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Klaus Heyne's Mic Lab / Re: N52a question
« Last post by uwe ret on Today at 04:06:27 pm »
The original N52a did uses two banks of Stabilyt cells for filament regulation, followed by a dual (adjustable) series resistor to provide the desired 4 V filament supply. Stabilyt cell have a life expectancy of up to 2 years and sure have expired decades ago. I assume that section has been modified? When Stabilyt cells were discontinued in the mid 1970s Neumann briefly offered a modification kit SK 33d with Zener diode regulation.  Modern voltage regulator ICs allow for much more effective circuit substitution with precise regulation and tube saving soft filament supply ramp-up. The later version N52t used solid state (transistor) circuits which used selected resistor values to compensate for component tolerances and adjust for accurate filament and plate/bias supply voltages.
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Klaus Heyne's Mic Lab / Re: U67 Voltages And Other Questions
« Last post by uwe ret on Today at 03:38:06 pm »
Only the first generation (mid 1960s to early 1970s) featured a trim pot for adjusting the filament voltage. Later versions rely on pre-regulation to 15 V with a Zener diode and 3 series resistors to drop the excess 8.7 V when the filament of 200 mA is drawn. If desired, resistors of marginally different values may be substituted to get close to the desired 6.2 V to 6.35 V at the tube socket. The exact high voltage  measured at the microphone connector (careful, avoid risk of rather unhealthy shock!) does depend on the condition of the tube, and should be within ±5% of the nominal 210 V. It can be brought close by tinkering with either the series and/or shunt resistor values, or both. Observe the power ratings for these resistors!
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Klaus Heyne's Mic Lab / N52a question
« Last post by J.J. Blair on Today at 03:20:52 pm »
Had some random clicking in one of my M49s, so I replaced the tube, and everything is better after a couple days of burn in.  However, my heater voltage is 4.3 VDC .  I don't know if the extra .1 is going to be a problem or not, but I'm wondering if there's a way to adjust the heather voltage in those, without having to solder a resistor into the circuit. 
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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 ± 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 90º 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 Mylar® 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 haven’t 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.
10
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 it’s 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 I’ll do that to soon, and pay close attention.
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