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 step-down: ~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 single-ended 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.

Subject: Article Request "UltraPath"
   Maybe you could write an article about the Western Electric "UltraPath" circuit and it's variant forms. How it works, how to properly apply it, where are its applications. There's another method where you use a capacitor from B+ to cathode that forms a voltage divider with the cathode bypass capacitor and achieves B+ supply noise and hum cancellation as well as improves PSRR. This can be applied to any cathode biased stage with a resistor. Not sure how it can help a battery biased or zener biased cathode. There's actually a lot to this interesting topic and has been discussed by me on audioasylum.com. It would be nice to see your treatment of the subject and have it summarized concisely in an article.  One application that seems to work, but it's not intuitive, has been for push-pull stages.

Kurt

      The blame for the "UltraPath" topology does not  belong to WE, or at least I am not aware of it doing so; I am not sure who first started using it; in fact, one reader wrote that it was John Atwood's creation. It isn't. Here was my reply:

As for the Ultrapath circuit, the blame does not belong to John Atwood, as I believe it is from Jack Elliano of Electra-Print. I have heard good reviews of his amplifiers and transformers, but I am sure that it does hum, you see it is only half wrong, or rather, half right. The complete UltraUltrapath circuit is covered in the second issue of this journal in an article titled "Lowering the Single Ended Amplifier's Output Noise."       
        http://www.tubecad.com/april99

    This article might even predate Jack's article in VTV, but I do not care much about its history (and even less for the name...maybe from now on I will name all the circuits in this journal with whimsical names such as the "Gray Dove" preamp or the "Defiant Follower" or the "MegaFlow" voltage regulator) as much as I care about its purpose and functioning. And I can see why it is so hot a topic, as it is so simple: the power supply capacitor use go to ground, now it goes to the cathode; too simple.
   In contrast, the two cap solution I offered in the article, ladened with theory, math, illustrations, and

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