Signal Path Caps: Coupling & Blocking

 


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Last Edited:
12-Sep-2004

  
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. Scott.

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.

 

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