I've been looking for a thread that really addresses my situation, but I haven't found one so here we go. About 5 years ago I had the miracle happen that we are all waiting for. Because of my contacts in town, a guy called me up who needed to clear everything out of the house that his recently deceased musical-electronics engineering brother had left behind. The house was filled with all kinds of test equipment and audio gear. He wanted no money for it, and just wanted me to haul away as much as I could. I loaded up oscilloscopes, oscillators, cheap mics, stands, wire, parts, an Altec speaker, a Revox PR99, and an Ampex PR10. As I walked to the door to leave I opened a large old wooden box thinking it might be a turntable and saw the head basket of a U47 on a non Neumann housing. I held my composure, said I'd like this too, and left with a truckload of goodies. The first thing I did when I got home was open up that mic to discover a VF14 tube inside the housing. There is no M on this tube. The other parts did not look original, and the power supply is homemade. I figured out right away that whoever made the supply didn't know what they were doing, so I rewired it correctly, and replaced the electrolytics which were bad, for $33. I have attached pics of the schematic of the mic as found, the capsule which looks a hellofa lot like a M7(not reskinned), the tube and transformer, the capsule interface parts, and the resistors and caps. and the head basket.
The U47 build threads I've found on the groupdiy site are all oriented to not using a VF14, but I'm lucky enough to have one here even though it does not have the M (It actually sounds pretty quiet) Thats why I started this thread. I need opinions about rebuilding a U47 using a VF14. I'd like to assemble parts made by the original manufacturers or something very similar.
You'll notice in the schematic that there is no 1780 ohm resistor to the heater. He made a separate adjustable line from the power supply that sets the 36V which works pretty well. There is also a 0.002 uF disk capacitor to ground from the backplate instead of the correct 0.01uF cap. An extra 0.1 uF cap has been added in parallel to R8 which is 3.5 Mohm instead of the stock 3 Mohm. R7 is 2.25 Mohm instead of 2 Mohm. Finally, a Triad HS-60 transformer set for 150 ohms has replaced the original BV8 transformer.
The additional good news is that it works and sounds pretty good. I don't have another U47 to compare it to but I've used it on vocals, in front of a complete drum set, and as a room mic, and its got a great sound.
What I really want to do is to gather replacement parts, instead of the ones he used, that are as close to the originals as possible, including the transformer. After looking over the forums, I've found 60 Mohm resistors from Alipress, and I see that Moby's BV0.8 transformer is highly recommended for the U47. I noticed the U47 schematic on this site shows (2) 100 Mohm resistors going to the capsule instead of the 100 Mohm and 60 Mohm that I've seen on some of the earlier U47 schematics. Does anyone have strong opinions on what kind of difference that creates? Looking at photos of original U47s I see one capacitor made by Bosch near the output connector. Does anyone know the manufacturers of the original caps and resistors? I've had a hard time finding wirewound 2 Mohm and 3 Mohm resistors on Mouser. The ones in this mic are 2.25 Mohm which is pretty close, and 3.5 Mohm which is a little farther off. Has anyone fooled with the values on these resistors feeding the capsule?
The part that really annoys me is the 0.002 uF disk capacitor to ground from the backplate. Neumann must have used something better than that and this one is much smaller. I've got a dialectric absorption comparison tester that I'll go through the caps I'm going to choose, but it would be cool to find the original manufacturers.
I haven't included a photo of the casing, but it really doesn't look "Neumann". I'm thinking of seeing what micparts has for that, hoping that my head basket will fit.
Lastly, I'd like to rewire the heater voltage circuit to its original layout, but it means finding a 1780 ohm resistor that can handle the 2.67 Watts it uses. (105V-36V)^2/(1780 ohms) = 2.67 Watts. I'd like to use a 5W resistor for that so it runs cooler. There's nothing like that at Mouser. Anyone know where you can get a monster like that?
Well, any opinions, suggestions, and wild ideas about how you think I should tackle this are really appreciated.
In closing this is a reminder that dreams do come true. Treat people right and eat your vegetables and your dreams may also come true.
