Hi list, I have a nice U64 but since the last replacment of the nuvistor there is a microphone-effect when I knock (gently) on the corpus of the mic. It rings like a tiny bell. I checked the whole preamp and found an overaged 250MOhm-Resistor. When I replaced it, the Mic sounded better (and did not hum anymore), but the ringing refused to go away. I tried several other nuvistors and it seems, that the ones which are best in shape do ring the most. Now I wonder if it's just normal behaviour, and the real problem is the rubber mounting, which's gone to dust, long ago, as I can't find any other broken component...
So this is my question: has anyone an U64 and does it behave like mine? Or has anyone an explanation?
Damping down microphonics from the notoriously microphonic Nuvistor is very hard to do, especially in such a tiny housing where isolating external shocks or sound pressure waves from reaching the Nuvistor's filaments is close to impossible.
I am afraid, you may need to look for more Nuvistors to find a really low-microphonic specimen. Just make sure that, every time you change the Nuvistor, the timbre or pitch of the microphonics changes (indicating, the resonance really comes from the Nuvistor!)
If it remains always at the same pitch and timbre, regardless which Nuvistor you use, and if you can even trigger this resonance after removing the tube, and be able to hear it acoustically, when you hold the housing directly to your ear, then ping it, you have to quiet down another component on the circuit board which is ringing!
thank you for your answer! It's always a very similar ringing with any of my Nuvistors (I have checked 4 different Brands) - but I tried to locate the source by opening the housing and knocking on the components having the preamp on. It's the Nuvistor, this is very definite. Since I don't have the best Instruments to measure components with high resistance and low capacity, I don't really believe in my measurements. So there is another explanation at hand: a wise guy from the vac-tube-era told me (if I remember right), that this effect could come from a misalignment of the operating voltages/currents the nuvistor is working on. He says, that it would explain why old nuvistors with a more flat characteristic curve ring less. He stated that Nuvistors are used in Tornados (you know, the dogfight-vessel), because transistors would disfunction when there is an EMP beside an atomic-bomb explosion (then, I think having an airplane crash is not the worst choice going to death, but that's rather off-topic). He says he doubts, that such a nuvistor is sensitive to vibration and physical shocks in normal circumstands. But he also states, the 7586 Nuvistor is originally built for RF-Frequencies, so maybe the unintended use for low-Frequency purposes can make it ring?
So this is my next question: could there be an alignment problem of the anode current or screen voltage that can cause this behaviour? When I measure the voltages at kathode and anode they seem pretty okay (used an Oscilloscope with 10MOhm restistance-probes, which is not much for this kind of cicuit, I think. The anode voltage is roughly 5% too low, but this could come from the Oscilloscopes probe the cathode voltage is inside specifications).
One information I didn't check regarding this is the heater voltage of the nuvistor - I'm a little confused right now. I had a look into the forums archive where someone states the heater voltage of the nuvistor is 5.5 V. Ok, similar info in the circuit-description of the mic (5...5,9V), but the circuit description of the N6 Power supply says 6,1...6,3. The Nuvistors data sheet talks about 6,3V and the overview of power supplies here on the Neumann website says it's 6,2V which also says the brochure of KM64/U64. Right now I feed the mic with 6,2V Heater voltage. But I would really like to know which was the original designed heater voltage, since between 5,5 and 6,3 is a very big step and I don't want to make my nuvistors aging too fast.
AKG, which used the 7586 Nuvistor longer and in more mic models than Neumann (as if that's any badge of honor!) prescribes a heater voltage of 5.0VDC for most of its mics.
I have always used this voltage with good, long-term success and a minimum of heat buildup inside the mic. I suggest, you trim down the heater voltage to 5,0VDC and observe whether the ringing is affected.
I don't know anything about the esoteric and supposedly resonance-inducing interaction between certain components and a Nuvistor. I was under the impression that your Nuvistors only rang when agitated?
Microphonics certainly were a hallmark of the Nuvistor, and gave its reputation as a dud in microphone application a black eye from which it never recovered.
I'll reduce the heater voltage, thanks for the info!
If there is an oscillating circuit by mistake, then it's 'just-before-resonant'. I agree that this seems rather unlikely, especially when it's known as a common 'feature' of these nuvistors being microphonic.
When I had my talk with that wise guy I mentioned in one of my previous posts, I already had the impression that the U64 was a kind of compromise... Despite of it's very good sound (I like it very much for strings and acoustic guitar) it seems odd to me to use a component which isn't particulary designed for that purpose when there are so many good alternatives at hand.
hmmm, why is there such a high failure rate? maybe because the nuvistor was never built to work in AF-applications? Sometimes the decisions of industry designers even of such a great company like neuman is based on economical reasons. 40 years ago it was easily possible to select under hundreds of nuvistors. Today it's not that easy.
Nevertheless, as I stated before, the Mic is a really good one in questions of sound, but sooner or later it's maybe better to reutilize it with a more common type of miniature tube, since it's too much of a hassle to get these old nuvistors (especially when I have to buy 10 to find maybe one more or less appropriate).
it's as simple as this: the 7586 rings, very often. It always had to be selected. Nuvistors were intended as the tube industry's answer to transistors, but it never worked out, in microphones. Accordingly, the U64 was the only Neumann to employ the 7586. Ringing is typically mechanical, coming quite simply from internal windings which are not properly welded / fastened to the tube housing.
Best regards, Martin Schneider / Neumann Mic. Development