Once the grid voltage is known, it is divided by the idle current and the result is the cathode resistor value. The general rule with resistor is to use a resistor wattage twice as great as is needed. For example, if the plate resistor will see 150 volts across its leads and current conduction of 5 mA, then it will dissipate .75 watts of heat; thus, using a 1.5 to 2 watt resistor would be wise.
Subject: Headphone Amplifier Inquiry Sorry to bother you, but I have no one in my area to ask. I am trying to work out a headphone amplifier design, based on an old RCA microphone preamplifier. It is wonderfully simple, and seems to make running one tube a side a possibility. Problem is, high impedance on the output  220k Z is what the RCA design says. ( This is with a 12AX7. I am likely to substitute another tube which will bring the impedance down some, but it is still likely to be high.) I'd like to use this Preamp. to drive my 250 ohm headphones. So am thinking of using an impedance matching transformer on each side. Problem I foresee is a large stepdown: ~220K to 250 ohms. That's near "1000 to one" for the transformer! I've been looking around for such a transformer, and I haven't found it yet. So am thinking of winding my own. Do you believe this feasible? Any ideas/opinions/suggestions welcomed.
Not having seen the schematic, I can only guess what is going on (i.e. please send a copy). It sounds like the circuit is only a grounded cathode amplifier that uses a 12AX7. The 12AX7 does provide a good deal of gain, but very little power. An optimally designed resistor loaded singleended amplifier delivers only 12.5 % of its power dissipation into the load. In other words, even with a coupling transformer, the power output may be insufficient. A better choice might be the 12AT7, as it has a lower rp and healthy amount of gain. The impedance matching transformer winding ratio is the output impedance divided by the input impedance, that result is the impedance ratio. The winding ratio is the same as the voltage ratio (and current ratio as well), which is the square root of the impedance ratio. Therefore, the winding ratio you need is only 32 to 1. The big problem with coupling transformers is not finding the right winding ratio, but finding a high quality one.
