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

Testing Supplies Without Killing Mic's Tube?

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Jim Williams:
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.

KaiS:
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

industrial arts:
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

Jim Williams:
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.

Klaus Heyne:
...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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