Ideally, you drain the capacitor first with the same resistor you're about to install as a bleeder.
Much less ideally, you can short a cap with a screwdriver or something, but the degree to which that's a bad idea scales at least linearly with the value of the cap.
I did that once with a photoflash cap, while trying to troubleshoot a failed Nikon SB-R200 ring-mount flash head - expecting the failure to be in the control circuitry, I wasn't sure whether it had died with charge on the cap. It had! Luckily I had the good sense to point the thing away from me, because it melted a chunk out of the screwdriver tip and distributed it as slag across my worktop. Even as it was, my ears took most of an hour to stop ringing.
A fair question at this point is: with a capacitor big enough to be that dangerous, why wasn't there the kind of bleeder resistor we're talking about? In this case, it's also a design feature, because flashes run on batteries and you don't want to waste charge, or have to wait all over again for the battery to charge the cap every time you switch on the unit. Too, these flash heads have no externally accessible contacts through which the cap might discharge into the user, and the charge circuit uses a MOSFET to switch battery power to the cap, so even if you go poking fingers into the battery compartment, it still won't light you up.
Nonetheless, it serves as a good example of why you want to be very careful with high-power capacitors. The one I'm talking about is only about the size of the second joint of your thumb, small enough to fit into a flash head that itself fits into an adult's palm. Even so, at full charge it had enough juice to blow up a screwdriver and injure my hearing - and if I'd been even more careless and discharged it through my actual hand, I don't doubt I'd have ended up with a permanent scar.
Be smarter than I was! Discharge your big caps through a high-value resistor before you do anything else with them.
I inherited a stereo amp from the 70s, and I'm pretty sure one of the capacitors has gone bad. I've managed to find a schematic and PCB layout for it (remember when service manuals were a thing?) but I've been too worried about discharging capacitors to poke at it.
For now, I just deal with a mains hum that's only audible when nothing is playing.
Amp power caps aren't usually bitey enough to be dangerous, although they will certainly hurt if you're careless with them. They also usually have bleeders, since a line-powered unit doesn't need to economize on power the way something like one of my flash heads does. I usually just poke a screwdriver across them when I'm doing a recap job, and I've never had them so much as spark. But for maximum caution, I'd use a 100K or so 1/2W resistor, connected with clip leads across the capacitor leads, and left for a minute or two prior to desoldering.
For an amplifier of that vintage, I'd probably be more worried about the fact that it'll likely be neither grounded nor double-insulated, meaning it's possible for an internal isolation failure to present a potentially lethal line potential on any metal parts of the case.
Much less ideally, you can short a cap with a screwdriver or something, but the degree to which that's a bad idea scales at least linearly with the value of the cap.
I did that once with a photoflash cap, while trying to troubleshoot a failed Nikon SB-R200 ring-mount flash head - expecting the failure to be in the control circuitry, I wasn't sure whether it had died with charge on the cap. It had! Luckily I had the good sense to point the thing away from me, because it melted a chunk out of the screwdriver tip and distributed it as slag across my worktop. Even as it was, my ears took most of an hour to stop ringing.
A fair question at this point is: with a capacitor big enough to be that dangerous, why wasn't there the kind of bleeder resistor we're talking about? In this case, it's also a design feature, because flashes run on batteries and you don't want to waste charge, or have to wait all over again for the battery to charge the cap every time you switch on the unit. Too, these flash heads have no externally accessible contacts through which the cap might discharge into the user, and the charge circuit uses a MOSFET to switch battery power to the cap, so even if you go poking fingers into the battery compartment, it still won't light you up.
Nonetheless, it serves as a good example of why you want to be very careful with high-power capacitors. The one I'm talking about is only about the size of the second joint of your thumb, small enough to fit into a flash head that itself fits into an adult's palm. Even so, at full charge it had enough juice to blow up a screwdriver and injure my hearing - and if I'd been even more careless and discharged it through my actual hand, I don't doubt I'd have ended up with a permanent scar.
Be smarter than I was! Discharge your big caps through a high-value resistor before you do anything else with them.