Hi all,
I have recently completed a new amp and have used some vintage paper in oil caps for the outputs. One is showing a bit of DC leakage (I had it connected up to a friends DC coupled solid state amp and it did some interesting things to his speaker cones).
I can just about live with it because my main amp has input transformers, but obviously I would like to sort it out. Unfortunately I haven't got any spares.
I understand that PIO are self healing, but I have been running these biased at about 180V for over a week and things haven't changed. There is absolutely no sign of leakage from the metal cans.
Does anyone have any ideas as to how to assist them to heal. I thought the Oil might have become a little stiff over time so was thinking that baking them in an oven for a bit might re-liquidize them and fill out the short. What does anyone think.
Shoog
I have recently completed a new amp and have used some vintage paper in oil caps for the outputs. One is showing a bit of DC leakage (I had it connected up to a friends DC coupled solid state amp and it did some interesting things to his speaker cones).
I can just about live with it because my main amp has input transformers, but obviously I would like to sort it out. Unfortunately I haven't got any spares.
I understand that PIO are self healing, but I have been running these biased at about 180V for over a week and things haven't changed. There is absolutely no sign of leakage from the metal cans.
Does anyone have any ideas as to how to assist them to heal. I thought the Oil might have become a little stiff over time so was thinking that baking them in an oven for a bit might re-liquidize them and fill out the short. What does anyone think.
Shoog
Don't bake them as this will make all the oil wax
the only thing i can think of which may help is on the leak tl12 amp they have a cap block ie. multi caps soaked in a low viscosity car engine oil (by low viscosity i mean runny) so the electrolyte should hopefully be something of the like
may be vacuum the cap and feed in some oil but then you would need to reseal them afterwards.
the only thing i can think of which may help is on the leak tl12 amp they have a cap block ie. multi caps soaked in a low viscosity car engine oil (by low viscosity i mean runny) so the electrolyte should hopefully be something of the like
may be vacuum the cap and feed in some oil but then you would need to reseal them afterwards.
The term self-healing is a much exaggerated phonemena that applies only to the latest capacitors. And then is limited to minor overloads of certain particular types. When an insulation is punctured by arc over, it is damaged and has a permanent weak spot. I'm of the strong opinion that "vintage" capacitors, especially the old paper jobs, are useless once they become leaky. The internal paper insulation has aged out and they should be replaced. Salvage is futile.
I also fail to fully understand the love affair with PIOs. Paper has the worst dielectric properties of any insulation material used today. It's absorption rating is high and it's release rate is poor making it a comparatively slow capacitor. Musically this translates to poor high frequency response producing a rolled off characteristic sound. Is this what people like? I would think the better plastics or teflon dielectrics should perform better, especially where transformers are in the signal path. But to each their own ears I guess.
I also fail to fully understand the love affair with PIOs. Paper has the worst dielectric properties of any insulation material used today. It's absorption rating is high and it's release rate is poor making it a comparatively slow capacitor. Musically this translates to poor high frequency response producing a rolled off characteristic sound. Is this what people like? I would think the better plastics or teflon dielectrics should perform better, especially where transformers are in the signal path. But to each their own ears I guess.
Interestingly its exactly that sound which is quite appealing. Having said that, run solo they do have a rolled off sound. I had them bypassed with some 500pf silver mica caps and that made things a bit zingy. I now have them bypassed with some plastic caps which a friend gave me. My prefered option is PIO bypassed with polystyrene which seems to give a very neutral sound overall.
Its effecting the input transformers on my Headphone amp so it will have to be sorted. Unfortunately the case is drilled and mounted so cosmetically ditching them will be a disaster.
Shoog
HollowState said:
It's a strange thing -- people who don't like PIO caps and thus who don't use them often say the reason for this is that the highs are rolled off. However, people who do like them often note the reason for this is due to the the sparkle and extension in the high frequencies.
My experience is that Vitamin Q's have good high frequency response (as do the Jupiters -- the other cap you see the same dichotomy about). The Russian stuff does too, though I dislike them for other reasons. On the other hand, many of the audiophile film caps have comparatively terrible high frequency response.
As for repairing toxic leaky antique caps? Dispose of them, but please do so in a responsible way.
dsavitsk said:
Yo Dave,
While it's true that I don't use PIOs very much, I can't really say that I don't like them. I have mixed opinions regarding them. But here is an example of why I said what I did.
About thirty years ago I built an Ampex 351 recorder from scratch. (Well almost as I bought motors and heads.) I couldn't afford even a used unit, but I was working in a machine shop and had access to the equipment. The electronics used all Sprague capacitors that were oil and paper like the originals. Resistors were carbon comp. I still have the working machine.
Years after that I got to reading how plastic capacitors were much better for audio from the Walter Jung artical in Audio Magazine. So I decided to replace some capacitors to improve the sound. After replacing most of the coupling caps with polyesters, I was shocked at what happened. The high frequency response was peaked so much that the playback compensation adjustment had to be set full over to make it reproduce correctly. And the record preemphasis was similarly too high. All because of going from paper to plastic capacitors in the signal path.
Sound crazy? Not really, because other parts of the circuit were designed for the response of the slower paper caps. They can't be that bad because all of those old Mercury Living Presence and RCA Living Stereo recordings that audiophiles swoon over were made on those vintage Ampex machines.
I had them and like the sound when appropriately bypassed. I stupidly forgot to check their integity before I installed them on the top of the case. It is no big deal to replace them with some 4.7uf MKP's I have, but the case is riddled with holes from them. I did have spares but I gave them to a friend.
I will probably mount the MKP's under the case and leave the PIO on for show. Lessons learned - check your caps.
Shoog
I will probably mount the MKP's under the case and leave the PIO on for show. Lessons learned - check your caps.
Shoog
i've used vintage PIOs on lots of my projects. the ones i use are high grade stuff though. sprague vitamin Q or NOS military stuff. havent had a problem yet. I've read that PIO's can be the longest lived of all caps next to silver micas providing the seals aren't compromised.
to address the self healing comment...
self healing caps are NOT new. i have a QST (amateur radio) magazine from the early 1930's that has a full page ad for aerovox capacitors that self heal.
to address the self healing comment...
self healing caps are NOT new. i have a QST (amateur radio) magazine from the early 1930's that has a full page ad for aerovox capacitors that self heal.
Shoog said:
Make a case for the new caps and hide them in there. If your caps weren't PCB, I'd remove the guts and put a more compact modern cap inside. Maybe you can find a cheap non-pcb cap to serve as a container donor.
sonata149 said:
A simple way is to put a resistor in series with the cap, put DC voltage across the two, and measure the voltage across the resistor. Calculate the current by ohms law. You want to use at least the voltage the resistor will see in the circuit. You can use the circuit supply for this. Use a resistor value around 5/C. You can use less, but the resolution will be lower.
Here's a more detailed description of a similar method: http://www.angelfire.com/electronic/funwithtubes/Testing_caps.html
Sheldon
If you have a good cap to compare it with a simple method is just to check it with a digital multimeter set to the 2meg ohms scale. The l,eaky cap will take a lot longer to rise than the good cap.
So I replaced the oil caps with some big juicy 4.7uf MKP's. The overall tone is similar, but everything is a bit tighter sounding. At the moment they are just bolted to an available corner of the case and I will work out where they will end up when I have sorted out what I am going to do with the PIO caps. I was thinking of adding a VSPS on top of the case. I built one of these before and was really impressed. I am currently running a Tube phono but never really got it quiet enough for my liking. I could try the FVP5 phono stage which would partner with the SLCF preamp.
Cheers all.
Shoog
So I replaced the oil caps with some big juicy 4.7uf MKP's. The overall tone is similar, but everything is a bit tighter sounding. At the moment they are just bolted to an available corner of the case and I will work out where they will end up when I have sorted out what I am going to do with the PIO caps. I was thinking of adding a VSPS on top of the case. I built one of these before and was really impressed. I am currently running a Tube phono but never really got it quiet enough for my liking. I could try the FVP5 phono stage which would partner with the SLCF preamp.
Cheers all.
Shoog
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