The amp has two inputs, one “Rock” input for high-gain overdrive, and the more modest gain “Roll” input. Since I often use an external preamp, the Roll input is perfect, but I also use the Rock input without the preamp. The Rock input stage is modified to increase the cathode bypass capacitor C3. The stock cap with R5 creates a bass roll-off starting above 100Hz, the first of many bass cuts in the amp circuits. Replace the 0.68uf/250v cap with something in the 5uf-10uf range. You don’t need a high-voltage cap here, (the cathode voltage is only a few volts), but I happened to have a 10uf/200v Mylar cap in a parts bin, and there is room for it, so that’s what I used. You can safely use a 50v electrolytic. This change flattens the 1
st stage response and eliminate one of the bass-cut stages.