[AI2V]

The distortion is important. The 6A7 is not moding properly and at the point where the distortion occurs, the oscillator frequency will go out of track all the way down the tuning dial. I've seen it several times and it came down to wrong voltages on the tube. For a quick solution, tack a resistor across each resistor in the 6A7 circuitry. At some point the addition of the resistor will make the problem worse or less. That's going to lead you to adjusting a resistor value up or down. Don't be afraid to make some changes in resistance values. Not all tube perform perfectly in any given application. Tweaking doesn't hurt.