One approach is to "reform" these capacitors by initially applying a low voltage to them and over a period of time (several hours), slowly increasing this voltage until the operating voltage is achieved. Some people use a variac (variable transformer) to apply this controlled voltage to a tube amp (with all the tubes except the rectifier removed), but I prefer to use a separate, high-voltage power supply. By using a power supply on each capacitor, it's easier to monitor the current the capacitor draws while reforming, to ensure that it isn't internally shorted and building up to an exciting explosion. Exploding capacitors will splatter an unpleasant, odiferous material on you that you'd really rather avoid. :-)
Given that these old electrolytic high-voltage capacitors are prone to failure, replacing them before they fail (especially if they're very old) is probably not a bad idea. Replacing them after they fail is not an option. :-)
However, high-voltage electrolytic capacitors can be expensive and hard to find in most junk-bins, thanks to the low-voltage requirements of the more common solid-state stuff around today. An interesting type of replacement capacitor to use are "Photo Flash" capacitors, which are typically 160 uF, 330v electrolytics. A surprisingly good source for these that I've recently discovered are the disposable cameras with a built-in flash. It turns out that after developing the film, the film processing centers normally return these to the manufacturer for recycling. Since they don't get anything in return for this, they're perfectly willing to give them to customers who politely ask for them. Of course, tube geeks are very polite.
I made a stop at my local supermarket and went home with a dozen of these cameras a few minutes later. Each camera contains one of these capacitors, providing me with a dozen capacitors. While opening up the cameras, I noticed that many of them still held a charge. In a sampling of the various capacitors, I found them to vary at between 0 volts and 190 volts!, so you definately need to discharge them before handling. I found a 100 ohm resistor to work well.
By putting two of these capacitors in series, you achieve a voltage handling capacity of 660 volts, which is usually more than adequate for the B+ circuit in a tube amp, but remember that capacitors in series produce a total capacitance of:
(C1 * C2)
Ctotal = ---------
(C1 + C2)
resulting in 80 uF at 660 volts. Since there is no guarantee that the
voltage will divide evenly across the capacitors, a shunt divider should be
connected across them as well to ensure that the 330 volt rating of either
capacitor is not exceeded.
+ ---------+----+-----+
| | |
C1 C2 R C1 = 160 uF/330v
| | | C2 = 0.1 uF/330v Polypropylene film cap
+----+-----+ R = 330 Kohm, 1/4 watt
| | |
C1 C2 R C2 is optional, but improves quality
| | |
- ---------+----+-----+
The optional film capacitors (C2) across the electrolytics helps to reduce
the impedance of these capacitors, but some opinions on this disagree. Make sure to orient the + leads on each of
the photo-flash capacitors towards the positive terminal of the aggregate
capacitor being built. Reverse-biased electrolytic capacitors explode even
quicker than old capacitors...Back to the Speaker description.