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Post subject: Fender Bassman 100 Head Silverface 1974 - Blows Fuse
Posted: Sun Sep 02, 2018 6:11 am
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Just go this amp. Trying to figure out what is clamping to ground. My bulb limiter is full bright as I crank up the volts.

Everything looks to be original, maybe even the RCA tubes. All tubes tested good and are out of the amp. All filter caps have had + side lifted and all test good.

The following do not seem right...

I measure 6 ohms to ground before and after the set of 3 series rectifier diodes and everywhere along that path.

I have continuity across both sets of 3 series diodes at under 1 ohm.

I am going to remove all the diodes and was wondering what to use as a replacement. I have a bunch of 1N4007 but I think I need something with a higher current rating.

Thank you for your help! MC


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Post subject: Re: Fender Bassman 100 Head Silverface 1974 - Blows Fuse
Posted: Tue Sep 04, 2018 1:51 am
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Disconnect all secondary of power transformer wire ; did fuse blow or bulb lamp is full bright ?
Full bright ; power transformer is burn

6 ohms reading look not right
Continuity ( both side ? ) both set of rectifier diode is not good. You must disconnect one side of each diode to read ohms; one side reading is around 220 k other side Over Load, or infinity, open

1N4007 are good for this amp


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Post subject: Re: Fender Bassman 100 Head Silverface 1974 - Blows Fuse
Posted: Tue Sep 04, 2018 5:21 am
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Turned out to be that all the rectifier diodes were shorted. Thank you for the help!


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Post subject: Re: Fender Bassman 100 Head Silverface 1974 - Blows Fuse
Posted: Tue Sep 04, 2018 5:42 am
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misterc57 wrote:
Turned out to be that all the rectifier diodes were shorted. Thank you for the help!



You have to find why all diodes where short, they don't come short by itself ; often Power tubes are short and a too much powerfull fuse, check for that.

Tube tester is no good to test power tubes, use only know good tubes


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Post subject: Re: Fender Bassman 100 Head Silverface 1974 - Blows Fuse
Posted: Tue Sep 04, 2018 5:52 am
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Yes good point. I do not know the history on it other than it blows fuses. They may have had the wrong fuse in it. Thank you!


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Post subject: Re: Fender Bassman 100 Head Silverface 1974 - Blows Fuse
Posted: Tue Sep 04, 2018 6:05 am
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misterc57 wrote:
Yes good point. I do not know the history on it other than it blows fuses. They may have had the wrong fuse in it. Thank you!


You can check, so easy.
A right fuse will protect amp, nothing will blow . Most of the time it's a Power tube(S) fail,
Put new tubes


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