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Post subject: Replacement Filter Caps in a 67 Twin Reverb
Posted: Wed Mar 03, 2010 7:34 pm
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Hi all,
Servicing a 67 Fender Twin Reverb as my first real project and want to change the original filter caps. Not sure with what, though. Five old Mallorys, two are 70MFD 350VDC and three are 20MFD 500VDC. What are the possible replacements? Do I look to match these values? Go higher? Thanks.

Name's James


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Posted: Thu Mar 04, 2010 12:56 am
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James, if you want to keep the character of the amp the same, you'd want the same mfd(uf) values. But since the wall outlet voltages are higher now, it's best to up the cap volts some. I usually go with 80uf/450vdc or 100uf/450vdc for that first bank and 20uf/500vdc for the rest. Xicon, F&T, and Sprague Atom are fine, but be sure to measure the capacitance of each one before installation(capacitance meters are relatively cheap-$40). Also be aware that the physical size can be misleading. The Sprague Atom is a small cap packaged in a larger shell to replicate the OEM caps. So a smaller high quality cap is fine. Lastly the temp rating will give you an indication of how long the cap will last(higher temp= longer expected life). BTW the Mallorys are not necessarily junk, if left to sit for 6-12months, they can have the electrolytic reformed and checked for leakage. Oh yeah be sure to check the (2) 220K bleed-balance and dropping resistors on that board. Art

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Posted: Thu Mar 04, 2010 5:15 am
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Art,
Thanks for the info. These Mallorys puzzle me. Even though they date from 67 (original) they look in great shape, no bubbling, leaking, etc. I'll go ahead and replace them but how do I store them for the 6-12 months? Also, what does it mean to have them 'reformed?' And (sorry about all the questions) where do I find the bleed-balance and dropping resistors?
James


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Posted: Thu Mar 04, 2010 2:22 pm
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The key is high quality caps. Go with US made caps. The import
Taiwanese Illinois stuff is prett bad.


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Posted: Thu Mar 04, 2010 2:41 pm
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Sounds good. I was thinking of going with the Spragues. And I'll use the values that Art recommended. Shold I replace the bias cap as well and if so, do I use the same value or go higher?


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Posted: Thu Mar 04, 2010 2:49 pm
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Art is the man with amp repair. If it was me and I was already in there, It couldn't hurt.


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Posted: Thu Mar 04, 2010 2:56 pm
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You MUST replace your bias cap. The voltage applied on this cap is higher than he can hold.

Put 100 volts an same or litle more capacitance.

Remember if this cap "burn" your outputs tubes go away too.


If you do not well know tubes amps, be carreful, they can kill you , voltage is always there unless you discharge the bigs caps you want to change.

If you do not know what I mean , don't touch to that caps


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Posted: Thu Mar 04, 2010 4:19 pm
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Yeah James, the bias cap replacement is a good idea. And as Stratele points out, it's best to bump up that VDC rating to 100. You could go with a 50uf or 100uf/100VDC here. Also check that 470ohm 1watt resistor on the bias board. The other resistors that I mentioned are located on the filter cap board. When you remove the filters the 220K resistors are real accessible. You might want to draw a picture of the cap board before removing any parts( polarity is critical and there are wires running underneath the top board). Just a question, do you have a three prong power cable on that amp? Art

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Posted: Thu Mar 04, 2010 4:28 pm
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I think I have it discharged so I'm confident working on her. I already had to put in a new fuse holder and I'm taking it real easy. So, the bias cap that's there is a 50@50vdc and the catalog I'll order from doesn't have a 50@100. The closest cap is the 100@100vdc. Will that one be okay? I also noticed that the first filter cap (large one on the left end) is reversed from the others as far as the polarity. Is this common or was it put in backwards? I'm told the amp played 'okay' until it was put away some years ago. Hey. I really appreciate all your advice. This is a cool site.


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Posted: Thu Mar 04, 2010 5:39 pm
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Hey Art,
Thanks again. I'll be sure to check the resistors. I have a three-prong plug and that's on my list as well. I have a Gerald Weber dvd where he services a Deluxe Reverb. I'm using it as a basic guide and following what he does step-by-step. But, this forum has given me great direction.I'll keep you posted on my progress.
James


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Posted: Thu Mar 04, 2010 8:40 pm
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Yeah, those first two filter caps are in series and should be replaced in the same way. That's why I still make a sketch of how the parts are in the amp before removing any parts(in a lot of cases),or I write out the procedure. If it's my amp, then I have a record. If it's for a customer, it really helps to explain what was done to their amp and then they have something for their records. The 100uf/100VDC is fine. BTW I've been buying parts from Antique Electronic Supply for many years, but for power tubes I use other vendors. That dvd should be very helpful. I've just about worn out my copy of his"Desktop Reference of Hip Vintage Amps," over the years. I'm guessing that you have a decent multi meter. That cap reforming would be a subject for another day, just don't clip the leads so short that they can't be tested and possibly used later(put them up on a shelf). Good luck, work safely and patiently. Art

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Post subject: Ducati cap?
Posted: Wed Mar 10, 2010 12:43 pm
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Coming along with the cap job on the Twin Reverb and have run into a roadblock. I've changed the filter caps and the three 25/25 electrolytics on the other side. There is another cap? there, a little silver one marked "Ducati" and it is a 25/30 volt. Do I change this as well and what can I replace it with? I don't see any 30 volt caps in the catalog I'm looking at. Thanks
James


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Posted: Wed Mar 10, 2010 12:58 pm
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James, you're replacing the electrolytics on the main circuit board, right? That cap can be a 25uf/25vdc also. How'd it go installing the filter caps? Art

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Posted: Wed Mar 10, 2010 2:29 pm
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Hey Art,
It went pretty well. I took my time and drew a diagram of what steps I wanted to take. I thought soldering was going to be rough but I caught on pretty quick how much to use and where to put the pencil. Put one pair of the circuit board caps in backwards (twisted together the wrong ends, duh) and had to redo them. I'm getting there. I really appreciate your help. I'm just hoping the amp doesn't blow up in my face when I eventually fire it up. Thanks for the info on the cap. I thought Ducati made motorcycles.
James


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Posted: Thu Mar 11, 2010 8:13 am
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IT WORKS! I feel like a kid on Christmas morning. Got all the caps in, had to replace a resistor I broke in half, put the tubes in and took a chance. Fired right up, I even biased the tubes with no problem, it runs quiet with no hum, no microphonic noises. I plugged in a Les Paul and, man. This is one sweet amp! I didn't think I could do it but all the thanks goes to this forum. For a first cap job, it was a great experience. Gotta find another amp now. I'm hooked!


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