedback in the balanced domain but the technique for doing it single-ended is identical. The issue is phase and impedance.

Phase obviously so its negative rather than positive. You'll find that feeding the grid requires higher resistance values which should be unsurprising.

We had a customer send us a Leak Point One preamp for which he needed a power supply; it was our job to build that supply since the Leak runs an umbilical which normally plugs into a Leak amplifier for its power. The Point One employs EF86 pentodes with feedback applied to the grids. This allows for a single tube to operate as a phono preamp for MM cartridges and a second EF86 as a line stage with tone controls. It turned out to be a very musical preamp- flying in the face of the traditional 'triodes only' and 'zero feedback' assumptions. It seemed a nice reinforcement of the ideas that Crowhurst wrote about. I really wish I could find that article again!

What Crowhurst mentioned, IIRC, is that the feedback applied to the cathode resulted in the generation of higher ordered harmonics as well as IMD. That correlates nicely with the bad rap feedback has gotten in some circles, such as the SET community. I don't think the SETs guys would like it though simply because as you clean up your amplifier power its very natural to use more of it. So people that say '7 Watts is plenty', if using feedback may find that they need more like 70 Watts (of course it would be playing louder but not sounding louder).

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