Note the DC Voltages in the ovals on the Green channel. Try to get the same Voltages on the Red channel. Chances are the Drain Voltage is way too low. Increase the resistance of the resistor between the Source and ground, R22 or R26 to try to get the Drain Voltage into the right range.
There is no protection for input JFETs Q1 and Q5. To add protection, connect a SIGNAL diode like a 1N914, 1N4148 or 1N4152 between the Gates of Q1 and Q5 and the -12V source point D. Connect the end with the band (Cathode) to the Gate and the Anode to -12V.
Note: It's a good practice to always plug the guitar cable into the guitar (or effects board) first, then plug the cable into the amp. If you hear that loud BUZZZZ! out of the amp, you're doing it wrong. Turning the amp off or the Volume down won't protect the JFETs you can kill them even when the power is off.
There is no protection for input JFETs Q1 and Q5. To add protection, connect a SIGNAL diode like a 1N914, 1N4148 or 1N4152 between the Gates of Q1 and Q5 and the -12V source point D. Connect the end with the band (Cathode) to the Gate and the Anode to -12V.
Note: It's a good practice to always plug the guitar cable into the guitar (or effects board) first, then plug the cable into the amp. If you hear that loud BUZZZZ! out of the amp, you're doing it wrong. Turning the amp off or the Volume down won't protect the JFETs you can kill them even when the power is off.