Power filter caps can sometimes literally blow up. Sometimes they can cause weird noises to come out of your amp, or they can cause a burning smell, or sometimes they just lose their capacitance and quietly overload the transformer.
Any amp over 20 years old really should have these replaced. They dry out as they get older and are an important part of the power supply circuitry, so they should be replaced if need be.
1. Here is the old
cardboard covered filter caps in my '74 Twin Reverb.
These were manufactured by Mallory. They seemed to still work ok after 27 years,
but any one of them could have blown up at any time.
2. Here are the new filter caps just after installation.
These are Sprague "Atoms".
These improved the amp's tone. Everything sounds a little more defined
since replacing them.
3. Here is a view of the underside of the chassis.
The new power transformer is on the left. Next is the choke.
The output transformer is in the middle.
The one for the reverb circuit is furthest to the right.
This amp has ten tubes: 4 power and 6 preamp.
4. Don't forget the coupling capacitors!
I replaced all of the "chocolate drop" caps with Sprague Orange Drops.
The originals all looked like they had seen better days....
For replacement filter caps
My Twin Reverb Page