[TA Forbes]

Something useful to make careful note of when replacing the "can" electrolytics is to note if the can has an insulator sleeve around it or a washer-insulator that it sits on. If so this means that the negative side of the cap does NOT go to chassis. In these cases there will be some kind of lead that is connected to the can's external body.

In the case of your model 18, C53, 54, and 55 all share a common (negative) side. Note that this is CT of the plate winding for the rectifier, which will have less potential than that of the filament winding, where the positive sides of C53 and 54 are connected. C55 has it's positive side to ground.

Frequently the negative side goes to a potential that is relatively negative to the potential on the positive side. Example: positive side goes to 100V, negative side goes to 50V. In this case the 50V is negative in respect to the 100V. BUT the negative side of the electrolytic must still be insulated from chassis.

Philco sets (and some others) often have an electrolytic with the positive side going to ground. As was well expained in a previous entry, this is usually to provide some form of negative bias.