I've just done that modification last week for a fellow
not easy to do however...
i've activated the rear diaphragm (just unscrew the center screw and put a connector):be careful very fragile....
stock, the psu gives +70V and -70V (measure it on the wires inside the psu, mic plugged)
i've soldered a 61v zener to the -70v to obtain -60V
put a 100nF/100v across the zener
due to a floating ground design, a zener on the +70v doesn't work, i've installed a little resistors network to drop to 60v
then, in the mic, i've kept only the first tube stage : remove C4
remove also R4 R5 T1 and put a 47kOhms resistor from +60V to tube plate : this should give you 37v on the plate
on the molex connector there a unused -70v wire: put a 51Mohms with 10nF(to ground) then 1Gohms to the rear diaphragm
create a third hole on the metal plate (where the capsule mount is screwed) and put a connector through it, for the rear diaphragm (from the 1gohms resistor)
Important: put a 1nF between the front and the rear diaphragm soldered on the capsule connectors
remove filters C2 and C17
REMOVE 32V FROM PSU
remove cathode resistor R3 and replace it with a 2,2kOhms : measure around 1,3v
from the tube plate connect a MKP 1uF and after it plug an output transformer (around 10:1) : there some free space inside the body...
replace output wire in the mic and connect to the transformer
unsolder and disconnect the 2 hot pins of the XLR output (in the psu) and connect it directly to the transformer wires
Put a 3positions switch inside the PSU wich connects to the rear diaphragm components : +60V ,ground (via a 4,7k resistor min) and -60v
this will give you a 8, cardio and omni : wait 15sec after switching
enjoy a true K47 sound...
cheers
Fred
