Electrolytic and Polarized Capacitors: The Long One is +

99% of Electrolytic Capacitors are Polarized, meaning they have a + and a - and they must be installed with the + to positive and - to negative in the device. If a polarized cap is installed backwards, not only will the device not work correctly, often the cap goes POW and very large caps often go BOOM. In amps, the HV caps are large and easy to see, however with old tube radios, they have a lot of smaller electrolytic caps, and many people who still play with tubes, are oldsters like me. Of course, the eyes are going, and trying to figure out which lead is the + and - on an electrolytic or polarized cap sometimes can be a challenge. However, there is a gimme. On radial caps, which are the caps with both the leads coming out of the bottom, the long lead is always the +. I had been doing this for many years until I had a class on switching power supplies and the instructor told us that one. Hey, nobody knows everything. However, this gimmee doesn't exist on Axial lead caps, can caps and snap in caps. However, most of those are large and well marked anyway. This does exist on other polarized leaded caps other than electrolytic caps that are polarized, like Tantalum. However, be aware that many small value low voltage Tants and others are Bipolar (like me) so they can go in in any direction.
When replacing caps in tube radios, I normally go with high temperature caps. Normal temp caps is 85 and high temps are usually 105. Voltage needs to be at least the same or higher, but don't go crazy with that. Also be mindful of what the caps purpose is. Many caps are used to filter out hum and to smooth out the DC voltage. With that purpose, goung up in the UF value is fine, but again, don't go crazy with it. However, often small value caps, like the D201 series uses a bunch of 1uf caps, those caps are used as blocking or bypass caps, like I have mentioned often in amps. Those are used to block the DC and pass the AC, hence if you up the UF's in those, it will mess with the signal and operation of the radio.