Almost there. Insert the leads of the new capacitor (an F&T 20 µF / 500V here) through the holes in the cap ends. Check to make sure the polarity is correct! The positive end has a ridge around it. The new capacitor is marked, of course. Slide the assembly back into the cardboard sleeve - it is a little fussy - take your time. TRIPLE CHECK that the polarity of the old cardboard sleeve matches the new capacitor polarity!
A spot of glue will fix the aluminum caps in place in the tube.
crimp the end of the tube over your restored capacitor. Use needle-nose pliers. Do a neat job. If needed, touch up the ends with amber shellac; it perfectly matches the tube color.
Congratulations! You are done.
Your ends should look like this now.
When it is time to replace these capacitors, technicians usually just cut out the old ones and solder in new caps. Which is fine.
Except there is something about those old Astrons - they are handsome and exemplify the vintage quality of those old amplifiers.
I like to restore these capacitors by placing a new capacitor inside the old shell. Now, lots of other people do this, too, especially old tube radio people, but I have developed a method that is fairly simple and is a very accurate look.
Cut the lead wires off, as close to the metal rivet as possible.
These filter caps are from a 1958 Fender Tremolux. The ground lead of each capacitor runs through a hole in the metal cover and is soldered to the outside. Later on, Fender ganged all the grounds to a single wire and ran it inside the chassis to a ground.
This early style is a little more work to take apart, and you need a large soldering iron to melt those solder joints on the metal cover.
Once the crimp is fully open, grab the positive lead of the capacitor with pliers, hold onto the cardboard tube, and pull. The aluminum can should come right out.
Clean the insides really well. Use a wire brush.
Now cut the big end with a pair of wire cutters, vertically, and peel the aluminum back as shown so the negative end is only about 1/2" long, the same as the positive end. Don't skip this part; the caps are going to go back into the capacitor shell and they must not touch.
Wear nitrile gloves at this point in the process.
I have heard supposedly knowledgeable technicians refer to the stuff inside dry capacitors as "Sulphuric Acid". It's not. It is a solution of ethylene glycol and boric acid, or, more recently, dimethylformamide (DMF), dimethylacetamide (DMA), or gamma-butyrolactone (GBL), all of which are anhydrous organic solvents. The white goo that sometimes leaks from old electrolytic capacitors is usually crystallized borax electrolyte.
Wear gloves. The capacitor goo should be considered toxic.
The Astron Minimites on the circuit board of this 1958 Tremolux have also been restored. Here is where the labor really pays off - I love the "look" of an original eyelet board with period components. There is just something wrong about seeing modern capacitors soldered in there. I know it doesn't make a difference functionally, but for me, the aesthetics are important.
Drill a small hole close to the rivet on both cap ends.
You can do this with any size capacitor, but the bigger ones are easier than the small ones.
Here is a nest of Astron Minimite electrolytic capacitors.
Here they are, removed. Note "Big Bertha" soldering iron. I use this iron when I have to solder to a chassis or other big heat-sink, like the cap cover in this case.
The factory foam strip has melted onto the capacitors, which is unfortunate.
Here are the restored Filter capacitors from the above 1958 Tremolux. Looks completely original. Yes, no one but a technician will ever see them, but I know they are there.
Start the process: the blue stick in my hand is the end of a cheap small plastic paint brush. It is very flexible, and the tip is rounded.
I insert this tool under the crimped edge of the POSITIVE end of the Minimite, and then slowly push it around the circumference to "unfold" the crimp.
We are going to open up the capacitor, so get a tubing cutter, like mine, here (or similar). If you have never used a tubing cutter before, practice with it for a while until you get the hang of it. The important thing is to keep what you are cutting parallel to the blade - if you are off just a bit the blade will wander, and you'll make a mess.
Don't try to cut the case with a knife or a saw or wire cutters, or anything but a tubing cutter. I have tried them all, and only the tubing cutter works.
With the tubing cutter, cut the aluminum case 2/3 or so of the way towards the Negative end. Once it is cut through, you can pull it apart like this.
It smells weird, but I kind of like it. It smells like old radios.
Remove the capacitor guts - the paper/foil roll and the foil connectors from both ends, and throw them away.