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Testing Supplies Without Killing Mic's Tube?

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industrial arts:
I am working on a few older Neumann supplies for a friend.  I would like to substitute a resistive load for testing the supplies.  Does anyone have a suggestion for values I could use that would approximate the load of the microphone?

One supply is the NN48.  The other is a supply for a U47.

The U47 is failing under load, it appears the selenium rectifier is at fault.

The NN48s both have problems with the filament supplies.  One I plan to put a regulator in, the other I would like to keep closer to stock, so that one would need me to load it properly to check the voltage.

I'm still a neophyte at tube repairs, so if any of this seems impractical, it's cuz I'm near the bottom of my learning curve

Thanks for your help,

Mark Springer
industrial arts

Klaus Heyne:
To simulate the heater voltage/current drain of an AC701, you need to connect a 5W 100 Ohm variable resistor between pins 4 and ground. Start with 100Ω, then reduce the load until you get ca. 3.95VDC. On the B+ side,I would start with a 2W 10KΩ resistor across Pin 5 and ground. trim out until you get ca. 120VDC.

I would not try to trouble shoot the U47 power supply that way. Why do you feel the NG's rectifier is defective? I have rarely if ever encountered such on these power supplies.

KaiS:
Hello Klaus, I think he wants it the other way round- define an exact resistor value as substitute for the mic.
This is - IMO - the best way doing it, it keeps you from frying an valuable mic.

For the AC701 type mic's it might be different from type to type, e.g. KM56:
Filament: 100mA @ 4V makes 40 Ohms / 0.4W (use 2W resistor or burn your fingers).
Anode: 0.61 to 0.53mA @ 120V makes 197 to 227 kOhms / 0.06W
(choose 200kOhms / 0,6W).

For the U47 it's simple:
40,5 mA (heater + anode supply) @ 105V makes 2590 Ohms / 5W
I suggest 1K2 + 1K4 Ohms /5W in series, so they don't get too hot.

For testing AC701-type PSU's remember to load both filament and anode supply, as the voltage drop inside the transformer will change if you only load one of those.

This is for fixing faulty supplies, not for calibrating them to a certain mic!
It seems that specially the heater voltage of AC701 mics need to be carefully calibrated to gain long tube life.

Have a scope and at least 1 electronic multimeter handy for faultfinding.
Monitor your line-supply voltage, if it's far off specs don't expect the PSU voltages to be in specs.

Check the ripple right after the rectifier - it shouldn't be too big, and it should be 120 (100) Hz.
If it's 60 (50) Hz one leg of the rectifier is blown.
If it's too strong, the 1st capacitor is dried out or blown.

Kind regards
Kai

Klaus Heyne:
KaiS wrote on Sun, 06 February 2011 10:27
Hello Klaus, I think he wants it the other way round- define an exact resistor value as substitute for the mic.


I gave him the method that I use to get a power supply" into the ballpark" and that I find LEAST likely to kill a AC 701 (read my 'sticky' about that tube): approach it from a safe distance, and trim out voltages to roughly the value(s) encountered once he re-plugs his mic with the tube inside, then uses these values when installing temporary  fixed resistors

Either way to do it is probably fine, if he swears to watch the rise in voltages like a hawk, once he re-plugs the mic into the power supply thus prepared, and hits the 'kill' switch the moment the voltages go past 3.95 VDC to 4.00VDC. or significantly higher than 120VDC on B+.

KaiS:
Klaus Heyne wrote on Sat, 05 February 2011 17:22
...On the B+ side,I would start with a 2W 10KΩ resistor across Pin 5 and ground. trim out until you get ca. 120VDC...
Isn't 10K a bit low?
The PSU has to put out 12mA - 20 times what a mic uses.
Specially the old PSU designs with the zehner-regulation aren't capable of such a high current.

Regards

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