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It's occurred to me that I am overdriving the 6BG6. The screen voltage is 290v - the expected voltage is 255v. I think that the purpose of the 6BG6 is to amplify the sawtooth pulse coming from the horizontal discharge. The rising and falling voltage associated with the amplified sawtooth wave passes back and forth through the primary coil of the flyback transformer, creating a magnetic field which passes through the secondary windings. If the 6BG6 is being overdriven, wouldn't the sawtooth wave clip (at the maximum gain of the tube)? And if it clipped, would the nominal difference between the top and the bottom of the wave be shorter, hence less voltage change, hence smaller magnetic field generated by the flyback, hence less voltage generated in the secondary winding? Well, that's the theory. Tomorrow, I'll test it by increasing the value of the 10k resistor in series with the 6BG6 screen. I'll drop the screen voltage to the recommended 255v, and see what happens.
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