Your thoughts on PIO caps?

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Bowie

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Jun 22, 2012
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368
  Though I'm big on NOS tubes and resistors in my builds, I've never bothered much with vintage caps as I'm pretty happy with modern selections.  However, when buying a large lot of vintage tubes this week I found myself with about 150 NOS, US-made PIO caps.  Sprague Vit C, West Cap, and TWO.  I know that people make a big fuss about these so I wanted to ask for opinions here since I can trust you guys for down to earth responses, rather than doing a search which brings up all kinds of nonsense from hi-fi and guitar forums.  What applications do people prefer these in and what are their benefits/compromises when using PIOs?  Yes, I will try them myself but I would like to get opinions before implementing them.  Thanks!
 
They have a sound.  You like it or you don't.  I wouldn't take working ones out of old audio equipment. 
 
There may be a niche market for NOS to use in vintage repairs/restorations. Be aware very old caps can dry out so be worthless. Also larger caps may need to be formed in. (slowly brought up to voltage with current limiting). I am not an expert about care and handling of antique caps so perhaps research "forming" or "reforming old caps".

My only experience with decades old cap inventory was while at Peavey, and I discarded those rather than risk investing more cost in reforming them, then using old parts that are known to deteriorate over time in new builds.

I probably have some caps in my back lab that are from the '70s.  I have seen dramatic improvement in capacitor technology and reliability over the decades since I bought those.

JR

PS: There is a bit of wisdom that rings true, Saying new is always better is just as ignorant as saying old is always better. In the case of electrolytic caps new may actually be better, and very likely different in marginal circuits, so YMMV.
 
I know that old electrolytics will "dry out", but is there a similar (or other) failure path for PIO caps, such as Vitamin Q's?  Just curious....

Bri

 
I seem to recall that Vitamin Q caps were in a metal can with perhaps glass (???) seals on the ends.  I have some around here that I need to dig out to refresh my memory.

Bri

 
Yes, the hermetic seal types do better with age.  The beeswax coated types and some plastic types are frequently bad. 
 
Well I have nowhere near the technical expertise of any of the above posters, so I can't speak on that front.

But I have made a couple of observations based on listening (earbservations?) with a couple of projects:

For gtr, I changed out the tone cap on my G&L ASAT years ago with a vitamin Q, definitely made a difference for the better. Sounds like it has a slight resonance thing going on with the filter which is nice. I can roll the tone pot all the way back and it has a very nice, very usable sound, not really the case before. There's about two or three really sweet spots to me that weren't there before.

On my PRR 176 i used a switch for the timing cap, goes between stock tantalum, solen fast, and russian pio, all the same value. Definitely a difference there as well. And the pio setting gets used more regularly than the others. They all sound good, just different in a subtle way. The tant seems to have kind of a midrange push, the solen sounds more hifi on the top and bottom, and the pio is kind of a mix of the other two, very rich, full response.

Not peddling snake oil, I've never done any real measurements and I'm sure anyone else could better explain what's happening technically. I do understand the concern with longevity on old caps, which is why I got a handful of them. But I have yet to have one go bad on me. lucky i guess.
 
Well, what I found were actually NOS Aerovox hermetic PIO caps....1.5 uF @ 400 VDC.  Date codes from 1962.

I still may have some Vitamin Q's somewhere.  My stuff was in a bit of disarray before I moved from Oklahoma to Kansas...and then everything got "shaken well"!  lol


Bri

 
Back to basics - take a few of each value and each manufacturer to someone who has a decent cap tester and check for value, ESR, and leakage.  You will likely find that some are still good (or great) while others have not passed the test of time.  I have seen bad batches of caps in 1950s gear and 1990s gear.  I have also seen caps that are still good in 1950s gear and 1990s gear!
 
mjrippe said:
Back to basics - take a few of each value and each manufacturer to someone who has a decent cap tester and check for value, ESR, and leakage.  You will likely find that some are still good (or great) while others have not passed the test of time.  I have seen bad batches of caps in 1950s gear and 1990s gear.  I have also seen caps that are still good in 1950s gear and 1990s gear!

Yeah..I have eleven (???) of these Aerovox PIO caps and plan on doing that.  Perhaps there is some "Ebay Fodder" in the Ziplock freezer baggie.....


Bri


 
One aspect of old paper in oil and similar caps is leakage. When used as an interstage coupling cap, it's conceivable that this leakage could change the bias of the stage with the leaky cap on its grid. So, while this is not the 'normal' function of a coupling cap, it's clear that this could happen, and cause some of the audible effects that are outside of the normal scope of what a dielectric can do to a circuit's sound.

In my mind, this effect could be what makes certain instances of amplifiers 'interesting' or 'unusual in a good way' but it's really hard to control using old parts. A more honest approach would be to simply parallel a high valued resistor across the interstage cap so that the leakage can be adjusted and perhaps optimized. Thoughts?
 
Monte McGuire said:
One aspect of old paper in oil and similar caps is leakage.

Monte,

I remember you from the old R.A.P. days.

I seem to remember you hooking me up with some Nuvistor tubes for my pair of AKG mics.

Welcome, good to see you again.

Mark Plancke

 
tryin to avoid starting a new topic,
does anybody have experience 'reforming' pio caps?--if thats even what is going on here

I have had trouble with the couple USSR "Paper'n oil" in a (anode dc blocking/output) mic build 5 years ago—they seemed leaky , anode voltage way down,no output , dont remember details but a solen worked fine.

I picked up a sprague tel-o-mike cap tester from 1950s at a swap meet a few weeks back, it
is old but seems to work. With newer and nos caps of all types, leakage seems to be fairly accurate and capacitance is right on with labeling— fluke multimeter is a little off printed value. The sprague tester applies dc values up to 600v to test leakage.
Question:
While every russi a pio (green and silver bodies) fails capacitance test (bridge or wheatstone bridge i think)on sprague tester, they are within spec on fluke dmm ...
Leakage/insulation test is also a fail on every ussr cap tested—at least at first:

It seems if i let them sit for some minutes 'soaking' they build up a charge/stop leaking. But capacitance reading is never right.
While I don’t have complete faith in the meter Other types test as they should.

Am I observing the dielectric reforming?
Is this due to the dielectric/construction type?
 
Don't forget dielectric absorption, which is akin to memory effect. A fully discharged cap, left sitting, ultimately reacquires some charge. Organic dielectrics, like PIO, are more prone than are the most prone to that effect.
 
I have been quite interested for some time in non/low inductive caps.

Arnt the paper caps in this category?

And the oil impregnation, isnt that used to prevent vibrations which can modulate the capacitance..?
 
shabtek said:
tryin to avoid starting a new topic,
does anybody have experience 'reforming' pio caps?--if thats even what is going on here

I have had trouble with the couple USSR "Paper'n oil" in a (anode dc blocking/output) mic build 5 years ago—they seemed leaky , anode voltage way down,no output , dont remember details but a solen worked fine.
....

Am I observing the dielectric reforming?
Is this due to the dielectric/construction type?

There’s another thread from maybe 2018 where I go into detail, but yes, I’ve reformed/attempted to reform many electrolytics and PIOs. My experience with electrolytics has been fairly successful, with a correlation between brand/type and successful reforming. Sangamos for some reason almost never reform successfully, while Mallory, Sprague and Aerovox in the metal cans often reform well as long as they aren’t defective.

As for PIOs, the success rate is a little higher for hermetically sealed ones like Westcap, Sprague Vitamin Qs, Goodalls and  their ilk. The higher quality russian K-40 types (hermetically sealed, silver body) reform about as well as these. But the lower end K-42Us and K-75-10s (green bodies) are a completely mixed bag. Even when I see significant improvement in capacitance and ESR, the leakage is still high, which makes me hesitant to use them.  I think I’ve maybe ended up with a 20% success rate with those. I have used the green ones that ended up with satisfactory measurements in projects, and they are still working well.

I stopped buying any of the green ones a couple of years ago because of the low quality and low success rate with reforming.

my 2 cents anyway.
 
5v333 said:
I have been quite interested for some time in non/low inductive caps.
It's a matter of construction, not of dielectric. PIO can be of both types.

And the oil impregnation, isnt that used to prevent vibrations which can modulate the capacitance..?
Not only. Its main purpose is actually to increase the permittivity, which in turn allows making the cap smaller for a given value, and increases the dielectric strength, so the cap has a higher voltage rating.
 

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