Hello Guys,
Sorry for being that late, but currently I am limping behind because of too much work and not enough time for fun things.
Regarding your questions, the circuit is a modified version of a
classic C12, with a few improvements in noise and depth. There are forums to be filled with opinions about what is the best, all original or implementing better parts and circuit to get less noise and more tonal quality. But here is something to try.
The bias of the classic C12 was done pretty cheap with adding a bunch of noise via a negative dropping resistor of the main supply; to implement this type of bias, a major modification would have been necessary.
Also with that style of fixed bias there is no way to bias the tube properly, so only with a selected tube two mics would sound the same. In the end the fixed bias sounds pretty dark with most CK12 capsules.
Changing it over to cathode bias with a bypass cap decreases the noise floor, makes the mic less sensitive to the tube used, and opens the door for individual fine tuning, capsule to circuit.
Just by playing with the value of the cathode resistor and cap, it is pretty easy to find the sweet spot of that particular capsule/tube/x-former.
The setup of the capsules for remote control done in the historic AKG way is a bit noisier than the Grosskopf style remote, and the coupling cap before the grid is quite complicated with reference to sound. So I chose the M49 remote developed by Mr. Grosskopf; in a way this VR is a C12/M49c hybrid, retaining the sound of a classic C12 with the noise figures of an M49c.
Rating that project on a scale of 10 to 1, this is definitely a full 10 with reference to difficulty; and taking the cost and time in account, not the average DIY project. For everyone that is brave or insane enough, attached you find the schematics with parts list. I will try to get a PCB layout done that I can upload in a form of a jpg. I still do all my designs and PCB analog, but layout and most of all assembly is curtail to get a good sounding mic.
All parts needed are available online. Ohmite makes the best high impedance resistors, caps is matter of taste, resistors are metal film, the Teflon standoffs are hard to find. The Hiller style T14 that my company makes is available via Mercenary. Most important: "I do not offer a C12VR mod, so do not send me your mics!" I talked with Terry to assist him to come up with an out of box C12VR modded mic.
Best regards,
