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!
