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[kats]

Well, I got this u47 as part of a purchase lot of gear. there are a few oddball things about it and am wondering what any of you fellas can make of it!

First off, there is a clarostat 3 position switch (accessible from outside) that has me baffled. The middle position disconnects audio, the green position seems normal, and the third full clockwise position (red) doesn't seem to do much - however in the red position there is a 2dB drop in signal and a very subtle roll off in the sub lows. Seems too subtle as a HPF and was wondering if it could be some sort of impedance selector? Any of you fellas come across something like this?

The second thing that throws me for a loop is that it is a long body mic, but with a matte head grill (K47 inside). It doesn't come as a big surprise that the M7 may have been replaced with a K47, but replace the whole head basket? Odd to me, although this mic dates to 56 or 57 (I forget what the person told me) - was there any possibility of a matte basket being used on a long body during a transition period?

Some pics:






 

[klaus]

RE: Clarostat switch: follow the red and yellow wires soldered to the switch to their starting point (these are the only wires I can see, if there are others follow them as well.) That will give you a clue what the switch does.

Preliminarily, you can also perform some tests with the unplugged mic: test resistance between Tuchel pins 1 and 2 (audio out) as you click through the three switch positions. if the resistance deviates from the ca. 60 Ohms nominal on a stock U47 with BV8, you know the switch intersects at the secondary of the transformer.

RE: Matte grill on 1956 long body: that would be stock. From mid 1955 to the advent of the short body in 1958 all U47 had a matte grill.

[klaus]

I had another thought, looking at the red and green position markers on the rear of the mic: Maybe someone installed a remote pattern control, whereby the red position is "cardioid only" an the green "remote patterns"?

[kats]

Interesting thought. I turned the mic on this morning and selected omni on the switch. Then I switched back and forth between the "clarostat" red/green positions to see if it disabled the rear capsule. Discovery! The rear capsule does not pass audio at all, regardless of positions. Luckily we have another U47 here so I switched baskets. No matter what the red/green position is, the rear capsule does not become disabled. Now this may prove nothing, if my assumption on how the "true cardioid" is implemented is incorrect.

Re: Rear capsule not passing audio. So I opened it up and noticed the contacts on the omni selector switch were black with grime. So I cleaned them and now am passing, what sounds similar to, white noise.

Oh, I also tried the suggestion of checking the impedance between pin 1 and pin 2 using alligator clips, I have 14 ohms in green position and 27 ohms in red. I should add that this reading is a bit unreliable, I notice when I move the mic the reading changes and that the xformer is slightly loose in the body.

[klaus]

Bingo! The switch obviously taps the secondaries of the transformer (there are two secondaries on the BV8) to lower the output.

So, this is either done by connecting the two secondaries in parallel, yielding a ca. 50Ω output impedance, or one of two secondaries is cut off in one of the two switch positions.

[kats]

Okay, a had a few more hours to suss things out and here's what I find. I reversed the capsule to see if  it is in fact the rear of the capsule not functioning or if it is a connection issue. So with the capsule revered I got signal in cardioid, but it was was thin. Then the white noise in omni. I then reversed the capsule back, and somehow omni worked... for about 10 minutes, then back to white noise (cardioid works fine). So test inconclusive.

I then proceeded to check some voltages because I found the mic a little strident (I know that sound, it's when something isn't quite right). Step 1, pin 5 to check if it is pushing 105 volts. Nope, it was pushing 115v. I took out another PSU that I own, and measured 105v. I did a sound check, and it sounds smoother as it should.

I know the fellow I got this mic from just put fresh caps in the PSU, I wonder how he screwed up...

More work to do!

[klaus]

This may not be just the fault of new capacitors, but NG power supplies tend to drift upwards over time.

That's why it is critical for the life of tube and capsule (let alone other components and materials in the amp cavity, which must fight the considerable heat of the tube and wire-would resistor) to trim out the B+ to maximal 105VDC.

As to the capsule's erratic performance every time the rear side is added: You never revealed the type of capsule in the mic. If it is a PVD capsule, the answer would be obvious: deteriorated PVC membrane. But that remains pure speculation on my part, until you show or tell us what type and brand the capsule is.

[kats]

Here are some pictures that will hopefully help. And as you can tell the cap is a K47. A few more things to clear up is that the extra switch on the PSU acts as a 20dB pad on the audio out. The caps are hot glued in upside down because the holes were too small to mount and the previous owner didn't want to drill holes. Another thing to note is that I took a measurement of the PSU today (with a different u47 amp) and it clocked in at 109-110 volts (down about 5v from yesterday). On the "good PSU" it clocks in at 102.5v (down about 2.5v) using the amp we're working on.

Re: Capsule back side and omni. For a test I took the basket off another U47 that had a Theirsch PVC M7 and put it on the amplifier of the mic in question. Omni works. I replaced the basket with the original and re-tested and the when switching to omni it was good for about 1 minute of speaking  until the noise slowly ramps up to white noise. If I switch it back to omni and let it rest 5 minutes and can repeat the problem starting from no noise. (as an aside, the k47 in cardioid outputs exactly 4dB hotter than the M7).

And lastly, the BV8 has close to 1mm of play mounted. Years ago it looks like someone put kleenex and tape to secure it in (see original pics above). I removed it and noticed the play. I was wondering why they would not just tighten the screws that hold the xformer in? Is there something I should be aware of before doing that? It looks like I'd probably have to remove the vf14 to do it properly though.

[kats]

PS, I would also like to test the voltages of the other points in the mic, but only have a shock mount right now. Is it okay if I power the mic without the headbasket on and stand it up upside down without the amp cover tube?

[klaus]

The caps are hot glued in upside down because the holes were too small to mount and the previous owner didn't want to drill holes.

Right. Would be a shame to despoil the beauty of an ocean of hot glue!

Regarding the failure of the K47 (by the way: one of my favorite versions of that capsule!):
The rear looks contaminated, and I would bet that proper (professional) restoration of the capsule would remedy the problem.

I'd recommend: Tighten the two transformer screws. There is no direct pressure applied to the bobbins when doing so. All you do is compressing the laminations a bit (hand tight with a small screwdriver will do!)

Down the road I'd also recommend to start taking a look at some of the inferior replacement components that were used in the audio circuitry. Among others, the 0.01 mfd green block comes to mind.

[kats]

Thanks Klaus,

Before I pass go, I'm going to have to do something about the PSU. When you say the values drift, what components are left for me to replace to bring it back to proper order? Aside from the xformers, I only see one resistor and that black tall skinny cap. Or am I supposed to introduce a new resistor to brign it down the 10v?

Re: Cheap components, I only see the green block and the Mallory that look unoriginal. I'm pretty sure I have the correct value in polystyrene around for the green block. Do you see any other?

[klaus]

There are two ways to bring the voltage down to proper levels:

1.Replace the bridging resistor (green Rosenthal) with a slightly higher 10W value.

2. Add another 10W resistor BEFORE the first choke coil.

[kats]

I guess the question begs: Would it be more prudent to just replace the PSU with a modern one? I'm also worrying about the rectifier and if that is a ticking time bomb?

Thanks again for your guidance!

[klaus]

I would not replace this power supply. The early NGs, like yours, are very well designed and yield very clean DC. Though the NG lacks voltage regulation, it's a minor issue, compared with the often mediocre implementation of regulation found in many modern supplies.

In the professional studio environment, where the user has some idea and influence over what comes out of the wall, AC-voltage fluctuations are usually kept small enough that they don't matter (±2VAC).

[kats]

There are two ways to bring the voltage down to proper levels:

1.Replace the bridging resistor (green Rosenthal) with a slightly higher 10W value.

2. Add another 10W resistor BEFORE the first choke coil.

After pulling the PSU apart  bit more, I noticed a 1K resistor on pin 5 as well. Can I just up the value of this one, or were you being very specific on the 2K resistor that is shown on the pictures above?

 

[klaus]

Just check the heat developing on these dropping resistors. Try to even out the heat (and power dissipation) over all resistors involved, and determine the resistive value that way. If one resistor does overtime with dissipating heat (and, in the U47 application, soaking up current) over another, it will fail sooner and voltages will drift more.

[kats]

Thanks Klaus, your aces!

[klaus]

If the existing resistors don;t seem to be getting too hot, you can slightly lower the output  voltage of the unit by increasing resistor value of one of them. Go to the one which is least hot first. If they are both very hot, add another 10W resistor at the very beginning of the circuit, right after the rectifier.

This all assumes that you are versed in working on high voltage electricity. Work with one hand behind your back when testing or manipulating the power supply when on, or pass the job to someone who has experience with voltages in excess of 300VDC!

[kats]

Well, I am going to have to put the mics down for now because I am booked until the middle of November before I have a break. So here's my last entry until then.

PSU: I decided to remove both resistors and check their values. The 1k tested perfect, the 2k gave me an OL reading. I am thinking that the 10V drift can be due to this. So I'm just going to replace the resistors with stock values and re-test.

K47: I did the "breath test" on the back side, and instant fuzz! I'm sure your right about the contamination. But one thing I have been doing is some extensive testing on the front side because it seems a little brighter than one would expect judging by anecdote. Compared to an old M7 with it's original skin, this k47 is more pronounced in the 10k region. There's an impression that it has ever so slightly less weight in the low end - although it's possible that the extra top end may be influencing the perception. I also compared it to a Theirsch made PVC m7 with the same impression. I have read many times the perception of the k47 to be a darker capsule than a M7, but in my case it seems to be different.

VF14: I decided to kill two birds with one stone and replace a T-USA VF14K (glass tube in steel) with the spare Vf14 that came with this mic. I wanted to make sure the VF14 works, and as an aside compare the difference in sound between the two tubes all else being equal. The good news is that the VF14 (not NOS) works great and is very quiet!

Now I'm sure some may find this interesting regarding the difference between the two tubes. Tonally I found them quite similar. The main difference in tone was a subtle extra presence in the Vf14 (I allowed an hour for each tube to warm up). The impression I got though was that the VF14 seemed to be a little "punchier" (for lack of a better term) or some may say a little more dynamic. The overall impression was that the voice felt bigger or livelier. These differences were subtle and I wouldn't lose much sleep if I had to replace a VF14 with the T-USA VF14k. That said, although the differences were subtle, I had two other producers listen to the files "blind", and they described the differences in similar terms. In other words, subtle BUT noticeable.

Again, thanks for reading my latest adventure and an extra thanks to Klaus for his guidance.