You know that "hum" noise that comes out of old electronics, especially tube powered radios and amplifiers/ This is usually caused by failed electrolytic capacitors in the power stage from the transformer. Replacing these old parts will usually resurrect a dead tube set or eliminate a constant hum in a functioning set.
This is a method for replacing failed multi stage capacitor cans to get old radios working again. These things only last about 25 years so most of them have failed at this point. New ones are not generally available and are expensive if you can get one at all. Follow along as I show a quick and dirty way to replace these pesky caps. Some people will re-stuff the cans with new parts to preserve the look but I don't and I don't care how it looks, my amps are not for showing to people, they are for making music!
Step 1: Cut Off the Old Can!
Using a hacksaw blade and taking care to not damage the tubes, cut off the can right above the crimp line, saw right through the sucker and you are left with a stump. Remove tubes if you need to in order to get a clean shot at the can. Be sure not to damage the info stamped into the side of the can, it will tell you what value to get for the replacement parts and what terminal to use.
Step 2: Take Note of What You Need to Buy , Get Shopping
For this one I need
60 MFD at 350 volt
40MFD at 350 volt
30 MFD at 350 volt
10MFD at 350 volt
I can go up in voltage, substituting a 10 MFD at 450 volt for instance but I can't change the MFD by very much-10% is the furthest I'd push the specs. If I had no time to wait for a new part and I had a 42MFD on hand I'd try it, but be prepared to pull it off if the amp sounds funny.
I buy mine from Justradios.com but you can get caps just about anywhere on the internet. I'm not a proponent of buying super expensive caps for old tube radios. My ear does not hear a difference and I'm not shelling out $20 for a cap in a blue wrapper that functions the same to my ear as a $4 black capacitor.
Step 3: Note Where the Caps Go
See those little symbols next to the value designation? the triangle, box, half circle and "nothing" are etched into the bottom of the cap, we are looking at the same chassis here, but from the bottom, you can just make out the square symbol and half circle in the pic. Now CAREFULLY transfer that to the top of the chassis, be sure not to mix up the position while flipping the amp over
Step 4: Drill Next to the Cut Stubs of the Old Cap, Install the New Caps
Yep, drill right next to them, this will allow you to pass the replacements down to the stubs and solder them on! and you wrote down which position was what so if you destroy the symbol drilling through it, no problem! See how the new caps fit right in? Be sure to observe polarity, most cans are wired negative to ground against the case but there are exceptions. this one is can ground so I'm passing the positive leads through the holes I drilled. BE SURE the positive leads don't touch the rim of the can you cut off.
Step 5: Finishing Up the Electrical
Solder the Positive leads. connect all the ground leads together and solder a bit of wire to them, run it though another hole , drill if you need another hole, and connect to ground. there is usually a leftover tab from mounting the can into the chassis you can use. Don't try to connect to the original solder point on the chassis, your iron probably won't be able to get the metal chassis hot enough fast enough, just find a new ground nearby. Now you can test it out!
Step 6: Glue!
Use hot glue to stick all the caps together up top and then squirt some glue into the gab between the cap and base, this will insulate and physically support the caps so they don't bend over and ground out. YOU ARE DONE!
Please note in the pics there are only two, I've yet to install the other two caps.