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Capacitor Replacement

1/12/1998 9:13 PM
Jason Borders
Capacitor Replacement
This is probabably one of those subjects that's been covered a hundred times. Sorry if I haven't been paying attention. I'm looking for opinions regarding intially charging caps up for the first time (Vox AC-50). Is there a safe way that doesn't require a variac or put any stress on 33 year old transformers.  
Thanks for any help.
1/13/1998 7:32 AM

Try the poor man's variac...  
Wire up an electrical outlet in SERIES  
with a 40 watt light bulb. Put a switch  
in there too for emergencies.  
Check you work by plugging something  
(say a drill) into it to see that the  
lightbulb dims when you operate the drill.  
The bulb will take some of the edge off the  
initial amp power up, and the caps  
should be happy.  
It's actually described in Kevin O'Conner's  
Tonnes of Tone, to give credit where it's  
... Whit
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1/13/1998 8:21 AM
John Kos

You could put a large value resistor (1 meg or more) in series with the B+ before the first filter cap, remove all the tubes (accept the rectifier), and then turn on the amp while observing the voltage across the caps. They will charge slowly. Just make sure they don't go over their working voltage.  
John Kos
1/13/1998 8:29 AM

R.G. detailed a method in a previous post, you may still be able to retrieve it from the archives of this BBS. Seems to me it was back in Nov'97 or thereabout. There may be a method on his web page in the tube amps section.  
Also, there is a suitable method detailed at Angela's website (  
I think it's best to read all of them for a better understanding of the subject, and then pick a correct method which is easiest to accomlish with the parts & apparatus you will have on hand.  
1/13/1998 10:11 AM

Just curious here... does the 40 watt  
light bulb in series method have  
I'd hate to think I'd given out bad info...  
... Whit
1/13/1998 10:04 AM
The standard method used to form the caps in the first place is to connect them to a voltage source through a high resistance. This lets the low amount of current that leaks through form the oxide layer without localized heating that un-forms it.  
Sprague recommended using 100K in series with the capacitor and a voltage source equal to the final voltage you want to form to. You watch the voltage across the 100K, and when it declines to 10% of the voltage source, the capacitor is formed.  
One implication of using this method is that the leakage resistance of the cap has to be 10X the resistance in series as 9/10 of the voltage is dropped across it, so you get out of this a cap with leakage of about 1M.
1/13/1998 10:12 AM
Jack Orman

Seems like I recall a simple method using a 6X4 tube in series with the B+ as a soft start (if memory serves). Was in an old issue of Glass Audio.  
regards, Jack
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