Crawls Backward (When Alarmed)

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Removing and Restuffing Mallory Can Capacitors on Fender Amps

Now it's on to the recapping of the Vibro Champ.  This is the Main Event - the aluminum multi-section Mallory 'can' capacitor that contains the three filter caps.  Lots of pictures in this post, and lots of fun.  I'm going to show you what I do to get them off and replace them.

You will see these a lot on radios and amplifiers built in the mid-1950s through the 1970s.  They may be most well-known on Fender Champ, Vibro Champ, Princeton and Princeton Reverb amps, but I have seen them on gear built by Zenith, Hallicrafters, Dynaco, Pilot, Eico, Heathkit and others.  And not just American junk - I have Eddystone, Grundig, Marshall, Diora, Luxor, and other European gear that use this type of capacitor.  This approach can be used with any can capacitor.

It's pretty obvious how these caps came to be popular.  By the 50s, circuits were typically more complex, tubes became smaller, and chassis followed suit.  High-voltage electrolytics were historically large, bulky components.  But can caps allowed them to be made smaller and easier to install.

Most of the time, you can just bend some retaining tab back, or undo a clamp and the can will come out.   But in Fender's case, they soldered two (or more) of the tabs to the chassis.

The chassis, of course, acts like a big heat sink, so it takes a lot of heat to melt the solder.  I've used different approaches - this time I tried two 140-watt soldering guns at once.

But one of the tips is broken, and with not a lot of surface area to get the heat down, I was largely unsuccessful.  What to do?

You need big heat?  How about a propane torch?

This should really be done with a smaller torch, but this is what I had.  I dialed it back to the smallest flame I could get (smaller than the one in the picture), and used foil to cover anything that might melt.  Don't try this at home!

Then I put one of the guns on high heat right on a blob of solder with a piece of solder wick under it, and hit it directly with the torch.

It worked. Ha.  (A really big soldering iron is on my list of tools to buy).

The solder got broken up enough to the point where I could use an old bench chisel to chip it off, and then raise the tabs.

Sounds like a lot of work, but it took about 10 minutes in all to get the can out.

Here's the can removed from the chassis.  In the US, Mallory was the company that made these.  Again, I've seen other brands on English and European stuff, but the concept is the same.

Note how the different sections are indicated on the can.  The 40uF/450 volt section, for example, is indicated on the bottom by a half-circle.  The others are marked with similar symbols. 

Now, you can get new cans from CE Engineering (they use the old Mallory tooling), or JJ and F&T.  They are all high quality parts and will work well.  However, they cost a bunch of money.  The CE can with these same values runs about $35 US.  Rather than spend the money, I have another process I generally use for restorations:  I cut the can open and put modern radial caps in it.

First step is to open the can.

You *may* be able to open the crimp at the bottom and peel back the edge that's crimped down and remove the innards from the bottom.  I've done this in the past, but this particular can was crimped down to the point that I couldn't get a tool into the seam to open it up.

This is actually what I wind up doing 8 out of 10 times: cut the can open along the rim.  You can use a hacksaw, or, as I did here, a Dremel or other rotary tool with a heavy-duty cutting disc.  The can is light aluminum and will cut easily.  Try not to hack it up too much.  You'll soon see why.

Well, goll-lee.

The top of the can slides right off, exposing the innards.

Snip off the aluminum strips that connect the tabs on the bottom to the caps.  The long strip you see is the ground.  Unfortunately, it's hard to solder to, otherwise I'd reuse it.  I just snip them all off.

I have these super duper, high performance Panasonic caps I'm using as replacements. If you think these are too expensive, you can save $5 and get some more generic caps. 

With a little trial-and-error, I come up with a configuration that will leave me with all of the grounds tied to one point, and the three positive sides laid out so they'll contact the appropriate tab on the can for its value.  Depending on your can, you may have 2, 3, or 4 caps inside it.  Radial lead caps tend to be easier to work with for restuffing than axials.

In this case, the grounds are tied together in the middle.  The two caps on the bottom are 22uF - the second and third sections of the can, and the cap on top is the first section.  The older Champs used a 20uF cap; the later ones like this used a 40uF.  I decided to split the difference and use a 33uF.

Drill holes through the bottom of the can near each of the tabs, and also a hole for the ground.

You can see what I mean by trial-and-error.  I had to arrange the caps so that each positive lead would wind up near the appropriate tab.

Just run the leads through the holes you drilled, and solder them to their terminal and trim the leads.

You probably see where this is going now.

Another angle of the tabs on the bottom of the can.  You can see how I ran the ground lead through a mounting tab.  I'll bend that tab down and solder the ground once it's back in the amp.

Next, we mix up a batch of J-B Weld epoxy.  Spread some around the rim of the can and cut edge on the bottom of the can.

Stick it together and...

We have a newly restuffed can cap ready to go back in our Vibro Champ.

I like doing this because it more or less keeps the original appearance.  You will see a lot of these amps where new filter caps have been cobbled together above the cap inside the amp.  I just don't like that look at all.  Sure, you can tell the can has been redone, but I think it's a far better thing than using discrete caps.


Post a Comment 4 comments:

  • Sven Nyström said...
    May 29, 2013 at 5:08 AM
    My, that is really cool. Sometimes I regret not learning anything about electronics. At all. But reading posts like this still intrigues me.
  • Paul Rayment said...
    February 10, 2014 at 10:23 AM
    Thank you! This is really useful.
  • Anonymous said...
    March 21, 2015 at 8:35 PM
    Great instruction. I will give this a go.
  • Anonymous said...
    June 16, 2016 at 6:13 PM
    It was interessant and I had fun, great

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