Vishay
RS here have Metal Film at 100M
I tested the originals using the Neumann ext psu 408 VDC with no load.
The 100M indicates 130M
The 60M is reading 125M
Test Trumps Theory eh?
RS here have Metal Film at 100M
I tested the originals using the Neumann ext psu 408 VDC with no load.
The 100M indicates 130M
The 60M is reading 125M
Test Trumps Theory eh?
I would not trust a measurement of very large resistors without using a clean laboratory test fixture and an electrometer.
BTW what do you mean by noisy, per published spec or your perception. 60Meg is very small for a condenser mic and contributes considerable noise, I have no idea about the grid current of the VF14 but the bias resistor would be better at 1-5G Ohm if possible.
BTW what do you mean by noisy, per published spec or your perception. 60Meg is very small for a condenser mic and contributes considerable noise, I have no idea about the grid current of the VF14 but the bias resistor would be better at 1-5G Ohm if possible.
> 60Meg is very small for a condenser mic
I believe that is appropriate for a tube. Grid current is low but not as low as a small JFET. Grid current in >>100Meg makes hiss. I do not have parameters for a U47 or its parts, but a Sony and an English tube condenser mike got optimum in the 10^2Meg area.
I am reluctant to listen to mike hiss without objective calibration. There is always hiss!! A modern reference could be AKG 414, which is usually in good shape and reasonably low hiss. Adjust both mikes on speech so A/B switch matches level, clear the studio, crank gain (same on both!).
I believe that is appropriate for a tube. Grid current is low but not as low as a small JFET. Grid current in >>100Meg makes hiss. I do not have parameters for a U47 or its parts, but a Sony and an English tube condenser mike got optimum in the 10^2Meg area.
I am reluctant to listen to mike hiss without objective calibration. There is always hiss!! A modern reference could be AKG 414, which is usually in good shape and reasonably low hiss. Adjust both mikes on speech so A/B switch matches level, clear the studio, crank gain (same on both!).
Right, best to leave it alone even if the original capsule and body would be far less noisy with a complete redo of the circuit it would have no value to a collector.
Noise
There are two main elements to the noise. A constant hiss, which now seems likely to be exaggerated by the grid resistor now drifted to 125M, over twice the intended. There is another noise, intermittent bursts, I think someone called it discharge noise. Several experts have suggested that often comes from an oxidised valve socket. I have cleaned that, but the reassemble will have to wait for resistors
There are two main elements to the noise. A constant hiss, which now seems likely to be exaggerated by the grid resistor now drifted to 125M, over twice the intended. There is another noise, intermittent bursts, I think someone called it discharge noise. Several experts have suggested that often comes from an oxidised valve socket. I have cleaned that, but the reassemble will have to wait for resistors
I meant a redo that dumps the tube, if that makes it more clear. 1" capsule mics exist at 4dBA or so but they don't use tubes.
@SoundSound - I just remembered, my No. 1 guy on Neumann is Martin Kantola of Pearl Mics. He's a great guy and if you find him on social media I'm sure he would give the best advice. See below his full ambisonic mic made with genuine NOS Neumann capsules and plenty of VF-14's $$$$.
Attachments
Last edited:
VF14
Yup Unicorns. Only 29,000 manufactured. 6000 selected for Neumann VF14M
I have sought advice from a couple of the world's experts. I hear that these valves very rarely fail. Also, 80% or more of those that get a bit noisy or microphonic can be regenerated. So the Fat Lady ain't singing yet. There are a couple of replacement options from Phaedrus and Andreas Grosser.
Neumann will still service a u47, up to spec. with a 2 year guarantee.
Yup Unicorns. Only 29,000 manufactured. 6000 selected for Neumann VF14M
I have sought advice from a couple of the world's experts. I hear that these valves very rarely fail. Also, 80% or more of those that get a bit noisy or microphonic can be regenerated. So the Fat Lady ain't singing yet. There are a couple of replacement options from Phaedrus and Andreas Grosser.
Neumann will still service a u47, up to spec. with a 2 year guarantee.