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I have a couple Triplett 3413A tube testers in need of calibration. Before I dive into this, more-or-less blindfolded, I figure I should replace their rectifiers. As described on the Triplett parts list, they are "copper oxide, 1/2 wave, two lead" rectifiers. Have never heard of copper oxide rectifiers before. Are these essentially the same as selenium rectifiers? Can they be replaced by diodes? The B+ circuitry in these testers is very primitive -- no tubes or electrolytics (the only cap in the tester is a .1 MFD 400V bypass). Any advice would be greatly appreciated!