> demands a submini triode based microphone preamplifier with X-former input, the 5719 tube(s)(or similar)... I've got a Jensen JE-115 input transformer
5718... yawn. It's just a half a 12AT7, not even as good. You don't save any power. You hardly save a bit of space.
OK, the input transformer is fine, excellent, for tube grid.
> I assume you have to tune the plate and cathode resistors for the lower level signals found in mic pres.
What lower levels? "Line level" runs from 1V down to zero. We expect line amps to handle the weak sounds sweetly. OK, for mike-amp we might favor low-noise over high-output, but for other reasons there isn't a whole lot you can do for the noise, and live recording mike amps can be asked to carry signals much higher than most line amps.
You can use the tables tk pointed to, or any 12AT7 design, or probably 12AX7 or NYDave's 12AY7 designs. You may need to tweak for ab-best performance but they will work.
BTW: one big disadvantage of these micro tubes is: in any batch of tubes, most will have noise near theoretical minimum for the cathode impedance and temperature, but a few will have excess hiss. With socket tubes, you swap a few tubes and quickly weed-out the hissers. With solder-lead tubes, is a pain.
When stealing generic values, stick with 100K or 270K plate resistors. They show values of 470K or higher, with more gain and output, but the treble response tends toward AM radio or servo-amp quality. OTOH, using low-value plate resistors increases power consumption, which means you need bigger filtering. And B+ ripple will be your biggest problem (unless you use a voltage regulator, which seems silly to me).
The real problem is that a naked tube will NOT drive modern studio inputs well. The output impedance is around 10K, so it might barely drive 10K inputs, gasping. The elegant answer is an output transformer, but this has to be a costly unit. The cheat answer is to close your eyes and put in a hi-voltage MOSFET cathode follower.
