# re-capping a twin reverb



## Marcus0000 (Apr 17, 2007)

I bought a late 70's silver face twin recently and trying to get it up to snuff myself instead of taking it to a tech. The point being to learn about tube amp maintenance in the process. So I've replaced all the tubes and am in the process of recapping it. I've heard that Sprague Atom caps are the norm. So I'm wondering where in Canada I can find these? And if I can't, is the brand all that important? If I go to my local electronics supplier will they be able to provide perfectly fine alternatives?


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## jcayer (Mar 25, 2007)

Marcus0000 said:


> I bought a late 70's silver face twin recently and trying to get it up to snuff myself instead of taking it to a tech. The point being to learn about tube amp maintenance in the process. So I've replaced all the tubes and am in the process of recapping it. I've heard that Sprague Atom caps are the norm. So I'm wondering where in Canada I can find these? And if I can't, is the brand all that important? If I go to my local electronics supplier will they be able to provide perfectly fine alternatives?


For the caps, You can check at "the tubestore" http://thetubestore.com/index.html . The brand is not that important. But look for what they call orange drop capacitors. Check for the voltage they can tolerate...

Along with the caps, you can measure the resistors. I once refurbished an old amp ( http://www.jcayer.com/bricolages/renoamp.htm ) and some of the resistors were over 50% their rated value...

You might also want to check this page: http://www.paulrubyamplifiers.com/info.html#FirstPowerUp
there's a lot of good advices...

Good luck,


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## Wild Bill (May 3, 2006)

Marcus0000 said:


> I bought a late 70's silver face twin recently and trying to get it up to snuff myself instead of taking it to a tech. The point being to learn about tube amp maintenance in the process. So I've replaced all the tubes and am in the process of recapping it. I've heard that Sprague Atom caps are the norm. So I'm wondering where in Canada I can find these? And if I can't, is the brand all that important? If I go to my local electronics supplier will they be able to provide perfectly fine alternatives?


Normally the only caps that age and fail over the years are the electrolyics. These are the ones in the power supply (under the metal cover in a Twin) and also the small 22 mfd caps that bypass the cathode resistors in the preamp stages. The other caps are made of different materials and do not need replacement, barring a catastrophic failure.

Most mainstream industrial electronic distributors only carry radial electrolytics, which means that both leads come out of one end of the cap. These are a pain to mount in old tube amps. They are designed for modern printed circuit boards. The volume of sales for industrial distributors in axial leaded caps (out each end like Sprague Atoms) is so low that few get involved in stocking or carrying them. You can try - hey, people sometimes win on lottery tickets too!

In fact, at most modern industrial distis you probably will have problems finding a salesperson who even knows what a tube is and where it's used! If they were born after 1965 they likely have never seen or heard of one.

Fortunately there are a number of Net-based distis that specialize in parts for tube heads. http://www.thetubestore.com is one that carries Atoms and some other brands, along with tubes of course. They are the only Canadian source I know. I've dealt with them for some years and they are good people with great service.

http://www.tubesandmore.com is the site of Antique Electronics in the States. They carry a ton of stuff for music amps and old radio restoration parts.

Instead of Atoms you might consider Illinois. They are more modern in construction and tend to come in smaller sizes for the same C/V, or capacitance to voltage ratio. You'll find Illiinois in most modern American amps. I don't know yet if they last longer - I've only seen them for a little over 10 years so I've had few failures!

Hope this helps...

Wild Bill


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## Wild Bill (May 3, 2006)

After I made my post I saw that Jean from Quebec had beat me to it!:tongue: 

I agree about Orange Drops. Good caps for a reasonable price but again, these are coupling caps and not electrolyics. Replacing them would be a treat but not necessary as preventative maintenance.

As for resistors, be careful here! Silver Face Fenders likely use carbon film resistors. They are kind of a dumbbell shape. The older amps used carbon composition resistors, which are true cylinder shaped, like somebody chopped a piece out of a small rod.

Some audio hifi sites claim that there's a sonic benefit to the old carbon comp resistors. They also want you to pay $1000 for a "sonic enhanced power cord" and claim there's benefit there as well. Pretty well any techie who's not trying to fish you into buying such parts thinks this is just mojo crap from the ignorant. One bona fide "guru" in the States named R G Keen did discover that if the resistor had some serious current flowing through it there might be a trace of measurable sonic difference (the only place in a guitar amp where this would apply would be the plate resistors of the phase inverter that feeds the output tubes) but even then the difference was too trivial for all but the greatest of ears to notice. As soon as you stepped out of clean jazz tones and cranked up some George Thorogood I defy anyone to be able to tell the difference.

If your amp does have the old carbon comp resistors you should know that they are fantastic hiss, spit and noise generators! They also tend to drift far away from their proper ohmic value over the years. So replacing them with more modern carbon film or metal film is always a good idea.

BUT! If it's not your amp be careful. Sometimes over the years those resistors have drifted away from their marked values but the player LIKES the tone of his amp! If you replace those resistors with the stock values the amp will sound like the day it left the factory but NOT like it did for the player today!

I always measure each resistor and make a note of it's actual value. Then I pay attention to what it actually does in the circuit and in tone critical areas I use the "drifted" and not the stock value. That way the player keeps his tone the way he likes it!

Just more FYI...


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## jcayer (Mar 25, 2007)

Wild Bill said:


> BUT! If it's not your amp be careful. Sometimes over the years those resistors have drifted away from their marked values but the player LIKES the tone of his amp! If you replace those resistors with the stock values the amp will sound like the day it left the factory but NOT like it did for the player today!
> 
> I always measure each resistor and make a note of it's actual value. Then I pay attention to what it actually does in the circuit and in tone critical areas I use the "drifted" and not the stock value. That way the player keeps his tone the way he likes it!
> 
> Just more FYI...


VERY GOOD POINT :food-smiley-004:


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## Marcus0000 (Apr 17, 2007)

thanks for the tips. I'm going to order from thetubestore. Couple questions:

I'm replacing all the electrolytics under the metal cover... and the tube store doesn't have exactly the same values of caps available. what was in there:

Mallory
1. 2x220 MFD 285 VDC
2. 3x20 MFD 500 VDC
3. 2x70 MFD 100 VDC

I can order:

1. Illinios 220 uF 300 VDC ( I assume uF is the same as MFD)
2. Atom 20 uF 600 VDC or Illinois 22 uF 500 VDC (way cheaper than the atom)

3. the closest are:
(atom or illinois) 80 uF 450 VDC or
atom 50 uF 50 VDC


What are my best choices? thanks


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## Wild Bill (May 3, 2006)

Marcus0000 said:


> thanks for the tips. I'm going to order from thetubestore. Couple questions:
> 
> I'm replacing all the electrolytics under the metal cover... and the tube store doesn't have exactly the same values of caps available. what was in there:
> 
> ...


Interesting about the 220/300vdc. Is that the stock schematic value? It looks to me like someone had subbed them in over the years. Unless you've got the later "UltraLinear" Twin. I seem to remember those values in that one.

There were a LOT of Twins!:tongue: 

Anyhow, you can ALWAYS go higher in voltage rating! It just means the cap has more safety factor available. So [email protected] volt is fine.

You can also go a bit higher in capacitance value, to reach a standard available value. Those 2 pcs of 220 mfd actually will be in series in your power supply circuit, which will result in a combined value of half of one cap, or 110 mfd but at twice the voltage rating i.e. 600 volts. The actual voltage at that point in the circuit will be less than 450 vdc so there's no problem.

Either brand will be just fine. Some folks just feel more comfortable with a larger, old-fashioned style cap, that's all.

The last choices are for the cap(s) in the bias supply. You absolutely CANNOT use a 50 volt rated cap here! The bias voltage could exceed 50 volts and the cap would likely short out.

The Illinois is ok at 80 mfd but 450 volt is sheer overkill and likely kinda big. You might want to pick up something from that mainstream industrial distributor! A radial leaded cap is not too much trouble to mount in that spot and they likely have something over 70 vdc available, like 100 vdc. Much smaller!

If there's nothing close to 70 mfd then you could go for 100 mfd, which almost certainly is available. Or you might go a bit lower, down to 47 mfd. If it were my amp that's what I'd do. I don't like the idea of filters being too big a value in the bias supply. Bigger values take longer to charge up to the needed voltage and while the voltage is too low during warmup the output tubes will see too low a bias voltage and draw excessive current. All you need is a big enough value to keep the bias voltage hum free. Since there's no current to speak of in the bias supply yu can get away with MUCH lower cap values! If this were a Marshall you'd see 8-10 mfd instead of 70! This is another example of how the engineers that designed the Silver Face Fenders were kinda goofy...

Pay attention with that last cap! The bias supply is the only point in the amp where the negative lead is NOT tied to ground! All the other filter caps are tied negative to ground but in the bias supply you ground the positive!

If you make a mistake you'll find out pretty quickly...


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## Marcus0000 (Apr 17, 2007)

hey thanks! based on your advice I'm going to order:
All illinois: 
2x 220uF 300VDC
3x 22uF 500VDC
2x 47uF 500VDC (I'll squeeze them in...I should have extra room if the other illinois' are smaller than the originals)

:banana: 


here's the schem BTW. Looks like those 220uF caps are in there:


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