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Post subject: 1976 Fender Twin Keeps Blowing Fuses
Posted: Mon May 23, 2011 2:43 pm
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i bought a '76 twin on EBay about 3 weeks ago... it was working fine until the other night it blew a fuse. so i just replaced it with another 3amp slo-blo buss fuse... as soon as i turned it back on, the intial power blew the fuse again. Tried checking the power tubes and they all look fine. There is no rect. tube on this twin cause its a solid state.. I've heard there are several things that it could be but I am wondering if anyone has had this problem before and what was it that needed to be fixed... IM HOPING that its not the power transformer.

Chase


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Post subject: Re: 1976 Fender Twin Keeps Blowing Fuses
Posted: Mon May 23, 2011 3:13 pm
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If the amp hasn't recently been re-capped, you've likely got a filter cap heading south. It could also be the bypass cap in the bias circuit.

Arjay

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Post subject: Re: 1976 Fender Twin Keeps Blowing Fuses
Posted: Mon May 23, 2011 3:23 pm
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actually the guy i bought it from said he just replaced all the caps before he sent it out to me. i pulled out the chasis and the caps actually looked brand new...


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Post subject: Re: 1976 Fender Twin Keeps Blowing Fuses
Posted: Mon May 23, 2011 3:34 pm
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I would still suspect a power tube. Do you have access to a spare set? I had a 76 for over 30 years and the only time it blew a fuse was when I had a shorted power tube. The originals lasted over 25 years. LOL

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Post subject: Re: 1976 Fender Twin Keeps Blowing Fuses
Posted: Mon May 23, 2011 3:51 pm
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yes i just bought a new pair of 6l6's but im not sure how to bias it without killing myself. so i just put it in where the "bad" one was and it just blew another fuse... ARRRGGGG


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Post subject: Re: 1976 Fender Twin Keeps Blowing Fuses
Posted: Mon May 23, 2011 7:42 pm
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ChaseTVW wrote:
yes i just bought a new pair of 6l6's but im not sure how to bias it without killing myself. so i just put it in where the "bad" one was and it just blew another fuse... ARRRGGGG


Does the amp blow fuse without the power tubes installed?


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Post subject: Re: 1976 Fender Twin Keeps Blowing Fuses
Posted: Mon May 23, 2011 8:07 pm
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Yes it does


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Post subject: Re: 1976 Fender Twin Keeps Blowing Fuses
Posted: Mon May 23, 2011 8:13 pm
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That's the way to do it. Just be sure that the speakers remain connected to the amp. If the fuse doesn't blow without the power tubes = you have a bad tube(s) in the original set.

If the amp still blows fuses with the power tubes out, you prolly have issues with the power supply or power tranny.

Did you ever hear this amp play, after the tech replaced the caps? Polarity is an issue with replacing caps under-the-hood and in the bias power supply circuit.


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Post subject: Re: 1976 Fender Twin Keeps Blowing Fuses
Posted: Mon May 23, 2011 8:24 pm
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Yes it was working great for the first week then just went out. I'm taking it to a guy this week to see if he can fix it. I appreciate all the input guys!


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Post subject: Re: 1976 Fender Twin Keeps Blowing Fuses
Posted: Mon May 23, 2011 9:32 pm
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ChaseTVW wrote:
Yes it was working great for the first week then just went out. I'm taking it to a guy this week to see if he can fix it. I appreciate all the input guys!


It's possible you have a bad diode in the rectifier circuit. If you are tech savvy, you can disconnect the secondaries from the power transformer and see if it still blows fuses. If so, the transformer is bad. However, it is best to take it to a qualified tech if you do not have the knowledge. You can kill yourself, ya know. :D


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Post subject: Re: 1976 Fender Twin Keeps Blowing Fuses
Posted: Tue May 24, 2011 1:14 am
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Before pronouncing that power tranny as DOA I'd check the primary and all four secondaries for continuity and resistance with a DVM. That's a pretty costly component to needlessly deep-six.

Arjay

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Post subject: Re: 1976 Fender Twin Keeps Blowing Fuses
Posted: Tue May 24, 2011 4:55 am
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Retroverbial wrote:
Before pronouncing that power tranny as DOA I'd check the primary and all four secondaries for continuity and resistance with a DVM. That's a pretty costly component to needlessly deep-six.

Arjay


+1 In all the years I've been playing, I've never lost a power tranny.

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