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Adjustable negative feedback

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I wanted to play with adjusting the negative feedback on my Radio Craftsmen (Williamson) amp.

Pretty simple RC feedback pair

going into the cathode bias.

So, I figured I would just put a replace the 5.1k resistor with a pot and have at it.

Then I stumbled across a paper from Claus Byrith on a modern Mullard amp and he has a little twist.

Instead of adjusting the feedback resistor, he split the bias resistor into a fixed 390 ohm and a 100 ohm pot and injects the feedback above the pot.

Quote:

You might ask, why I do not inject feedback directly to the cathode. The answer is, that the feedback resistor is part of the cathode to ground path of the 1st stage and if you want to experiment with the amount of feedback, you would have to change both feedback and cathode resistor in order to keep DC conditions constant. Now as feedback is inserted over the 100W bottom resistor, the feedback resistor has a negligible influence on the cathode to ground resistance. You could even make feedback continuously variable, by changing the 100W resistor to a 100W potentiometer and inject feedback to the wiper. With the feedback resistor shown, feedback will be variable from 0 to 20dB.

Seems reasonable! I was curious if anyone else has any experience adding this type of adjustment before I get too deep into it.

Jim
Date: Dec 16, 2013   


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