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Author Topic: Testing Supplies Without Killing Mic's Tube?  (Read 4217 times)

industrial arts

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Testing Supplies Without Killing Mic's Tube?
« on: February 05, 2011, 06:13:39 pm »

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 Smile

Thanks for your help,

Mark Springer
industrial arts
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Klaus Heyne

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Re: Resistive load for microphone testing?
« Reply #1 on: February 05, 2011, 06:22:24 pm »

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.
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Klaus Heyne
German Masterworks
www.GermanMasterworks.com

KaiS

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Re: Resistive load for microphone testing?
« Reply #2 on: February 06, 2011, 01:27:40 pm »

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

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Klaus Heyne

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Re: Resistive load for microphone testing?
« Reply #3 on: February 06, 2011, 03:57:40 pm »

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+.
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Klaus Heyne
German Masterworks
www.GermanMasterworks.com

KaiS

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Re: Resistive load for microphone testing?
« Reply #4 on: February 07, 2011, 06:52:35 am »

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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Jim Williams

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Re: Testing Supplies Without Killing Mic's Tube?
« Reply #5 on: February 07, 2011, 12:32:59 pm »

On the AKG C-12VR, they use a constant current source design off the 317 regulator for the heaters. You will need to have the tube in circuit to measure and adjust that heater voltage. Remove it and it will read 15 volts.
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Jim Williams
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KaiS

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Re: Testing Supplies Without Killing Mic's Tube?
« Reply #6 on: February 07, 2011, 05:16:43 pm »

Even then you could replace the tube filament with a resistor for testing.
You have to know the current and voltage the tube should be operated at:
V / A = Ohm

If it's a constant current source AKG used it's limiting the warm-up current to reduce stress on the tube filament, which otherwise draws more current as long as it's cold.
I'm not sure if this makes a difference for the tube life, but for sure it doesn't hurt.

Regards
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industrial arts

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Re: Testing Supplies Without Killing Mic's Tube?
« Reply #7 on: February 08, 2011, 01:53:55 am »

Thanks for your in depth replies, both will be useful.

Yes, first I need to simulate the mic load while tshooting the supplies.  Using the mic as a test load was not a viable option, IMHO,

Once the supplies are close, I can attach the mic and variac the line voltage, keeping a close eye on the voltages out of the supply.

The reason I think the selenium rectifier is defective is the output looks pretty much like a half wave rectifier.  I am going to replace it with a silicon bridge and see if I'm right.  

And I will carefully watch my voltages when I attach the actual mic.  From what I understand, I will most likely have to put in some additional resistance if I replace the selenium rect.

Just to clarify, none of the supplies are functioning well at this point, so I have some repair work to do before I would want to attach them to a valuable microphone.

Thanks again,

Mark Springer
industrial arts
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Jim Williams

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Re: Testing Supplies Without Killing Mic's Tube?
« Reply #8 on: February 08, 2011, 11:24:32 am »

KaiS wrote on Mon, 07 February 2011 14:16

Even then you could replace the tube filament with a resistor for testing.
You have to know the current and voltage the tube should be operated at:
V / A = Ohm

If it's a constant current source AKG used it's limiting the warm-up current to reduce stress on the tube filament, which otherwise draws more current as long as it's cold.
I'm not sure if this makes a difference for the tube life, but for sure it doesn't hurt.

Regards


It is a good design, but guessing what the tubes resistance will be before and then after warm up is dicey. Each tube I place into that circuit needs a fine trim as they all vary a bit.
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Jim Williams
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Klaus Heyne

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Re: Testing Supplies Without Killing Mic's Tube?
« Reply #9 on: February 08, 2011, 01:24:00 pm »

...and that is why it's so very important to monitor and fine-trim voltages, especially of AC701 tubes, and especially their heater voltages. A 10% oversupply there will practically reduce the tube's life span by half.
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Klaus Heyne
German Masterworks
www.GermanMasterworks.com
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