The 6BK5 has a much lower gm2 than the 6W6. Can compare the g2 curves on the GE datasheets for them both. The 6W6 is rated at Mu_triode of 6.2 and I see Mu 24 scribbled in on my 6BK5 datasheet. I haven't tried this tube, even though I've got a few in a box. So probably was a calculated Mu from the curves.
Looking on the left/ bottom of page 3 of the GE datasheet for the 6BK5: the plate current versus grid voltage allows one to check Mu versus plate voltage (for constant current). Looks like the Mu drops some at low plate voltage (spacing between curves increases some for the 100V curve). But these curves are measured at constant (250V) g2 voltage, so this may just be an affect of the increasing screen current in pentode config. at low plate voltage. The plate characteristics curves at the top of the page show an obvious upswing in screen current around 100 Vp.
Don
Looking on the left/ bottom of page 3 of the GE datasheet for the 6BK5: the plate current versus grid voltage allows one to check Mu versus plate voltage (for constant current). Looks like the Mu drops some at low plate voltage (spacing between curves increases some for the 100V curve). But these curves are measured at constant (250V) g2 voltage, so this may just be an affect of the increasing screen current in pentode config. at low plate voltage. The plate characteristics curves at the top of the page show an obvious upswing in screen current around 100 Vp.
Don
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