For the desired plate current in my 12ax7 circuit, I found I have to adjust the grid voltage significantly more negative than predicted by the curves (-2.8 Vdc as opposed to -2.1 Vdc). After puzzling over the circuit for a while, I noticed that my filament voltage was around 13.7 Vdc. Is it possible for a modest deviation like this to disrupt the operating point of the tube so drastically? I discovered the issue too late last night to make any adjustments and I can't think of any other variable that could account for the mismatch between prediction and measurement. The power supply is sufficiently stiff and the tube only has a few hours on it. Any suggestions would be greatly appreciated.