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Author Topic: Variac for slow ramp up of tube mics?  (Read 130 times)


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  • Real Full Name: Dan Boner
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 micís 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.
Dan Boner
Associate Professor/Director
Bluegrass, Old-Time, and Country Music Studies
East Tennessee State University


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  • Real Full Name: Ruud van Steenis
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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