Hi Pokus,

The basic way to understand the triode gain stage is that gain (A) = -u RA / Ra + ra assuming cathode is fully bypassed, as your example is by the 10u cap.

u ('mu') = amplification factor

RA = plate load resistor

ra = plate AC resistance, a property of the tube itself

The "unknowns" - u and ra - can be determined from the load line for best accuracy, but data sheets also give some 'generic values', which we'll use here.

12AX7 has a u of 100. ra is given as 80k for 12.6V heaters, and 62.5k at 6.3v heaters....let's just say 70k as an approximation since the listed value is also approximate, and for the speed of getting this to you

The plate resistor (NOT ra, the actual resistor) here is 100k.

So, - 100 * 100k / 100k + 70k results in a gain of about -58. "Minus" simply meaning the stage inverts the signal.

If the cathode resistor is not fully bypassed, the gain will be lower and you have to account for the cathode resistor; the formula just changes to include Rk.

This comes from HERE:

http://www.valvewizard.co.uk/gainstage.html And you can find the other forumula with Rk, and read WHY, there. You need to 'get' gm, the load line, and where "mu" comes from, to get closer than the approximations...however, the approx. is usually JUST FINE!

HTH.