12AT7 datasheet question

[jeff]

 I was looking at this: http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12AT7.pdf

I don't get it. It says at 250V that the cathode resistor is 200 ohm the plate current is 10mA and the grid voltage is -12V
 If the bias resistor is 200 ohm and the currnet is 10mA then I*R=V : 200*.01=2 how is the grid voltage -12V?

[HotBluePlates]

I don't get it. It says at 250V that the cathode resistor is 200 ohm the plate current is 10mA and the grid voltage is -12V...

Look again...

It says, "Grid voltage, approximate Ib = 10 microamperes"

So what they're saying is that the grid voltage to reach (approximately) cut-off will be -12v. As you noted, the idle bias voltage is -2v. So another -10v of input voltage drives the tube to cutoff. Of course, only another +2v of input drives the tube to saturation and grid current.

There are reasons this could be important to know, but the typical conditions are often really "show-off conditions" for small-signal preamp tubes. When are you ever gonna idle a preamp tube at 10mA?

[PRR]

The approximate cut-off point is given because 12AT7 is intended as an RF Tuner tube. Radio signals range from very weak (you need 10mA plate current for gain and hiss) to very strong (you need teeny plate current so as you don't slam the next stage). i.e. your gain-control system needs to be able to apply -12V mis-bias to control strong signals.

Neither extreme is of any use in typical audio amplifiers. 12AT7 is handy for high gain into lower impedances, perhaps 47K plate resistor and 50K-100K audio load. Assuming 300V supply and tube biased halfway up the supply, that's 3mA at 150V. The left-hand 100V 3.7mA condition will be close enough for design.