In this video we look at the 50%, 70%, 'By Ear' rule of setting the bias point on a Marshall JCM800 50W EL34 Push-Pull guitar amplifier. We start with determining the bias point from the schematic, then we measure the actual DC operating point of the amplifier, this is the starting point.

Next we look through the Mullard EL34 datasheet and find a section that's close to our amplifier. However, there's a large disconnect. The published datasheet graphs aren't near our operating point. The plate characteristic given is for a screen voltage of 360VDC, our amplifier has a screen voltage of 450VDC. We discuss this using then do a complete load line analysis of the amplifier's 2 modes of operation, class A for low signal outputs and class AB1 for higher outputs. The load line analysis tool is again a Jupyter Notebook (python, available on my github page).

The class AB1 operation is clearly above the plate limit of 25W, now what? The next analysis examines this and determines that the amplifier is actually not exceeding the plate dissipation limit. This is discussed then graphically shown and calculated using numpy's integration function, where we integrate the instantaneous plate dissipation to give an average dissipation over a full wave cycle. Remember in AB operation, one valve is carrying the full load but only for half the cycle.

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