The amount of overall feedback depends on the value of one resistor and determines almost all the amplifiers characteristics. This resistor has the shirt number 25. The amplifier is born without this resistor and may function very good without it tough bass reproduction may be far from ideal and some distortion and hum may be heard.
Most engineers talk about feedback in decibels. The authors found it more practical to speak about the amount of times it reduces the sensitivity of the amplifier. In the initial sensitivity of the amplifier without feedback is 200mV and after applying feedback it is reduced to 600mV the feedback is factor 3.
Williamson specifies 20dB feedback which means nothing else as 10 times. The famous number 25 resistor returns the signal from the output of the amp to the input where it is compared with the input signal by subtraction. Any part of a signal that wasn't in the input is immediately corrected. By applying negative feedback the input stage has a reservoir of amplification it can draw from if anything goes wrong. The factor of feedback determines how big that reservoir is and how much we allow the amplifier to correct things. We must not forget that the system has no build in intelligence and it will also try to correct things that cannot be corrected causing even more distortion.
Apart from reducing sensitivity, it reduces distortion, increases the damping factor and widens the bandwidth of the amplifier. It does all these things in more or less the same ratio as it reduces sensitivity . Sensitivity however is the easiest thing to measure!
If you start experiment with feedback, make it a habit to measure always at the same frequency and to the same outputlevel. ( 400cycles and 3volt output are my favorites)
Well if feedback is such a good thing, why not tons of it? Because all that correcting makes the amp nervous. Its is better to leave it to do its job at some extend the way it wants to do it. That sounds better. The 10 times Williamson used is in my humble opinion way to much. A feedback factor of 3 ( or about 10dB ) is more to my taste. Distortion is low enough to be inaudible and the bandwidth sufficient. The only thing at feedback levels this low that can cause problems is the damping factor to the loudspeakers. Well, try any level of feedback by exchanging the horrible number 25 resistor, try values between 5kOhms and 100kOhms. If your amps starts oscillating at feedback factors under 20 stay away from high feedback levels till you found out why.
If you are measuring in your shack with a dummy load resistor on the output, try every now and then what happens if you remove the dummyload, leaving the output only connected to the oscilloscope with some square waves fed in the amp. If it starts oscillating when you remove the dummyload, chances are that it also oscillates when it is connected to your loudspeakers since the coils in your speakers have at an infinite frequency an infinite impedance, just like your dummyload when you remove it.
Experimenting with feedback means you also have to keep a constant eye on the capacitor with the shirtnumber 10. Its value mentioned in the original circuit is only the correct value when everything including the output transformer are the same Williamson used!
To adjust it to your amplifier, apply square waves to the input at 10Kcycles and adjust the value of C10 till your waves are almost square. A little overshoot may remain. If you alter the amount of feedback you must check C10 again.
Some amplifiers have a kind of C10 like capacitor parallel to R25 with the same purpose, adjusting the square waves. ( In fact adjusting the phase shift ) Some other have both. If you want to spend a few years or so in your shack finding a way to make such a construction stable please do so! E-mail me sometime!