Using capacitors that have drifted...

[tubedood]

Hi again,
just a quick question regarding my old stash of capacitors... Years ago I was having fun with some amp projects and I accumulated a bunch of old capacitors... The orange drop and some other similar drop types have aged well but the Sprague Black Beauties have all drifted in value, almost exactly the same amount. What are the general thoughts on using a cap that has drifted for a tone control in my Tweed Princeton project? The original value was .0022, the drifted value is .0047 which is what I was going to use for the tone control. The majority of them were printed as .0047 but are now .007ish!. Not being cheap just curious if a drifted cap can still have a purpose in its life hahaa.
Dale

[Chuck H]

I've honestly never heard of a film cap drifting. Have these caps been in use elsewhere? Anyway... I wouldn't use them because without more extensive testing we don't know what else may be wrong with them.

[Leo_Gnardo]

I've honestly never heard of a film cap drifting. Have these caps been in use elsewhere? Anyway... I wouldn't use them because without more extensive testing we don't know what else may be wrong with them.

Drifting measured value, yes I've seen that, mostly because some capacitance meters react to a very small leakage current by indicating a cap value well in excess of its rating. If the circuit isn't detrimentally affected by such a leakage, then you may be able to use those old drifted caps. If it's an application that will upset a tube's operation due to DC leakage, it's wise to avoid any questionable, possibly leaky cap.

[Malcolm Irving]

Yes. Places where a leaky cap shouldn't be used are where they have to block DC, e.g. inter-stage coupling, non-input grid to ground in a LTP PI, etc. Some places where a bit of leaking wont hurt are bright cap on a vol. pot, cathode resistor bypass, and caps in a tone stack which are in shunt or have another cap in series with them which can do the DC blocking.

[tubedood]

In the Princeton Tweed build I'm tinkering around trying the tone cap in question is right after a .022 that is a new modern capacitor so its function is just tone control, no high voltage on it. The Black Beauty caps are all new but been in drawers for a looong time, just Father Time had his way with them I guess! I will keep safety in mind and get a new tone cap!


Dale

[tubedood]

Does a "leaky cap" mean its actually oozing? These caps look to be in perfect shape... long new leads and the barrel I guess its called looks brand new with no oils or stains of any to be seen. Maybe I will try one in the circuit and compare it to a new modern one to see if it responds the same! I was wondering if the new value it has drifted to performs as if it was originally that value from day 1? Does a cap that has drifted from .0022 to .0047 sound like it was originally a .0047?

[Helmholtz]

Hi again,
just a quick question regarding my old stash of capacitors... Years ago I was having fun with some amp projects and I accumulated a bunch of old capacitors... The orange drop and some other similar drop types have aged well but the Sprague Black Beauties have all drifted in value, almost exactly the same amount. What are the general thoughts on using a cap that has drifted for a tone control in my Tweed Princeton project? The original value was .0022, the drifted value is .0047 which is what I was going to use for the tone control. The majority of them were printed as .0047 but are now .007ish!. Not being cheap just curious if a drifted cap can still have a purpose in its life hahaa.
Dale

A strong increase of capacitance typically means that the sealing is no longer intact and that moisture has crept in. Water has a very high dielectric constant of 80 (capacitor film materials having a value of around 3), which increases capacitance. I am familiar with this effect from bumble bee caps and Fender Astrons. All developed leakage from the resulting water content. I would only use them as tone caps in guitars.

[SoulFetish]

Hi again,
just a quick question regarding my old stash of capacitors... Years ago I was having fun with some amp projects and I accumulated a bunch of old capacitors... The orange drop and some other similar drop types have aged well but the Sprague Black Beauties have all drifted in value, almost exactly the same amount. What are the general thoughts on using a cap that has drifted for a tone control in my Tweed Princeton project? The original value was .0022, the drifted value is .0047 which is what I was going to use for the tone control. The majority of them were printed as .0047 but are now .007ish!. Not being cheap just curious if a drifted cap can still have a purpose in its life hahaa.
Dale

Doubled in value?? I would be more worried about leakage and/or failure. Personally, I would never use a cap that drifted that much.
But, I suppose if you could test the cap under high voltage conditions and verify the insulation holds up,... then maybe?
Even then, I would be suspicious that the epoxy casing had been compromised and moisture got in. I would question the reliability under the heat conditions commonly found in tube amp chassis. Respectfully, caps with sexy names don't mean shit. I would trash/recycle them or use them in low voltage non critical applications. But, IMO, no cap worth a damn has a tolerance of 100%

[SoulFetish]

Does a "leaky cap" mean its actually oozing?

Sometimes, it does. For instance, if someone were talking about electrolytic caps. But, mostly people are referring to a capacitor "leaking" DC voltages (which it should not).

[nickb]

I have just one thing to say: If in doubt throw it out.

You risk spending many miserable days tracking down obscure faults caused by bad caps. Don't risk going down that path.

[tubedood]

Cool everyone thanks... I wont risk their use in an amplifier. Maybe someone can use them in a guitar tone control. I'm no "cork sniffer" either regarding caps SoulFetish hahaa . An old friend was a navy ET that helped with radar development back in the 50s I believe! He had a an electronics shop and I bought a few of them way back in the day. What he had in the bins and drawers was what was available when he stocked it. Shame they didn't age so well!

[Chuck H]

I have just one thing to say: If in doubt throw it out.

You risk spending many miserable days tracking down obscure faults caused by bad caps. Don't risk going down that path.

Someone give this guy another thumbs up for me since I can only give one!

[tubedood]

Got him covered Chuck!

Thanks to all of you guys for your advice. I will start a build thread once things get figured out in more depth!

[gmaslin]

Does a "leaky cap" mean its actually oozing?

No. The leaking described is current through the capacitor. An ideal capacitor doesn't allow current through it but we all know nothing is ideal. As the cap dielectric breaks down, more an more current gets through it and then the cap stops being useful for its original purpose.

[J M Fahey]

Never ever saw film caps, even cheesy paper in oil ones drifting in value.
Out of tolerance? Yes, any day of the week.
And they stubbornly hold their "wrong" value forever, because they were "made" different from day 1, not that they "became" different.

But here the difference is so gross that Iīm thinking poor measurement instead.

DC leak which is equivalent to a resistor in parallel with the cap *might* explain a larger than expected *displayed* value, since "capacitance meters" typically apply a squarewave to the cap in series with a resistor, rectify AC going through and display the DC value as Capacitance (since higher DC means higher AC through so higher capacitance) BUT if a cap shows gross leak, high enough to double value with a mere 1 V RMS applied to it by the meter then that capacitor qill explode even with mere 9 or 12V DC applied.

But I donīt think thatīs the case, more likely the meter is grossly miscalibrated, out of batteries or cap is measured in circuit where other elements can affect measurement.