Vintage H.H. Scott gear uses a variety of non-polarized caps
for critical signal path coupling and blocking/bypass purposes. In older (fifties) H.H. Scott mono
gear, you'll typically find paper and foil "striper" plastic molded
caps. Later (sixties) vintage H.H. Scott stereo gear used the infamous American Radionic's
mylar and foil non-polar caps. These caps are notorious for failure and all of
them should be replaced if you plan on actually listening to your vintage H.H.
Before you rush out and buy the latest and greatest
"super caps," a note of caution:
Replacing the caps with expensive, premium foil and film
caps can alter the original sound characteristics of your vintage unit. Use a
good-quality, reasonably priced foil-and-film type like the Illinois
Capacitor, IC-MWR. They are
relatively inexpensive and sound much better than run-of-the-mill paper capacitors or the high-end "super-caps."
Sometimes, middle-of-the road is better.
Non-polar signal caps couple the plate of the preceding stage's tube
to the grid of the next stage's tube. The most common
problem associated with these capacitors is DC voltage leak. DC leakage adversely
changes the bias of the tube. Not good.
For example, if a grid-coupling capacitor
connecting the phase-splitter tube's output to the grid of the final push-pull output tube is leaking DC, it
will cause the output tube to run hot, and fail prematurely. In many
cases, the output tube's plate will start to glow red, the amp will sound distorted,
loose power and eventually blow the AC line fuse (if you are very lucky).
Unfortunately, this "runaway" condition can occur
very unexpectedly. If you do
not catch it quickly, your output tubes will "toast" and you could
also "fry" your output transformers. Sadly, if that happens, you'll
most likely be
looking for another amp.