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Old 19th March 2011, 06:17 PM   #1
treagle is offline treagle  United States
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Default Capacitor Sanity Check

So, I'm new to the forums and I'm starting on my first tube project which involves converting an old Grommes/Precision Electronics S35 tube PA into a guitar amp. It's current running two 6eu7's for the preamp, a 6c4 for phase inversion, and 7868's in PP for the output stage.

Before modifying anything I was looking over the schematic, it appears that one of the power filtering capacitors was spec'ed out to 250VDC when its rail is designed to operate at 325VDC. Could this possibly be correct??

full schematic (if interested):http://ampslab.com/SCHEMATICS/GrommesS35.gif
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Old 19th March 2011, 06:40 PM   #2
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You can do 2 things, measure the voltage to see if it exceeds 250v or just replace that capacitor with a 10uf/450v capacitor.

At first I thought maybe the 22K resistor dropped the voltage a bit but with 325 volts listed on the schematic, its possible the schematic is wrong.

Sal
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Old 19th March 2011, 07:00 PM   #3
DF96 is offline DF96  England
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Unless the rest of the schematic is wrong too, 325V at that point looks quite plausible. The other HT capacitors are specified as 450V. Maybe the schematic is wrong, and that capacitor is actually 350V or 450V?
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Old 19th March 2011, 08:09 PM   #4
treagle is offline treagle  United States
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I opened up the amp and verified the capacitor is actually a 10uF 250V cap but haven't had the guts to test the rail voltage with my radio shack multimeter.

Since I'm replacing caps anyway I'm going to put in a 10uf/450v and call it good. Thanks for the replies!
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Old 19th March 2011, 08:11 PM   #5
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Most likely it's a typo........A circuit designer/engineer or any ol' 'someone', like "us" that can read schematics...will find this error. Somebody didn't 'proofread' their schematic......till now.

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Old 19th March 2011, 08:27 PM   #6
treagle is offline treagle  United States
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^^Exactly. Better late than never though! That kind of error does make me a bit nervous about the rest of the design though.
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Old 19th March 2011, 10:06 PM   #7
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Get that 250Vcapacitor out of there! That's an accident just waiting to happen. According to that schemo, it's connected to a line that could go as high as 460V, with just resistor series drops to limit the voltage. It should be a 450V capacitor at least.

I don't know if that was an accident, or was done deliberately to keep the knowledgeable from building one of their own, but it's definitely wrong.
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