The N12A was designed to power the Nuvistor-model C12A. The C61 uses the same Nuvistor with same heater and B+ supply voltages, but a different connector.
B+ (high voltage) is only marginally affected in a C61 when you click through the N12A pattern selector, and only ifyou move the selector after you have calibrated the supply to deliver a perfect 120VDC for your C61. A slight voltage dividing effect will vary a couple of volts of B+ as you click between patterns.
A more permanent solution: disconnect R11 (variable resistor) in the power supply at the the junction of R6, C6 and R7. At that tap, a wire connects to the pattern selector. No connection, no voltage drain. You effectively have bypassed the circuitry for the variable voltage.
