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Old 29th January 2009, 01:24 PM   #1
MZ is offline MZ  United States
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Default Reforming Caps with Variac

Hello All:

I have two preamps which are powered off their respective amplifiers. Can both units (amp/preamp) be connected to each other and to the variac at the same time? Or is it better to do the amp separately and then connect the preamp? Appreciate your help.

Best, Mark
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Old 29th January 2009, 04:57 PM   #2
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Interesting question. To my way of thinking it is the power supply electrolytics that you are reforming with the Variac. These are in the power amp as you noted. It seems to me that only the power amp need be reformed. So to answer you question, I don't suppose it really matters. I'm no authority, so lets see what points the others raise.

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Old 29th January 2009, 05:56 PM   #3
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Should I bypass the rectifier tube with solid state diodes?

There ain't gonna be much emissions while the filament is
under voltage. And I don't want to begin the forming with
the Variac set high enough to effectively light the rectifier.
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Old 29th January 2009, 06:39 PM   #4
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Originally posted by kenpeter
Should I bypass the rectifier tube with solid state diodes?
There ain't gonna be much emissions while the filament is
under voltage.
Definitely yes. That is the proper way to do it. That's right-on thinking. I like to hook a voltmeter across the first cap and take the voltage up in 50V steps. The time interval depends on how long things have been dormant as well as general age. A milliamp meter in series with the rectifiers and first cap isn't a bad idea either. Any heating of the capacitors is crucial to watch for.
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