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Replacing Selenium Bridge Rectifiers

Some vintage H. H. Scott gear typically uses a selenium "stack" bridge rectifier for the preamp's DC filament supply and the negative grid bias supply for the output tubes, sometimes referred as the "C-supply". This is in addition to the full-wave rectifier power supply tube section which supplies high-voltage, B + supply.

You'll typically locate the selenium rectifier as square, finned device located under the chassis on a mounting bracket.

Often the selenium stack will fail unexpectedly, emitting noxious fumes. It is not a matter of "if" a selenium rectifier fails, but rather it is merely "when" it fails. Selenium rectifiers fail because they develop high, internal series resistance, resulting in lower bias or "C-supply" voltage.

What is going to happen to your vintage H.H. Scott amplifier's output tubes if all of a sudden it looses all of its negative grid bias or DC filament voltage in the preamp section?  You can probably guess.

Output tubes will draw too much current and overheat causing the output tubes' plates to turn red or orange. At the very least, you may be looking at replacing hard to get NOS (expensive) tubes. Allowing any amp to operate in this condition for very long will destroy the output transformers. That should get your accountant's attention!

Fortunately, there's an easy improvement (yes, Virginia there is a Santa Claus!).  The selenium bridge rectifier can be easily replaced with a suitably rated silicon diode bridge rectifier that is readily available at just about any local electronics supplier. And the cost for this "technology insurance" is a very reasonable, especially when compared to replacement of output tubes, transformers, and possible loss your entire unit. You most probably will also need to add a dropping resistor, to readjust the voltage slightly.

While it may seem strange to be making this simple "modern" modification, rest assured, it will not reduce the value or sound quality of your vintage unit. On the contrary, your contribution may permit your vintage H.H. Scott gear to continue to serve well into the next millennium and turn on the next generation of tube lovers!


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