Biasing Single-Ended Amplifier Output Tubes: A Basic Explanation and Demonstration

In this video I explain the principles of amplifier output tube biasing in very basic language. Topics include output tube design and function, how to measure Plate Voltage, indirectly measure Plate Current, and calculate Plate Dissipation. The effects of proper biasing upon headroom and tone are covered, as well as numerous tips and procedures to ensure personal safety while performing the measurements. Finally, the step-by-step biasing of a single-ended cathode biased amplifier is demonstrated using a nice, original 1960 Gretsch Model 6150 amp.

NOTE: In the video, plate voltage is measured between the plate (pin 3) and ground. A slightly more accurate measurement would be between the plate and the cathode (pin 8). Calculations for both methods are provided within the video.

NOTE: I have decided, after further research, that the graphs in the video at are overly simplistic and somewhat misleading. Please see this site for a more accurate, detailed explanation. Although it focuses on the pre-amp, the information also applies to output tubes: My comments regarding "hot bias = lots of headroom" are wrong. Hot bias = Early Breakup. Cold bias = Lots of Headroom. YT will no longer allow me to edit comments within the video, so I can only make the corrections here.

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