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Old 29th December 2013, 04:54 AM   #1
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Default Pop goes the bias cap.

I was firing up my modded ST-70 after changing tubes, and let some smoke out.

These are the guts of a 100uf 100V Sprague Atom. Time for a little clean up and ordering parts!



Griff
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Old 29th December 2013, 07:04 AM   #2
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Caps don't usually explode unless the voltage is too high. You might want to take a few readings after the new cap is installed.
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Old 29th December 2013, 07:49 AM   #3
SY is offline SY  United States
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Quote:
Originally Posted by astouffer View Post
Caps don't usually explode unless the voltage is too high.
Or the cap is backward. Or the rectifier is backward or shorted.
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Old 29th December 2013, 08:02 AM   #4
M Gregg is offline M Gregg  United Kingdom
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Or G2 to G1 short,
If its in fixed bias.

Depends what the cap is being used for? If its in the B+ it could be over voltage.
Low current draw on the B+ rail giving over voltage.

Or as above rectifier fail etc.
So you need to tell us where in the circuit the cap is connected? (and voltage rating)

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Old 29th December 2013, 09:08 AM   #5
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I'm figuring cathode bias resistor bypasses. Something in there caused excessive current, and too much voltage across the bias resistors. You need to find out what caused that. Might want some higher voltage bypass capacitors too.

(George poofed cathode bypass capacitors before trying for eleven a couple of times.)
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Old 29th December 2013, 02:45 PM   #6
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Poofed after changing tubes? Did you replace with the correct tubes?
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Old 29th December 2013, 04:47 PM   #7
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No, this is the first cap in the fixed bias supply, and was working for 3 years before. The bias was -46V with the controls all the way CC, and this was a 100V part. I cant explain why it blew up.
I have a Curcio auto bias board kit kicking around. Maybe this is a message telling me it belongs in here.

Thanks for the replies.

Griff
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Old 29th December 2013, 04:56 PM   #8
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Check the bias rectifier. Make sure its good with no reverse leakage.
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Old 29th December 2013, 06:33 PM   #9
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by ProfGriff View Post
No, this is the first cap in the fixed bias supply, and was working for 3 years before. The bias was -46V with the controls all the way CC, and this was a 100V part. I cant explain why it blew up.
I have a Curcio auto bias board kit kicking around. Maybe this is a message telling me it belongs in here.

Thanks for the replies.

Griff
Err NO!

It means you need to know whats wrong before you blow the board up as well.

G2 to G1 short what would happen to the bias supply?

Could be rectifier but isn't it strange it was after a tube change?

Then again don't be surprised that the fault has suddenly vanished...if there was an internal short in a tube now blown open.

Check the new tubes for internal shorts with a resistance test.

Check the components in the bias supply and get it working as normal before you do any mods!


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Old 29th December 2013, 07:17 PM   #10
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You are right. Good thinking. I will check out the tubes (new Winged C EL-34s) for shorts on my tester. The $5 in parts for a new bias supply would not hurt either.

Thanks and regards

Griff
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