Im trying to figure out how to Power Scale my Blackheart amp using the SB-2. The screen voltage is about 50V less than the plate voltage, so which one should I use for the Power Scale pot?
Most EL-84 output stages, including the Mesa F-30, the Valve Juniors, and all of the copies like the Blackheart, have significantly lower voltages on the screen than on the plate. The standard wiring for the SB-2 will cause a reduced maximum output power but will go down to unusably quiet levels. The alternative wiring for the PS pot will introduce a dead-spot in the control sweep, where the top end portion does not vary output, although this allows full output to be achieved and unusably quiet output when dialed down. The fix requires three steps: First, power the PS pot from the Va node. Second, control the Vs regulator using a voltage divider tied to the Va regulator output. Use 1k/V for the divider – often resulting in a 330k-1W to ground and 33k-68k between regulators. Third, add a 10k series resistor between the PS pot ‘X’ and the Va cap positive. Then add a 22uF-450V cap to ground from the PS pot ‘X’. This will provide a cleaner voltage to the Power scale pot, which in turn feed
