Negative feedback

[Isaac]

I was messing around with my 200S the other day. I decided to experiment with eliminating the negative feedback loop, switching it out using the disabled polarity switch.

I ran a line from the 16 ohm transformer output to the polarity switch and back to the feedback connection point. With the switch closed, the feedback line connected, the amp sounded the same as before. With the line open, the first thing I noticed was that there was noise. Not too loud, not noticeable when playing but apparent when the amp was idle. Turning the volume control down didn't affect it. The second thing I noticed was significantly more gain. Set for the same level, the tone was subtly different, but I'm not sure how to describe the difference. I'm also not sure that it's particularly useful, especially with the noise.

I'm still wondering about a couple of things, though. There are two feedback application points, one at the splitter/driver (6AN8) tube and another at one of the power tubes. Might it be a good idea to keep one of the application points? Greg, I'm looking your way. I seem to recall that you've done this kind of thing before.

[loudthud]

The 390pF cap was probably a refinement left over from the Dynaco circuit. You would need to look at the output with a scope in a HiFi context to really know what it does. My guess would be that it irons out the square wave response well below clipping. A guitar or bass amp would probably work fine without it.

Without feedback, the Ultralinear circuit has a moderate damping factor, probably in the single digits. That's higher than a normal plate output with no feedback but less than a amp with a normal amout of feedback. Guitar speakers seem to like low damping factors. That's why most solid state guitar amps are made that way these days. In bass amps, the low damping factor can result in really flabby and loose bottom end depending on what speaker is used. The so called resonance control lets you control the low end.