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Author Topic: U47/VF14 problem.  (Read 6876 times)

NVS

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U47/VF14 problem.
« on: November 12, 2012, 09:05:14 am »

Hello.

I have a Neumann U47 (M7/VF14M/BV8) and there’s a problem with the VF14. There is a “resonant/ringing” sound coming from the tube. Sounds like a sine wave. It is at around 390Hz and gets triggered by either movement, or sound around that frequency (for example if I sing a G-note (392Hz). I first heard it when singing a song in the key of G). It rings for 5-10 seconds. Slowly fading out. The higher the volume of the source or the harder I knock on the microphone body, the higher the ringing. I have also now powered the mic up, without the body tube on to measure inside and the lightest touch on the tube triggers the sound. The probe on the multimeter touching the f1 or f2 pins on the VF14 socket also triggers the sound.

The ringing starts pretty much as soon as the mic is properly powered up. The voltage in the PSU is at 107,9V. It actually varies between roughly 107,6V and 107,9V, up and down. I understand that this voltage, even though it is over 105V, is acceptable and will not damage a VF14. Correct?
The filament voltage measures 37,4V. Plate is at 33,7V and the k pin on the tube socket measures 1.14V

The whole thing might have started several months ago after I opened the mic for the first time and took the VF14 out. (I did this because the insurance company advised me to take pictures of everything.) I have tried cleaning all contact points in the mic. VF14 pins, tube socket etc… but it did not help.

I live in Norway, and it gets cold here in the winter. My studio is in my basement, and when not in use it gets a little cold there to (though never freezing). But, I have always made sure that all my microphones have reached room temperature before powering up. Never had any condensation issues.

So, do I have a defect VF14 with no hope, or is there anything that can be done?

Thank you.

Narve Vik-Strandli
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klaus

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Re: U47/VF14 problem.
« Reply #1 on: November 12, 2012, 01:30:26 pm »

This does indeed sound like a microphonic VF14. Though it does happen rarely, microphonics in VF14 are not unheard of. But it seems that at least during your ownership you took care of running the tube with correct voltages that did not cause or exacerbate the problem.

There is no known remedy for run-away microphonic feedback but one: knocking all sides of the tube with a blunt instrument (wooden mallet), and the possibility that the severe shocks administered will break a brittle filament and render the tube dead.

That's the final choice you face with a mortally ill patient: try one last, extreme thing to save him.
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Klaus Heyne
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Piedpiper

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Re: U47/VF14 problem.
« Reply #2 on: November 13, 2012, 02:54:47 am »

and what exactly does "knocking all sides of the tube" do, at best?
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klaus

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Re: U47/VF14 problem.
« Reply #3 on: November 13, 2012, 04:27:41 am »

When successful, the blows reposition (bend) the offending filament enough to suppress or reduce its run-away resonance.
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Klaus Heyne
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NVS

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Re: U47/VF14 problem.
« Reply #4 on: November 13, 2012, 05:05:01 am »

Klaus.

Thank you for taking the time to answer! At least now I have an answer, even though it’s bad news. I only had a little time last night to try out what you suggested. (These days I’m remodeling the bathroom and guess what my wife think is more important, the U47 or finishing the bathroom?)

Anyway, I did knock the tube using a pencil. I “closed my eyes” and gave it a few good knocks. And that kinda worked. I say kinda, because here’s what happened. I took the body tube off, powered it up knocked the tube and it worked just fine. Put the body tube back on powered it up and it was fine for an hour or so, but then the microphonics came back. Then I figured I’d give it another go thinking I should give it a harder knock, so off with the body tube, powered it up and waited, expecting the microphonics to come back, but it didn’t! I gave it about half an hour or so, but it was fine. Then, without knocking the tube, I put the body tube back on and half an hour later the microphonics was back. Could the problem be triggered by heat? I mean, a microphonic VF14 will be useable as long as it doesn’t get too warm? Or is what I experienced just happenstance?

I will continue the VF14 rescue mission after work today. Never say die! It ain’t over ‘till the fat lady sings… on a different microphone.

Narve.
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klaus

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Re: U47/VF14 problem.
« Reply #5 on: November 13, 2012, 05:14:52 am »

Knowing that the knocking works -for a while at least- I would now escalate:
pencil back into the drawer, and the back of a wooden screwdriver or similar, thumb-sized wooden tool next!

Good luck!
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Klaus Heyne
German Masterworks®
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NVS

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Re: U47/VF14 problem.
« Reply #6 on: November 13, 2012, 05:27:58 am »

Ok, thumb-sized wooden tool next.

Narve.
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NVS

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Re: U47/VF14 problem.
« Reply #7 on: November 14, 2012, 04:03:09 am »

Klaus

Yesterday afternoon I emailed Andreas Grosser asking about a VF14 for sale on ebay. I also gave him a quick description about my VF14 problem and he suggested I take the damping rubber placed around the tube out. This worked! Four hours of flawless operation.

Hopefully this solution will work in the long run. If not I will of course try knocking it.

Again, thank you for taking the time to help me here.

If anything changes I will post it.
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klaus

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Re: U47/VF14 problem.
« Reply #8 on: November 14, 2012, 06:10:44 am »

By taking out the close-cell foam ring around the tube, you risk tube and wiring damage: in a whiplash incident, for example, there would no longer be any buffer between tube and the housing tube. The rubber mount of the tube socket will also be stressed through extended range movement. Repeated movement of the tube socket could eventually also sever wiring connections to the socket (unless you NEVER moved the mic.)

Besides, you did not solve the original problem: the tube's hyper-vibrating filaments. By removing one of two transmission links, you simply buffered runaway microphonics.

I bet, if you have a loud enough sound source, the ping will still be audible.

Keep us posted!
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Klaus Heyne
German Masterworks®
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NVS

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Re: U47/VF14 problem.
« Reply #9 on: November 14, 2012, 09:22:13 am »

I realize the original problem isn’t solved, I just didn’t think about the things you now point out, except that I did in fact think about a possible strain on the rubber mount when laying the mic back down into its box.

I really tested it good last night though. Plenty of loud sources, but could not hear the ringing. It’s not really a ping type sound by the way. It’s like the sound coming from a tuning fork (only at around 390Hz).

Just curious, what is the worst case scenario if the wire solder joints brake?

I do take what you are saying seriously and I guess I will have to go for the “knocking the tube fix” after all, but I must admit the thought of completely killing it is kinda scary, now that it’s “working”.

I have of course also started looking into trying to find a replacement VF14. (A real one, because I have previously tried one of the “glass-tube-inside-plug-and-play” solutions, and it simply did not sound right.) 

And, I’m sorry, but I did not understand: “you simply buffered runaway microphonics.” The buffered part confused me. My English fail me a little here.
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klaus

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Re: U47/VF14 problem.
« Reply #10 on: November 14, 2012, 02:21:07 pm »

Just curious, what is the worst case scenario if the wire solder joints brake?
The mic fails, and you would need to repair and resolder.

Quote
And, I’m sorry, but I did not understand: “you simply buffered runaway microphonics.” The buffered part confused me. My English fail me a little here.
You covered the symptoms, but did not address the source of the symptoms.

I agree with you: if taking out the foam ring removed the microphonics, and if you do not mind some of the potential consequences of removing it, by all means, proceed!
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Klaus Heyne
German Masterworks®
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NVS

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Re: U47/VF14 problem.
« Reply #11 on: November 15, 2012, 04:27:34 am »

The idea of something else going wrong, if used without the foam ring, started to bug me more and more yesterday. Resoldering wires is in itself not really a problem, but obviously I want to avoid it. And, ruining the rubber mount would of course not be good.

So, yesterday I put the foam ring back on. However, I placed it as far down on the tube as I could, but still making sure it was keeping the tube secure, and that seemed to work.

I think I will leave it at that for now, even though the original problem is still there, and see if I can find a good VF14 (at least dream of it). If I do, I will try knocking the old one. Of course if I can’t find a VF14 and the one I have acts up, I’ll knock it.

Thanks again for taking the time on this.
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klaus

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Re: U47/VF14 problem.
« Reply #12 on: November 15, 2012, 01:11:35 pm »

You are welcome, and good luck!
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Klaus Heyne
German Masterworks®
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GYMusic

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Re: U47/VF14 problem.
« Reply #13 on: January 11, 2013, 12:42:38 pm »

Just a thought.  I have some experience with parasitic oscillation in RF amplifier tubes.  Is it possible that the same could happen with the VF-14?  Narrowing the Q in RF applications can filter out offending frequencies that set the tube off into an oscillation.

klaus

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Re: U47/VF14 problem.
« Reply #14 on: January 11, 2013, 08:47:10 pm »

The tube's hyper-microphonics will be triggered by vibrations of any frequency, mostly low frequencies or simple shocks- all  transmitted through the housing tube, rather than by anything electric inside the circuit.

So I am not sure how you would suppress the narrow band response from a wide band trigger.
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Klaus Heyne
German Masterworks®
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