| U47 voltage questions [message #337218] |
Mon, 28 April 2008 03:13  |
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 J.J. Blair
Messages: 10045
Registered: May 2004
Location: by the sea & sand |
Diamond Member |
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I've got two U47s sitting in front of me. One of them is supplying 3.4V to the backplate (mic #1), and the other is supplying 5.4V (mic #2). The schematics don't indicate what the proper voltage is, only the voltage before the capsule bias resistor: 63V.
Oddly, using the same PSU, the mic with 3.4V is measuring 68V before the capsule bias resistor, but the one with 5.4V is measuring about 55V.
What is the ideal voltage for the capsule backplate?
Mic #1 is T-USA U47M, with a VF14. R7 and R8 are using 1MΩ and 2.2MΩ respectively, where the schematic calls for 2MΩ and 3MΩ. What effect is this having on my findings?
Also, mic #2 is a Bill Bradley modded mic. All the caps and resistors inside the mic were changed out to high grade components, and the values are all correct. However, the NG PSU had an extra resistor added parallel to the 2.2kΩ cap, dropping the resistance to 1.7kΩ. The result was that 112V was being fed to the mic, and 40V being fed to the tube, rather than 36V. I have removed the resistor, and now the voltage is 107V, and the correct voltage is being fed to the tube.
Why would someone do that?
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| Re: U47 voltage questions [message #337339 is a reply to message #337323 ] |
Mon, 28 April 2008 14:00  |
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Klaus Heyne
Messages: 2027
Registered: April 2004 |
Platinum Member |
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Can I ask you what you find lacking in a stock U47 voltage setup?
Here are the issues and my recommendations:
1. Run the U47 with 36V heater voltage. That's about as high as the heatsinking in this mic is designed for. (See no. 2)
2. Running a VF14 with a 4 VDC higher heater voltage does not produce any audible changes, but it (moderately) shortens the filament's life span. It also creates more heat and that heat can not go anywhere. So, because the heatsinking through the metal dispersion is maxed already, the extra heat warms up the airspace around the tube, which in turn throws the operating parameters off (I mailed you some more technical details about this.)
Bottom line: You will not improve the tube's performance in a U47 by messing with its voltages.
You may experiment with the polarization voltage, but there again, I believe Neumann hit the jackpot with 60VDC. Yes, you can pump 70VDC or even 80VDC into an M7 capsule and it may hold up (because it's one of those thick 10µ PVC membranes.) But consider the sonic effects! The capsule tends to 'harden up' with too much polarization voltage applied, and gets increasingly ugly in the mid-range.
Klaus Heyne
German Masterworks
www.GermanMasterworks.com
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| Re: U47 voltage questions [message #337375 is a reply to message #337366 ] |
Mon, 28 April 2008 15:39 |
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Klaus Heyne
Messages: 2027
Registered: April 2004 |
Platinum Member |
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| J.J. Blair wrote on Mon, 28 April 2008 13:23 |
I'm a bit concerned about the (tube) life, though, as the tube output is measuring a little shy of 31V.
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You mean 31V plate voltage? If so, yes, that's a bit low, but not necessarily fatal.
First make sure that the correct R7 and R8 are installed. Then measure again. Is this the mic with the high heat, due to the missing wire resistor in the wall?
If so, the tube may be less noisy or even sound OK. Take it out and test it in a stock U47 environment. That will make it as clear as day what is going on.
Klaus Heyne
German Masterworks
www.GermanMasterworks.com
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| Re: U47 voltage questions [message #337927 is a reply to message #337884 ] |
Wed, 30 April 2008 13:28 |
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Klaus Heyne
Messages: 2027
Registered: April 2004 |
Platinum Member |
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If you are running your mic THAT hot, you should not even think of going forward, because you will harm other, much more costly components like transformer, tube, etc.
Why don't you just wind yourself a resistor patterned after the original? That to me seems to be the most prudent and durable solution.
Klaus Heyne
German Masterworks
www.GermanMasterworks.com
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