R/E/P > Klaus Heyne's Mic Lab

U67 Voltages And Other Questions

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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.)

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 90 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.

uwe ret:
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!

Thank you, gentlemen. Both of my nu67 PSU's have the trim pots for heater voltage and are now both outputting 6.20VDC measure at the mic.

On PSU number 2, I measured R3 resistor out of circuit and it was 8.05k instead of 8.2 so I popped in a fresh 8.2k resistor  (measured 8.18k beforehand) with no change to B+ voltage.  (Still 229VDC)

Any math wizards out there feel like guiding me on how to calculate what R3 should be to bring B+ from 229 down to 210VDC?

Regarding S2, when you snip at one of the right angles, do you slightly bend the resulting horizontal length of wire to prevent it from making contact with it's severed nub?

And my main PSU must be from the European market, because it has no pad network.  But on my spare PSU I see R11, R12 etc labelled on the board, but the resistors have been removed.

Thanks in advance,

With that high B+ go to 10k Ohms on R3 or reduce shunt resistor (May be a 150k or higher value, depending on original calibration) by the next major value increment. For example from 150k to 100k.

As you mentioned, bend up the S2 wire against the tube socket and stabilize to hold in place  with a dab of E6000 or silicon glue.


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