This isn't strange, as you saw the difference between the input and the output signals, which copes with the unlinearities of the output stage.
Best regards!
Yes, I realized this as I was falling asleep- the problem with posting late at night!! The whole point of NFB is to inject 'distortion' of opposite phase back to the input, so that the sum (at the load/speakers) is improved....at least that's how I'm thinking about it.
Beginner stuff, I know...baby steps. 🙂
I did some measurements of THD (measured at the output into resistive loads - both 4 and 8 ohm), using different amounts of feedback. I tried NFB into the 12AT7 cathode resistor divider (at a couple of spots) and also into the anode (RH style).
Distortion with low (0.5v) input signal levels was fairly good - about 3% with about 2.5W out with cathode FB or anode FB into 4 ohms (2k8 OT impedance). With increasing signal level, distortion increased - I think part of the problem is trying to use the 12AT7 input tubes with low plate voltage.
It sounds OK but the 'shop' speakers aren't very efficient, so it is hard to judge if there will be adequate and not-too-distorted volume. I think I'll try switching to EL84 tubes with higher voltage B+, using the same output transformers. Again, just breadboarding, using the bench power supply.
Beginner stuff, I know...baby steps. 🙂
I did some measurements of THD (measured at the output into resistive loads - both 4 and 8 ohm), using different amounts of feedback. I tried NFB into the 12AT7 cathode resistor divider (at a couple of spots) and also into the anode (RH style).
Distortion with low (0.5v) input signal levels was fairly good - about 3% with about 2.5W out with cathode FB or anode FB into 4 ohms (2k8 OT impedance). With increasing signal level, distortion increased - I think part of the problem is trying to use the 12AT7 input tubes with low plate voltage.
It sounds OK but the 'shop' speakers aren't very efficient, so it is hard to judge if there will be adequate and not-too-distorted volume. I think I'll try switching to EL84 tubes with higher voltage B+, using the same output transformers. Again, just breadboarding, using the bench power supply.
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