Further adjustments include a rectified and filtered heater supply to the triodes eliminating AC hum coupled to the signal path via parasitic wiring capacitances.
A power transformer with a 350V-0-350V secondary supply an increased voltage supply to the output section with is then stepped down to the 5F6-A’s voltage supplies to the phase splitter, cathode follower, and voltage amp.
A 27k trim-pot is included in the negative bias supply allowing for greater control over tolerances in tube conductance and resistor values ensuring desired bias current to the output pentodes. This supply voltage is sourced by an independent 100 VAC secondary tap from the power transformer for convenience.
These alterations were explored using Spice software simulation. Construction and testing of the modified amplifier allow for comparisons between simulated results vs. actual results. In order to keep test equipment safely removed from high DC bias voltages, source signal was applied to the very input of the amplifier, outputs were observed at the speaker output jack. Also lower volumes were tested in an effort to keep from overloading the signal analyzing equipment with the high AC voltages that this amplifier is capable of. See Appendix A7–AX for these comparisons.
Testing for Harmonic Distortion, two approaches were taken. The first involved flat EQ, low bright and normal volumes, and high presence. This drives the output section harder filtering negative feedback at test frequencies (1k) to ground. The second approach involved removing the presence control allowing for maximum negative feedback.