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Dear JPB, thanks for taking the job of reading, cropping and posting both relevant schematic parts for easier comparison.
Indeed in FM212 they tried to do everything with that mute; in FM65 they left it only as a thermal protector (fine) and added an extra dedicated mute ... which is silent by definition.
Dear Enzo: in this case I *guess* (may be wrong) that it depends on perfect (or near perfect) symmetry on everything to the right of Q12/Q13 since there is nothing active controlling it.
As long as parts come from the same batch, same parts bin, as is common in large scale production where they might have sourced 10000 of everything in large orders, reasonable symmetry is practically guaranteed.
But even for such a good Company as Fender, maybe one batch of , say, whatever they used for Q11/12/13 runs out in the middle of a production and the new transistor in the "machinegun ammo roll" loaded in the pick and place machine has more or less Hfe (which is normal) , now the mirror stage powered by them will not be that balanced any more, and main rails ripple won't fully cancel.
Just one imagined situation, not saying that's exactly what's happening.
Now in a normal/working amp, no big deal.
Since these amps usually have some 60 to 70dB open loop gain, which NFB pads down to some 20/26dB (10 to 20X gain) you usually have some "spare" 40dB gain reserve to iron out distortion, reject rail ripple, etc.
That margin disappears when you nuke the input stage, so I guess *most* amps will not have problems, but a few might ... or might develop it after some time or even worse, after parts replacement, even if nominally the same.
Just polishing the crystal ball, of course, but I guess Fender being the serious company they are, must have had a few "mystery hum" cases, enough to slightly modify the circuit.
Indeed in FM212 they tried to do everything with that mute; in FM65 they left it only as a thermal protector (fine) and added an extra dedicated mute ... which is silent by definition.
Dear Enzo: in this case I *guess* (may be wrong) that it depends on perfect (or near perfect) symmetry on everything to the right of Q12/Q13 since there is nothing active controlling it.
As long as parts come from the same batch, same parts bin, as is common in large scale production where they might have sourced 10000 of everything in large orders, reasonable symmetry is practically guaranteed.
But even for such a good Company as Fender, maybe one batch of , say, whatever they used for Q11/12/13 runs out in the middle of a production and the new transistor in the "machinegun ammo roll" loaded in the pick and place machine has more or less Hfe (which is normal) , now the mirror stage powered by them will not be that balanced any more, and main rails ripple won't fully cancel.
Just one imagined situation, not saying that's exactly what's happening.
Now in a normal/working amp, no big deal.
Since these amps usually have some 60 to 70dB open loop gain, which NFB pads down to some 20/26dB (10 to 20X gain) you usually have some "spare" 40dB gain reserve to iron out distortion, reject rail ripple, etc.
That margin disappears when you nuke the input stage, so I guess *most* amps will not have problems, but a few might ... or might develop it after some time or even worse, after parts replacement, even if nominally the same.
Just polishing the crystal ball, of course, but I guess Fender being the serious company they are, must have had a few "mystery hum" cases, enough to slightly modify the circuit.
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