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Author Topic: Variac for slow ramp up of tube mics?  (Read 1060 times)

BluegrassDan

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Variac for slow ramp up of tube mics?
« on: July 09, 2020, 09:48:03 AM »

Would it be beneficial for the health of a vintage mic’s rare tube and older passive components to slowly bring up the mains AC voltage during each startup? I assume older supply designs have no way of throttling inrush.
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Dan Boner
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Bluegrass, Old-Time, and Country Music Studies
East Tennessee State University

RuudNL

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Re: Variac for slow ramp up of tube mics?
« Reply #1 on: July 09, 2020, 01:57:40 PM »

Most original (older) power supplies have resistors in series with the filament output.
They function as dropper resistors, reducing the voltage to the right value for the filament.
A filament acts as a PTC. (Positive Temperature Coefficient).
This means that the cold filament has a lower resistance than a heated filament.
A cold filament will cause a higher voltage drop over the series resistors.
As the temperature of the filament rises, the resistance increases and the voltage over the filament will get higher.
So in fact, this is already some kind of 'slow ramping up'.
But it won't hurt to use a variac. No idea if it is worth the trouble...
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klaus

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Re: Variac for slow ramp up of tube mics?
« Reply #2 on: November 21, 2020, 04:47:44 PM »

Great post form RuudNL. I do not use a Variac for adding additional ramp up time. It's overkill, I think.
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Klaus Heyne
German Masterworks®
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Mike Owings

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Re: Variac for slow ramp up of tube mics?
« Reply #3 on: January 01, 2023, 11:31:09 PM »

Great post form RuudNL. I do not use a Variac for adding additional ramp up time. It's overkill, I think.

Would your advice be any different if the microphones in question (M49, U67) had not been powered up in a few years? It's a very long story....
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Kai

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Re: Variac for slow ramp up of tube mics?
« Reply #4 on: January 02, 2023, 03:30:09 AM »

Would your advice be any different if the microphones in question (M49, U67) had not been powered up in a few years? It's a very long story....
Electrolytic capacitors, mainly used in the PSU, need to “reformat” for full capacity after a long idle period.
This happens when they are powered with their normal use voltage again and may take a little time.

No extra measures need to be taken to do this.
You probably won’t even notice the effect, as the PSU filters have a significant margin for doing their work.


Another case are possibly broken / dried out caps, they stopped working and will not come back.
You will notice hum in the audio if this is the case.
Not very likely, as the big old caps are much better in this regard than modern small ones.


I’d make sure the mic’s and PSUs are free from humidity they may have collected over the time.
I had such cases here.
Warm them up slightly above room temperature (exact value doesn’t matter, don’t fry them) for some hours, then power them up the normal way.
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klaus

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Re: Variac for slow ramp up of tube mics?
« Reply #5 on: January 03, 2023, 01:29:52 AM »

Thanks for all your expert and well-founded advice through the years! This forum would not be the same without you.
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Klaus Heyne
German Masterworks®
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