For info, I found that some SPICE vacuum tube models have to be tweaked in order to work properly in SPICE simulation when a stage drives the grid of the vacuum tube. Marshall Leach is probably the source of the finest vacuum tubes models but even in his text he mentions that that a tweak has to be done. It consists of eliminating the resistor RG1 and the Diode in series between the grid and the cathode of the tube. Leach explains that it is in the model to deal with grid current when the tube is in the positive grid region. On the other hand, it has the effect of loading heavily the driving stage since RG1 is in the 1k range. I don’t know if you guys found that already but I thought that it would help you avoid the pain of finding why your driver stage is performing weirdly and why it reaches the cutoff so fast in an unexpected way.
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