From the manual for those interested in what the mod exactly is/does.....
"There are alot of ST-70 driver board mods out there but most have taken a classical approach to solving this amplifier’s deficiencies. Alan Kimmel’s ST-70 modification incorporates an advanced constant-current technique using a 6U8A tube to operate the 5751 triode voltage gain stage in the most linear manner possible. In addition, the output of the 5751 is optimally coupled to the rest of the circuit and this coupling is such that the 5751 sees virtually no capacitance and no loading at all. The amount of negative feedback is modest, and is taken from the 8 ohm output tap which is more effective than taking it from the 16 ohm tap, as most do. This circuitry results in a superior level of detail in both the midrange and top-end. Next, the output tubes’ bias supply is regulated resulting in a constant bias point with minimum drift. And finally, over 500uF of capacitance is added to the main power supply circuit resulting in a much tighter bass response and lower distortion throughout the frequency spectrum. The ST-70 driver mod features: a “main” printed circuit board consisting of the 5751/6U8A peripheral driver circuitry and the additional power supply filter capacitance (>500uF); a second bias-supply printed circuit board which includes the bias rectifier, filter caps, a zener diode regulator circuit and the bias adjustment potentiometers; and finally, a new aluminum mounting plate, with black anodized finish, is supplied which attaches to the top of the ST-70 chassis to support the rectifier and driver circuit tube sockets (with this design, there are no exposed components or circuit traces on top of the chassis). The existing output circuitry is retained. The existing power supply filter capacitor is removed and no longer used. In place of the original capacitor are five 100uF filter caps plus high quality polypropylene film bypass capacitors."
