> Here is the plate characteristics of 6550A in UL (black line) and triode (red line) connection from the datasheet by GE.
> When Eb=450V, Ec2 also equals 450V (blue line), so this condition is the same as triode connection.
> Ib of triode connection is Ib + Ic2 and the ratio of Ic2 to Ib is 44:455
> (from pentode characteristics).
> When Ec=-30V, Ib of triode connection is 280mA (point A),
> so the Ib of UL should be 280 * 455 / (455 + 44) = 255mA (point B).
> On the other hand, Ib of UL on the datasheet is 215mA (point C)
> and this value is too low in comparison with point B.
> So the GE's UL characteristics is suspicious.
> My pentode model is based on the triode characteristics.
> So the characteristics curve of UL connection by my model
> should pass through the point B.
Thanks for taking the time to review the 6550A datasheet and the explanation, it makes perfect sense now. So there is something not quite right about the GE datasheet, I will look at some other ones to see if they are any better.