STANCOR Williamson Ultra-Linear Amps

These mono Williamson amplifiers are based on the Stancor Ultra-Linear schematic. However, the output tube selection differs slightly from the original. I opted for 1625s, which are the 12V heater military version of the 807 tube. The amplifiers deliver approximately 30 watts before exceeding 1% distortion. The Williamson design is known for its instability if not implemented correctly. Therefore, numerous hours were spent fine-tuning feedback capacitor and resistor choices, as well as various other small adjustments, to ensure stability while maintaining the 20dB of negative feedback. The power supply section is my own design. It utilizes a 5AR4 tube, which produces approximately 460V B+ voltage. Since the amplifiers are cathode-biased, the plate voltage is around 410V, which exceeds the triode limit specified in the data sheet. However, the tubes can handle this voltage. We are just slightly over 100% plate dissipation, The cathode current is set to approximately 62mA per tube, I added a cathode current balance pot and a A/C drive balace pot. I plan to adjust the bias in the future. The ultimate test was whether the new Hammond output transformers had the bandwidth required for the Williamson design and they passed. The result was an exceptional pair of amplifiers. I will include the final square wave testing results.