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Old 10th September 2008, 11:33 AM   #1
Whiting is offline Whiting  Australia
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Default Keep Blowing Fuses

Hi All

Can anyone please help with suggestions as to why the amp keeps blowing fuses on power up.

I've checked the diode in the power supply section, as well as the resistors which are within spec.

I've attached the schematic of the PS section, of the amp if it helps.

Thanks in advance and cheers

http://img139.imageshack.us/my.php?image=img0300os5.png
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Old 10th September 2008, 11:58 AM   #2
AndrewT is offline AndrewT  Scotland
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power it up with a Variac and then measure (very carefully) voltages to find where the current surge is going.
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Old 10th September 2008, 12:09 PM   #3
Whiting is offline Whiting  Australia
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Hi AndrewT

Thanks for your reply mate, however I'm a newbie at this and don't have a Varaic.

This is only my second attempt at trying to diagnose & fix a tube amp, my first was only cap replacements based on visual inspection only.

This one appears to be a little more difficult.

Any other suggestion would be appreciated.

Cheers
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Old 10th September 2008, 12:29 PM   #4
AndrewT is offline AndrewT  Scotland
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will a tube amp with high current draw start using a high wattage light bulb tester?
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Old 10th September 2008, 12:39 PM   #5
Whiting is offline Whiting  Australia
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Thanks for that. I'll have to do some research on how to rig one of these up.

Cheers
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Old 10th September 2008, 12:53 PM   #6
AndrewT is offline AndrewT  Scotland
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but wait till a valve/tube guy confirms if the bulb tester works to protect/slow down the start up without damaging anything.
It certainly works with SS ClassAB and ClassA amplifiers and ClassA pre-amps. But, not with soft start circuits, these have to be shorted out.
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Old 10th September 2008, 05:38 PM   #7
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by AndrewT
but wait till a valve/tube guy confirms if the bulb tester works to protect/slow down the start up without damaging anything.
It certainly works with SS ClassAB and ClassA amplifiers and ClassA pre-amps. But, not with soft start circuits, these have to be shorted out.

Works fine with tube amplifiers too, and prevents catastrophic damage to the power transformer and other components in the event of a shorted cap or rectifier. Most likely to be either in this case. Something like a 100W bulb should be adequate for troubleshooting purposes to identify a failed component. Amplifier will not operate near normally with the bulb in series, a higher wattage bulb would permit that once the issue has been identified.
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Old 10th September 2008, 06:26 PM   #8
Zibi is offline Zibi  Poland
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http://www.geofex.com/ampdbug/fuseblow.htm
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Old 11th September 2008, 01:17 PM   #9
Whiting is offline Whiting  Australia
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Hi All

Thanks for all of the replys guys.

It's probably simple, but can anyone assist with with a scetch of how to built a tube tester and how to connect it.

I've googled, however noting of whats described was found.

Cheers
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