Can you explain this? I thought 16 ohm would make it most nfb and all frequencies come through, including low and mid. So it would mellow the amp out.
What happens if this was the 4 or 8 ohm tap?
And what happens with a 100k in parallel to make it 50k (47k)?
First you have to understand a bit of history.
In 1972 they used 16 ohm taps carried over from the late 60s. When a bypass cap was introduced across the first stage evolved from a shared cathode, the brightness increased. They adjusted it by dropping the NFB to 4 ohm for EL34 and 8 ohm for 6550 USA bound amps with the JMP line later on.
When you mod a circuit and add or modify bypass cathode caps, especially across the 820 ohm resistor in the 3rd stage, you emphasize even more highs than intended including whatever cutoff frequency is chosen to boost mids.
Later on when Marshall changed to 8 ohm and 4 ohm, they also increased the NFB resistor to further reduce NFB. Certain years of course have different values but that’s typical Marshall experimentation for the time.
The more NFB, the more stiff and linear the amp becomes across all frequencies including undesired ones if the preamp has been tweaked.
The less NFB the more raw it becomes and the less dynamic range, which means your loudness of highs to lows are not equally controlled and can differ in loudness because the dynamic range has been deceased. You’re changing the gain bandwidth product with the ratio of NFB from the NFB resistor, shunt resistor in parallel with the presence pot, and the tap used. 16 ohm tap provides the most NFB, 8 ohm is 3dB less than 16 ohm, 4 ohm is 6 dB less than 16 ohm. You can further reduce NFB by increasing the NFB resistor as mentioned earlier. Some prefer no NFB at all, but I do prefer it with a depth and presence.
So, since he modded yours to be to the speaker Jack, you’re using 16 ohm feedback to a 16 ohm cab. Your stages are bypassed, meaning youre experiencing ear bleeding brightness.
