![]() ![]() The new diode rectifier would be more efficient than the old rectifier, so there was a good chance that the voltage to the tube filaments would increase beyond the 12.6 volts DC specified on the Ampex 351 schematics. ![]() I also replaced the 6×4 rectifier tube used to supply high voltage for the audio tubes.īonkowski warned that replacing a selenium rectifier with new silicon diodes could create a problem. Luckily, this rectifier was easily replaced with two 1N4004 diodes, which I wired on a tag strip and connected between the power transformer and the power supply circuit board (see photo below). It turns out that this type of rectifier doesn’t age well: the DC voltage supply gradually deteriorates, they are prone to catching fire, and failure is imminent. Searching the web for info yielded an incredibly helpful tech paper called “Replacing Selenium Rectifiers” by Rich Bonkowski. I vaguely recalled rebuilding a Dynaco PAS 3 preamp where a selenium rectifier had to be replaced. Hmmm… Somewhere in the dark corners of my brain, near where I filed the number of tribbles that fell out of the grain compartment in “The Trouble With Tribbles,” there was a distant memory about selenium rectifiers. The photo at the head of the article shows this rectifier after it was unmounted from the side panel. ![]() While snooping around the 351’s power supply I noticed that one of the rectifiers-the components responsible for converting AC voltage to DC for supply to the tubes-was an old selenium type. Recognizing the age of the components, I expected that some of them would be out-of-spec, which could result in less-than-optimal audio performance and (more importantly) out-of-range voltages in the power supply. ![]() It was pretty easy to find schematics, an owner’s manual, and informative threads regarding repair and maintenance of these old beasties on the web. Replacing the tubes was definitely on the menu, given the service dates marked on the rear panel, and I thought it would be a smart idea to verify that voltages to the various parts of the circuitry were correct. At the close of the first episode of “Resurrecting an Audio Dinosaur,” my lovely Ampex 351 tape machine channel was alive and working-though I wasn’t sure if it was working properly, or what I’d need to do to make sure it wouldn’t blow up in my face. ![]()
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