Funny this old thread came up just now. This monday I looked at the U47 schematic while the TV was on with news about the recession. So I thought, well, how could I possibly make the cheapest U47 type mic? The Oktava MK319, at least a good one, does have a bit of a U47 flavor - so why not put a tube circuit in there? To keep things cheap I decided to use as much of the original mic as possible while getting as close to the U47 schematic as I could with that material.
The MK-319 transformer is about 8:1, which is an okay ratio for most tube circuits, so why not re-use it? I also used the 10M and 50M resistors from the low cut circuit in series for a 60M grid to ground resistor (i.e. original U47 value). I also re-used one of the 500M resistors for a hefty low pass on the polarization voltage (the high value resistors in recent Oktavas are 500M, not 680M).
For a power suply I decided to use a Chinese one which came with a t.bone Retro tube III, a cheap tube mic I recently bought (and already modded). It puts out 120V and 6.5V.
So all I really needed exta was a 7-pin XLR plug, a handful of resistors, a 1u film resistor (I could have re-used the 1u/160V electrolytic, for maximum cheapness) and a couple smaller caps. Since space inside the mic is a little limited, I decided to use a 5840W subminiature tube. Total cost about 10-15 bucks (I actually had all the stuff lying around).
Well, built the mic and it works very well. Sounds a bit more direct and forward than my (modded) solid state MK-319; it is hardly any noisier than my (pretty quiet) solid state MK-319.
I'm no tube genius, so I basically just tried replicate U47 values, i.e. I copied some values (1.75K cathode resistor, 100k plate resistor) and regulated voltages to U47 voltages. For some underheating I used a 4.7 ohms resistor to drop the heater voltage to about 5.9V. Initially I used a 47k resistor instead of the 30k resistor in the original U47 circuit as my supply voltage is higher. But I found that a slightly higher voltage sounds better with the 5840 tube, so I soldered another 47k in parallel (thus effectively 23.5k). Other than that, I haven't done any tweaking so far; and to be honest, I'm not sure it needs any further tweaking. It really sounds nice as is.
Anyway, it is a fairly easy and very inexpensive project. Will post pics later.