I can understand setting the plate resistor at 1/gm simply because the stage gain is gm.Ra which will be unity (as required) if Ra = 1/gm.

What I do not understand is your contention that one value will work for most anywhere on the 12BH7 curves. gm varies significantly with plate current which means Ra depends critically on standing current.

Cheers

ian

Ah I see. Yes I did say that, which is not correct. Sorry. Had been up all night... have had a nap now ☺️

I meant, we have a given point on the curves which we have defined based on our standing current, hence gm is reasonably known.

We set our value for Ra based on this and, within reasonable boundaries around this point, where Rp for the 12BH7 may be 5000 or 5300, and Mu may be 17.5 or 16.5, then the Ra will be quite reasonably within a few percentage points of being exact.

So, given the operating point of the 12BH7 as used in the Jensen schematic, I say that my value of 310 ohms is a better choice than the 680 ohms they chose. There is nowhere around that point at which the reciprocal of gm is approaching 680.

At low anode voltage use (as defined in the datasheet you provided), the 1/gm is lower, in which case Jensen's Ra value is even further off.

Again, sorry for the sloppy and incorrect statement. My intentions were in the right place, brain not so much!