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I agree with Dave - certain things should be changed, and a certain amount of modifications are necessary to keep the set operational and safe. Historically accurate is important to some degree, i.e., you wouldn't rip out the guts of an old console and stuff a modern solid state set in there, but replacing components is unavoidable. Capacitors have changed a lot, but new units need to be used, since almost all the old ones (NOS or otherwise) will have aged poorly. Selenium rectifiers are notoriously unreliable, and die slowly. When they do fail, they typically create a nasty smelling smoke, and could even start a fire. They also starve the set of power as they fail, becoming less and less efficient. I always replace them with modern rectifier diodes, disconnecting the original selenium stacks and leaving them in place. I'll typically use one of the terminals on the old selenium as a mounting point for the new diode. These kinds of modifications are easily reversed, simply by unsoldering the new diode and reconnecting the selenium rectifier.
As for recycling tube bases to create plug-in 5U4 replacements, I don't see much of a problem here - the 5U4 is an octal tube, which are very, very, very common. There were many different octal tubes made, and rectifiers are pretty common. Dead rectifier tubes are useless. And so are most failed tubes, with the exception of tubes that can be 'rejuvinated', or tubes that have collectible value because they were so unusual or early. 5U4's are much too common to become valuble, ever. OK, maybe in 300 years, but not any time soon. And there are LOTS of other tubes out there with octal bases, not to mention all those relays with octal bases, etc. If you _really_ wanted to be sure not to destroy a tube, you could always break up a relay, or find an octal tube with cracked or broken glass, or even just make something with a hunk of wood and some heavy gauge wire.
Basically, what I'm trying to say, is that there's a certain amount of historical inaccuracy with any repair or restoration. Every time you repair something, you have to modify it, melt the old solder off, remove a component, something. Modern components are much safer, more reliable, and can make a set preform as good as, or better, than when it was new. A certain amount of component replacing will be necessary, and you'll have to use new components if you want the thing to work, and work for a long time. Safety upgrades are important - silicon rectifiers, line fuses, polarized line cords, and making sure it's the hot line that's switched with the power switch and not the neutral - these are all important to ensure the set will function properly and safely, and prevent nasty accidents that could possibly destroy it. Just think of how many power transformers have been destroyed by shorted electrolytics, and how much damage can be caused by a shorted selenium stack. How many deadly fires were started because of a burning electronic component, and how many people were electrocuted by touching the metal shaft of a tuning knob on a hot chassis set?
-Ian
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