Leaky Presence Cap?

[Roaddogma]

During restoration of a Fender 5F6A Bassman I found the original Astron .1uf presence capacitor to leak 15uA DC @300V. Since the capacitor is part of an RC filter shunt to ground, I wouldn't expect there to be any ill effect from some DC leakage here. Thus, I am inclined to leave the original cap in place. Can someone let me know if there are any flaws in my thought process?

The coupling caps are getting changed out for Jupiter yellows, so it's not an issue to change it. I just thought it might be preferable to leave a few of the Astrons in less critical locations for the sake of keeping things more original. Maybe that doesn't matter so much when only a few are left in? The bass cap is fine, as are the ones on the stand-by and ground switches. Thoughts?

[maxkracht]

Since the capacitor is part of an RC filter shunt to ground, I wouldn't expect there to be any ill effect from some DC leakage here. Thus, I am inclined to leave the original cap in place. Can someone let me know if there are any flaws in my thought process?

I have left these in without noticeable issues. I'm guessing there would be an audible change in performance if/when the cap degrades further, but I wouldn't worry too much about it. My understanding is the failures in yellow astrons are just as much from environmental issues and plastic degradation as use. So, they might get leaky sitting on a shelf.

The bass cap is fine, as are the ones on the stand-by and ground switches. Thoughts?

Cap on the ground switch, colloquially known as the "death cap" should probably be taken out of circuit or replaced with a class Y safety capacitor.

Cap on the standby I'm not sure about. I don't know what the odds are of one failing short.

[martin manning]

The presence cap doesn’t have much voltage across it, maybe 10V DC and 10 VAC on top of that.

[Roaddogma]

I have left these in without noticeable issues. I'm guessing there would be an audible change in performance if/when the cap degrades further, but I wouldn't worry too much about it. My understanding is the failures in yellow astrons are just as much from environmental issues and plastic degradation as use. So, they might get leaky sitting on a shelf.

It has been my understanding that yellow Astrons suffer from environmental conditions, as well, except that it is the paper in the capacitor that degrades, which is obviously not as durable as plastic film. So you are saying that you have left leaky capacitors in this position without issue?

Cap on the ground switch, colloquially known as the "death cap" should probably be taken out of circuit or replaced with a class Y safety capacitor.

My plan was to leave the "death cap" in place and to bypass the ground switch altogether using a 3 prong AC cord.

Thanks for the comments!

[Roaddogma]

The presence cap doesn’t have much voltage across it, maybe 10V DC and 10 VAC on top of that.

Granted lower voltage potential across the cap should result in less DC passing through, but my logic had more to do with the idea that leaking a bit of DC to ground here should not be a problem. Do you have any thoughts around this?

[maxkracht]

It has been my understanding that yellow Astrons suffer from environmental conditions, as well, except that it is the paper in the capacitor that degrades, which is obviously not as durable as plastic film. So you are saying that you have left leaky capacitors in this position without issue?

I thought they were Mylar? My understanding was a loss of seal from the plastic case caused moisture ingress or something like that, but I'm just going on random things I've heard from the internet, so not too reliable... Yes, I have kept slightly leaky caps in the presence and tone cap positions that don't need to block dc. I wouldn't risk it on anything mission critical, and only when someone is concerned about keeping everything they can original. I don't think there is a tonal benefit to keeping them, also don't think there is much harm.

[martin manning]

The presence cap doesn’t have much voltage across it, maybe 10V DC and 10 VAC on top of that.

Granted lower voltage potential across the cap should result in less DC passing through, but my logic had more to do with the idea that leaking a bit of DC to ground here should not be a problem. Do you have any thoughts around this?

I would agree. It’s not feeding a grid where leakage would upset the bias point, and it’s not enough to alter the effective PI tail resistance. 15 uA at 300V is 20M parallel resistance.

[Roaddogma]

I thought they were Mylar? My understanding was a loss of seal from the plastic case caused moisture ingress or something like that, but I'm just going on random things I've heard from the internet, so not too reliable... Yes, I have kept slightly leaky caps in the presence and tone cap positions that don't need to block dc. I wouldn't risk it on anything mission critical, and only when someone is concerned about keeping everything they can original. I don't think there is a tonal benefit to keeping them, also don't think there is much harm.

I have been pretty determined to learn as much as I could about these capacitors, so I could replace them with something close if needed. It took some digging, but I finally ran across some info that seemed reliable. I even found an old product info sheet from Astron. It seems that the original Blue Points were paper in oil, then switched to a paper/mylar DiFilm around 1960. Someone put up a pretty cool list of vintage capacitors that he cut open to identify the dielectric and gave some commentary regarding his experience in terms of their reliability. http://crasno.ca/articles/capdielectrics.htm

My goal has been to keep everything as original as practical. The original speakers are gone so I'm not going to get to crazy. If I wanted to be persnickety regarding tone I would replace the problem caps with some Jupiter PIOs, but the Yellows win on cosmetics and price. At least they are mylar and foil which should be closer than a metalized film.

I'm not really sure if 15uA at 300 VDC counts as "slightly" leaky, but I have seen worse. I have just gone by the rule of anything over 1uA should be considered leaky enough to not be useful at blocking DC. If I was keeping the amp for myself I would just replace all of them, but I'll be looking to sell it soon, and figure buyers would prefer to see as more original components.

[Roaddogma]

I would agree. It’s not feeding a grid where leakage would upset the bias point, and it’s not enough to alter the effective PI tail resistance. 15 uA at 300V is 20M parallel resistance.

Thanks that was pretty much my logic on both points.