Functionally it doesn't make any difference, but if you put the resistor in series with the AC terminals of the bridge rectifier; it gives the transformer a chance if the rectifier fails short circuit. A failing selenium rectifier lets you know in no uncertain terms - by the time you smell a toasting transformer, the damage is done.I am trying to restore my Grandfather's old Telefunken Opus 6 (1955) to working order, and have a few questions. All the "experts" say to start by replacing all the electrolytic capacitors and the selenium rectifier. Since these are no longer available, they suggested using a silicon bridge rectifier with a dropping resistor rated at 100 ohms @10 watts. I assume (did I say that I am NOT an electronic engineer) that the resistor would be connected in series with the output of the bridge, but rather than do damage to the radio, I have chosen to appear stupid and ask someone that knows for sure. Also, there are a couple of 50MFD caps in it that I have been unable to locate so far. Would a 47MFD work as a replacement? Thank you in advance for your help with this!
Thermal fuse resistors used to be quite common in the days of valve TVs - usually square section white ceramic body with a spring loaded contact soldered to the stub of lead coming out one end of the resistor, if it gets hot enough to melt the solder, the contact springs open. If you can get one of those, its good protection - but you need a fuse of *SOME* sort in front of the rectifier.
On the subject of electrolytics, the tolerances are very wide - -50% to +100% are not completely rare! Generally speaking; you can use a higher voltage rated replacement - but don't over do it, in stored capacitors with no voltage applied for a long time; the oxide dielectric layer can decompose - this can still happen to a lesser extent if the capacitor is consistently operated too far below its rated voltage.