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Post subject: restoring/modifying a '78 twin
Posted: Fri Jan 16, 2009 8:35 am
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i have a 78 twin that's probably in need of some major restoration work. The reverb developes an awful hum after the amp has been on for 5 minutes, and the vibrato is very weak. All parts/tubes etc are original. Does anyone have a ballpark estimate of the costs involved? Can someone who is handy with a volt meter and soldering tools do it themselves?

Also, I remember seeing a article in GP in the distant past that detailed how to remove the master vol and feedback circuits. Has anyone out there tried this?


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Posted: Fri Jan 16, 2009 8:55 am
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Start with the tubes. It may cure a lot of your problems.


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Posted: Sat Jan 17, 2009 1:31 am
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I would also have the filter and bias caps replaced. I did it myself on my '79 Twin along with 6 other electrolytics on the other side of the chassis. The power filter caps hold dangerous voltage, you should let a qualified amp tech replace them for you.

Before:

Image


After:

Image


Small Electrolytics:

Image

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Posted: Sat Jan 17, 2009 7:11 am
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Ok--I will try the tubes first. Should it be re-biased? If so, any tips on doing that?


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Posted: Mon Jan 19, 2009 9:37 am
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If you change the power tubes you'll need to rebias. I would let a tech do it. Twins are a bit complicated. As the previous poster stated, filter caps can give you the symptoms you describe too. I'd still start with the tubes.

I wouldn't do any cap replacement unless you know what you're doing. Cost varies on a lot of factors.


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Posted: Mon Feb 16, 2009 7:20 am
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5er driver, Nice picks of the cap job. What brand caps did you use and was there any particular reason why? I'm really only familiar with sprague and mallory although I'm sure there are many quality ones out there.
Thanks for any info,
filerj


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Posted: Tue Feb 17, 2009 2:23 am
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Thanks for the comnpliment filerj. I used TAD caps. No real reason that I picked them over any others other than I had heard about quality issues with the Spragues that I would have needed and I don't live far from TAD's shop and if there were any problems I could get in their face.

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Posted: Wed Mar 04, 2009 12:27 pm
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Nice job. What did it cost you in materials to recap the amp?

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Posted: Wed Mar 04, 2009 1:10 pm
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Wow, I forgot how much the caps cost. They're not expensive at all. I don't think I paid more than €15 which is about $19.

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