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Klaus Heyne's Mic Lab / Re: u67 voltages and other questions
« Last post by klaus on Today at 12:39:40 pm »
Readjust both power supply heater voltages to measure ca. 6.2VDC at the tube. There are trimpots for this, accessible from the underside (printed side) of the power supply circuit board.

Reduce B+* on both supplies to 210VDC Ī 2 volts, by replacing and increasing one or both of the bridging resistors R1 and R3 by one value up, or reduce dropping resistor (at least a 1/2W rating!) by the next value down.

RE S2: Neumann made this bass choke (and a similar one in most M49) removable, at users' discretion, but I find the associated phase shift and sizable low end chocking in the U67 objectionable to high fidelity, and routinely snip the wire at one of its 90ļ angles (but keep it installed, just in case you want to resolder it).


P.S.: And make sure that the audio-pad network, standard for U.S. delivery U67, is removed.


* B+ is the incoming high voltage that is split up inside the mic to provide polarization voltage for the capsule, and, depending on the health of the tube, will determine a plate voltage of ca. 75VDC, usually a bit more.
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Klaus Heyne's Mic Lab / U67 Voltages And Other Questions
« Last post by afterlifestudios on Today at 10:57:42 am »
I have a U67 and two NU67 powere supplies.
PSU #1 provides 214v B+ and 6.34v heater, measured in the mic.
Spare PSU #2 provides 229v B+ and 6.24v heater, measured in the mic.
Both PSU are strapped for 117VAC line voltage
Mic has a Telefunken EF806s installed

I use PSU #1 all the time; is that heater voltage too high?
And what's the preferred method to pull plate voltage down to 210v on PSU#2?

I also noticed that the mic still has the S2 jumper in place.  Is it unanimous that it's better without?  (Mic sees a lot of vocal duties, but many other sources as well.)

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Thanks Klaus!
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Klaus Heyne's Mic Lab / Re: Neumann reissues the U67
« Last post by klaus on August 17, 2018, 01:22:21 pm »
Figures 3 and 4 of your linked article shows increasing shrinkage of Mylar with increased heat, in all cases. What am I not getting here about your counter argument?

Besides, I've tested this with Mylarģ capsules repeatedly: resonance frequency goes up with heat applied to the tensioned diaphragms.

Get well soon!
KH
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Klaus Heyne's Mic Lab / Re: Neumann reissues the U67
« Last post by gtoledo3 on August 16, 2018, 11:10:50 pm »
Mylar is pre-tensioned or stetched during manufacturing. Depending on the tension used in the capsule it's more likely that heat will increase the tensioning.
But - we do not know how Neumann treats the mylar in the manufacturing process.

More than you ever wanted to know about Mylar:

http://usa.dupontteijinfilms.com/wp-content/uploads/2017/01/Mylar_Physical_Properties.pdf

The long story short is that it appears that tensile strength goes down with heat increases and elongation goes UP. At extremes it finally becomes brittle.

Mylar can be preheated, which will serve to condition it, but up to that heating point, not above. I also read another abstract about UV exposure pretreatment fwiw.

I still havenít done the test on the timbales :-) Recovering from a slip and fall, which probably accounts for more than half of the time I spent chattering about this. (Sorry Klaus!)
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Klaus Heyne's Mic Lab / Re: Neumann reissues the U67
« Last post by Kai on August 14, 2018, 06:22:28 pm »
Heat could possibly decrease Mylar tension ...
Mylar is pre-tensioned or stetched during manufacturing. Depending on the tension used in the capsule it's more likely that heat will increase the tensioning.
But - we do not know how Neumann treats the mylar in the manufacturing process.
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Klaus Heyne's Mic Lab / Re: Neumann reissues the U67
« Last post by gtoledo3 on August 13, 2018, 02:33:24 pm »
When you have a drum head under tension and heat it, a dimple stretches and the Mylar thins. You can then tighten the drum head up more, heat again. Rinse and repeat. Until the drum head gets so thin that itís shot or breaks.

I thought the pitch relaxed when it thins out on the head, with it being under tension. But it goes up more like shrink wrap? Interesting. I have some timbale heads here that Iíll do that to soon, and pay close attention.
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Klaus Heyne's Mic Lab / Re: Neumann reissues the U67
« Last post by klaus on August 13, 2018, 10:57:22 am »
Heat could possibly decrease Mylar tension, but the problem in this case that is that it has gold bonded to it.
Heat does the opposite to Mylar: it tightens it. That's why drum shops apply heat to dented Mylar heads; the dents come out that way.
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Klaus Heyne's Mic Lab / Re: Neumann reissues the U67
« Last post by gtoledo3 on August 12, 2018, 02:53:03 pm »
Klaus, did you consider variability of the caps for the high frequency shunts as another thing to examine in differences of new vs vintage? (Not naysaying your point about the tension!)

It was occurring to me that what happens with the top end there, can also really pop out to the ear.
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Klaus Heyne's Mic Lab / Re: Neumann reissues the U67
« Last post by gtoledo3 on August 12, 2018, 02:49:15 pm »
Very interesting review! So again, the tensioning of the capsule membranes is the biggest problem.
Could you unveil a little how it is possible to relax the tensioning of the membranes?
(Not that I am even thinking about doing this myself!!!)
As far as I remember, the membranes were glued to the tensioning rings.
(Anyway, that is what I saw when I took a damaged Neumann U87 capsule apart.)

Heat could possibly decrease Mylar tension, but the problem in this case that is that it has gold bonded to it.

Any sort of pre-age/stress like heat would be best done before application of the gold, because with heating after the fact the gold and Mylar will not respond in the same way to heat, which may theoretically cause abberations in the bond, at least according to research.

That said, itís hard to think of other approaches to tweak the tension of an assembled capsule.
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