Hyrel PIO Capacitors - We Put Them in our Dynaco PAS!

A number of posts ago, I mentioned a "secret" that was going into the PAS.

Here we are...the secret is unveiled.  These are Sprague Hyrel paper-in-oil capacitors.  I have a bunch of these, as well as a bunch of Sprague Vitamin Q caps for future projects.  Basically, paper-in-oil capacitors have the reputation of having fantastic tone - warm, detailed, and generally yummy on your ears.  So I had to try 'em in our Dynaco preamp.

Paper-in-oil, or "PIO" caps, as they are known, have a dielectric composed of oil impregnated paper.  There is definitely a cult following for these things; try a Google search and see.  Sprague made a series in the 50s and 60s called "Vitamin Q" and these Hyrels.  "Hyrel" stood (...stands...) for "High Reliability."  I am not sure about this, but legend has it they were made for NASA applications.


These things are built like little tanks - they're pretty hefty and are hermetically sealed. The outside shell is covered with an insulating plastic. 

I tested them for leakage on my Heathkit capacitor tester at the rated voltage (400 volts) and they passed with flying colors.

Right before I went to put them into the PAS, I got the bright idea of measuring their capacitance too.  I figured I'd try to "match" two pairs for the phono and line circuits.  I actually have 10 of these puppies, so I got a pair that measured .948 microfarads and a second pair that measure .944.  Still within the 10% tolerance after 50 years.  That is quality.

The Hyrels are big indeed and I knew it would be a tight fit.  But it actually worked out better than I expected.  I just had to bend the leads under the cap bodies and I was able to use the stock Dyna eyelets.

So now we have only 2 caps per board that the signal goes through - a Sprague 716P "Orange Drop" poly-and-foil cap, and these Hyrel PIOs.  Here's to hoping we get good results.

The caps do lie on top of some other components, but I don't think they'll cause a problem.

I went with the red shrink tubing on these cause they're supposed to be so hot!