Thanks for the reply, but I beg to differ, I think a FET on the first stage is an obvious mod as it would increase the input impedance of the Pignose. Just thought of it then too. Thanks for the inspiration.
Sorry but you don't have a "first stage" there.
That 2SB172A is part of the power amp itself, look at the speaker out feedback being brought to its emitter .
And simply you can't slam a FET there, it won't work.
Now, you can add a Fet/op amp/transistor buffer before
the volume control, but that's something else.
EDIT: just saw the Hemmo version you attached.
Sorry but for one, it has errors.
He writes what he measured ... but he forgot to lift one resistor end "in the air" to measure properly.
In some the difference was so amazing (as in 47 ohms instead of 68K) that he wrote both, just in case.
In others where he found it "normal", he wrot only the measured value (in circuit, which is wrong).
Sometimes he apparently can't read the color code, as in the output transistors emitter resistor.
He measured 3 ohms (probably forgot to discount multimeter cable resistance), but he quotes 2K7 which is ridiculous, reasonable value would be 0.27 ohms .
He probably read 27K or something on the resistor body, but in that case, "K" (or "J") refers to temperature coefficient, not "K" ohms.
He also misread a *printed* label.
Everybody who wants to build his own Pignose goes on a wild chase for "TIP410" which unfortunately, and to confuse things, do exist, just they are a very different transistor.
The one he has was TIP41C, which he misread as TIP410
As of the presumed "C9014", same thing as before, you can't just slam a FET there.
Leave that power amp as-is and add whatever you want before that volume control, you'll get better results.