Data sheet confusion for 12BY7-A

[Pdavis68]

First off, here's the data sheet.

On page 2 the sample Class A setup has 250V plate, 180V screen and plate and screen currents of 26ma and 5.75ma, respectively. Cathode resistor is 100 ohms.

So a few questions:

1: where does the -11.6 grid voltage come from? I would think 100 ohm * (.026ma+.00575ma) = -3.175V.

2: On page 3, for Average Screen Characteristics, for plate voltage of 250V and screen voltage of 180, it looks like screen current is around 18ma.

Why am I so messed up?

[DRH1958]

I found the tube data in the RCA RC30 manual and it states the -11.6v is for 20uA plate current. I'm assuming this is for full cutoff but according to the plate characteristics on page 3,-7v to -8v should put the tube in cutoff. This is a sharp cutoff pentode, so I don't know why they give this number.
As for your second question, the RCA data sheet gives the exact same numbers for "typical operation" as yours does. This has me stumped also. With those voltages(250,180), according to the screen curves, you will be exceeding the 1.1 watt max rating by around double the wattage. But in the "typical operation" section, it gives 5.75mA screen current for these numbers which is well under 1.1 watts according to the screen curves.

[Enzo]

No circuit is specified, only the conditions at the tube. The -11v comes from whatever bias supply one might be using. A cathode resistor makes a positive voltage on the cathode, it does not make a negative at the grid. the grid may be negative with respect to the cathode, but that is not the same.

[Pdavis68]

True. I was just assuming that the grid voltage was relative to the cathode biased with a 100 ohm resistor. Invalid assumption, I guess.

If I look at the Average Transfer Characteristics for the screen current on Page 4, at about -3.3 or -3.4 V, you get around the 5.75mA with 250V plate & 180v screen grid, so I'm guessing the Average Screen Characteristics graph is just wrong.