Needing closest parts for Rebuilding a tube U47

[CalavoBob]

I would buy a body from Flea to use with the headbasket. I can't see the finish of the headbasket but if it's matte you should aim for short body otherwise go with long body.

Hi Tomas,
Thanks for the tip on Flea's U47 mic body. This mic body is 168 mm from the bottom of the headbasket to the end of the tube. There's an end cap that screws onto the bottom of the housing tube that extends an additional 5 mm past that. The headbasket is a matte black. I'll find out what size Flea's bodies are.

 

[tomas.borgstrom]

Hi Tomas,
Thanks for the tip on Flea's U47 mic body. This mic body is 168 mm from the bottom of the headbasket to the end of the tube. There's an end cap that screws onto the bottom of the housing tube that extends an additional 5 mm past that. The headbasket is a matte black. I'll find out what size Flea's bodies are.

Has it been painted? The U47FET exist in a dark finish but they don't have the pattern selector switch. If you buy new hardware from Flea you will have to rebuild the whole microphone. I don't know if that was you plan to begin with.

 

[CalavoBob]

I do think it was painted black, but this is a tube U47 not an FET.

 

[Drea]

“You're joking, right?”

No I have switch them record them switch them record and switch them many pieces and brands and compare. It costs me weeks . Just like switching silver wires / copper and stranded vs hole wiring.

 

[k brown]

You could make a fortune selling capacitors that give that 'tape sound'.

 

[tomas.borgstrom]

I do think it was painted black, but this is a tube U47 not an FET.

I've never seen or heard of a dark U47

 

[lp1000]

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.

Hi,
If you have resistors of 2M and 3M, the one closer to the ground, then the calculation of the divider at 105 is as follows. see the image. The voltage going to the capsule M7/K47 is 63V, but this is only a static voltage. If you have resistors of 2.25M and 3.5M, then the voltage slightly increases to 63.9V if I calculate correctly, assuming the larger resistor is closer to the ground. In the M49, the voltage is approximately 58V–60V depending on the type a, b, c M49 and the voltage to the anode is 116 or 120V; this voltage is also used as the static voltage for the capsule.

 

[lp1000]

Iv'e built a 1:1 clone with my own hand wound resistor. Not because it's better than using a PSU with separate heater supply but simply because it was a fun challenge.

Interesting that you made it yourself. Originally, there was a resistive wire in mica. What kind of wire did you use and what length did you need to achieve approximately 1800 Ohms? I myself use a ceramic resistor of 1800 Ohms. It's a challenge to make your own winding.

 

[vitopower]

The sealed flat Siemens, RFT, Bosch, SEL work fine but then you have to build the U47 like the early models with the transformer upright between the capacitors. Some axial Siemens I've used were within the specifications but they are hard to find. There's a lot of sealed axial MP of different brands but they are almost always too large.

If you are ok with moving the transformer mount up just a bit, there is room in the Studio 939 longbody to fit a flat Siemens MP (coupling) underneath the moby transformer and a smaller film for the filter cap and still have room for the VF14.

 

[Drea]

In the end is there a sound difference between a 10W 1780 ohm. Or a hardwired one.

 

[Khron]

In the end is there a sound difference between a 10W 1780 ohm. Or a hardwired one.

No.

 

[tomas.borgstrom]

In the end is there a sound difference between a 10W 1780 ohm. Or a hardwired one.

The hand wired resistor is pushed against the bottom of the microphone body and use it as a heat sink. You have to find a similar arrangement for the 10W resistor, otherwise the interior of the microphone will get even hotter than the original U47 and that might affect the performance and lifespan of the electronics.

 

[tomas.borgstrom]

Interesting that you made it yourself. Originally, there was a resistive wire in mica. What kind of wire did you use and what length did you need to achieve approximately 1800 Ohms? I myself use a ceramic resistor of 1800 Ohms. It's a challenge to make your own winding.

I used manganin wire that I believe is the same material Neumann used. The wire I used had a diameter of 0.06mm and a resistance of 182ohm/m. I did this about two years ago and in my notes it says a bit less than 300 turns around the flat core. I wouldn't recommend this project if you're not a microphone freak that wish to explore all the historic designs. It's time consuming and requires a lot of patience. You will probably fail the first and perhaps also the second time. The wire is very thin and fragile. It tends to tangle up, get knots and break.

I've made one (second try) and it still works fine. I will only consider making another one if an original U47 without one show up in my shop.

 

[CalavoBob]

The hand wired resistor is pushed against the bottom of the microphone body and use it as a heat sink. You have to find a similar arrangement for the 10W resistor, otherwise the interior of the microphone will get even hotter than the original U47 and that might affect the performance and lifespan of the electronics.

Tomas' comment above leads to what may be a sonic difference between a stock U47 with the 1780 ohm resistor (using the microphone body as a heatsink), and my "quasi" U47 that derives its 36 Vdc from an adjustable voltage divider in the power supply box. My mic body would be running cooler since the heat dissipated by the voltage divider winds up in the power supply box.
The downside to comparing the difference though, is that I would either need to put up with the hassle of winding the 1780 ohm resistor, or pay the $500 bucks to get one online.

 

[Khron]

Tomas' comment above leads to what may be a sonic difference between a stock U47 with the 1780 ohm resistor (using the microphone body as a heatsink), and my "quasi" U47 that derives its 36 Vdc from an adjustable voltage divider in the power supply box.

A "sonic difference" that amounts up to what, exactly? And how/why?

 

[Drea]

I have just glued the resistor deep in the bottle on the sides and a dot arctic silver 5 in the middle. The shell is after an 30 minutes 100 Fahrenheit.

 

[CalavoBob]

A "sonic difference" that amounts up to what, exactly? And how/why?

Hi Khron,
The sonic difference if any would be because of the heat added from the 1780 ohm resistor to the enclosed system. There could be an increase in noise if the heat is significant. More than a perceivable sonic difference though, would likely be shortened tube life and adjacent component life due to the increased temperature. Since I don't have two U47s to easily compare the difference I don't think I'll find out. So far Drea measured only 100 degrees F using the wound resistor, so the shell seems to be dissipating the heat pretty well.

 

[Khron]

There could be an increase in noise if the heat is significant. More than a perceivable sonic difference though, would likely be shortened tube life and adjacent component life due to the increased temperature.

That makes sense, actually. I guess i'm just a bit prejudiced against claims of (significant) differences between plain old passive components - usually the claims are for the better (especially in favor of the more "exotic" and/or "historically accurate" ones)...

 

[tomas.borgstrom]

A "sonic difference" that amounts up to what, exactly? And how/why?

Hi Khron,
The sonic difference if any would be because of the heat added from the 1780 ohm resistor to the enclosed system. There could be an increase in noise if the heat is significant. More than a perceivable sonic difference though, would likely be shortened tube life and adjacent component life due to the increased temperature. Since I don't have two U47s to easily compare the difference I don't think I'll find out. So far Drea measured only 100 degrees F using the wound resistor, so the shell seems to be dissipating the heat pretty well.

The most solid argument is that the heat will alter the tension/properties of the capsule membrane.

That makes sense, actually. I guess i'm just a bit prejudiced against claims of (significant) differences between plain old passive components - usually the claims are for the better (especially in favor of the more "exotic" and/or "historically accurate" ones)...

I do think that you can hear relevant differences between passive components but the problem with vintage pro audio and HiFi is that the enthusiasts often favor what you are suppose to like. Almost all of the coveted units have of the shelf components from a retailer in the same country. If they where produced over a longer period it changed with time. I enjoy using original or components typical of the time but I'm not so fond of carbon mass resistors and a lot of old capacitors sound really bad even though they measure as expected. I like excitement but not mud. My go to La2A is "upgraded" with Jensen transformers. Much more versatile than the original.

The problem with building clones is that every deviation in sound from the original is a failure even if the sound quality is as good or better, especially if the term "mojo" comes into play. Perhaps we should start the thread "which magic spell do you use when building a U47?".

 

[CalavoBob]

There really is something special about how mics like U67s, U47s, C12as, and Ela M251s sound that is very hard to quantify. Thats the reason we are drawn to them. Granted there are variations in sound even between like models but these particular mics rarely dissapoint if they are in good working condtion. Apart from whatever we might measure in them, my reaction to using them is "wow, that really makes my voice sound great." It's probably the mic's right combination of compression, THD, TIM, and frequency response that is responsible for most of their "sound". These are measurable, but awfully hard to create with a pile of resistors, capacitors, inductors, etc.

Like Tomas, I'm not too fond of carbon compostion resistors. As for caps, I've been testing them and selecting them for low dielectric absorption. They audibly handle transients better than ones with sloppy dielectrics.

,