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Be aware that tube equipment almost always has HIGH VOLTAGE, which could easily kill a person. So be very careful! Some radio designs (the "AA5" types) can have LETHAL VOLTAGE ON THE CHASSIS, ANY TIME THEY'RE PLUGGED IN!!! So ALWAYS unplug the unit, if you're going to be touching ANYTHING in it.

In older tube equipment, 99% of the time, the problems are just dried-out capacitors, rather than the tubes. After 30 or 40 years or so, they almost always just fail. The solid mica ones (more or less rectangular, usually, often brown, with "colored dot" codes) are almost always still OK. But, the main filter caps, and ALL of the other electrolytic caps, and any/all tubular caps, should just be replaced, right off the bat (usually before you ever even try to power it up).

You just have to write down the capacitance (uF, microfarads, usually) and voltage rating that's printed on each one, and then get new ones with similar (+ or - 20%, say) capacitances, and voltage ratings that are at least as high (higher is perfectly fine). Then, just snip the old ones' leads, near their bodies, and twist the new ones' leads onto the old leads and solder them. (Do them one at a time, so you don't get them mixed up! It's very easy, and cheap to do. If you have a silver "can" filter cap that is mounted on top of the chassis, that contains more than one capacitance, just get separate caps, instead of trying to find a matching can. Then just leave the can in place and solder the new ones to its snipped leads, underneath the chassis. Most, if not all, of the other capacitors will be found underneath/inside the metal chassis. Don't bother trying to test them. Just replace them ALL.

Electrolytic capacitors have polarity, you MUST replace them using the proper orientation. Also, allowable "tolerances" for capacitance values can be taken into account, when trying to find replacements. For example, for many electrolytic caps, the manufacturer's tolerance was -20%/+100%, meaning that a given cap, when new, could have a capacitance value between 20% lower and 100% higher than its listed value. So, obviously, when replacing them, you can get by with using a slightly different capacitance than the original. Paper caps, on the other hand, were often -20%/+20%, which gives you a little less leeway. And being too far off might have side-effects on the tone or sensitivity.

After snipping the old capacitor's leads, remove most of the old crud on them, before connecting and soldering in the new capacitor, by drawing your needle nose pliers across each lead, several times, to scrape them until they're bare metal again. This will help you avoid cold solder joints.