Single-ended power amplifier made with 6N7P tube

6N7P is a common cathode dual triode with excellent performance. It can be used for negative gate bias voltage amplification and zero gate bias power amplification. The tube has low internal resistance and high amplification factor, and is a treasure among low-power triodes. The filament voltage of 6N7P is 6.3V, the current is 800mA; the anode voltage is 300V: the maximum screen power consumption is 6W. The two triodes inside are connected in parallel. Under the screen voltage of 300V and the screen current of 35mA, the maximum output power can be about 4W. When used as a power amplifier, its tone has a sense of density and is unique. The circuit of this machine is shown in the attached figure. Its pre-stage uses the audio-specific pentode 688P for voltage amplification; the driving stage uses the single triode 6C5P (6C2C), which is equivalent to half a 6N8P; the final stage 6N7P works at zero gate voltage; the gate bias of 6C5P is -8V; the gate voltage of 688P is -1.5V (the working points shown in the figure are for reference only). Due to the discrete differences in the characteristic curves of each tube, they should be treated separately during debugging to ensure that each tube is adjusted to work in the best condition.