Some time ago I traced the 307a that kindly Vegard Winge sent me from Norway. As suggested by Vegard, I re-traced them with lower screen voltage and increased feedback to improve the linearity of the 307a in this mode.
The modification to the curve tracer is simple. A simple resistor divider (R1, R2) provides the feedback to the grid. The grid driver provides a constant impedance so works fine. With a potentiometer you can adjust the feedback ratio and look at the impact on the curves. Very handy!
With 15.5% feedback and 125V screen voltage I got the best curves as shown below:
Nice to see the anode resistance coming down to 1kΩ.I tried matching a triode model to these curves with a good result
