Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Geezer on January 13, 2011, 09:14:06 am
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I have a bunch of old caps I have pulled from old equipment over the years & finally got around to testing them for leakage. Most are at least 50 years old......I was surprized in my findings.
*Sprague "Bumble Bees" > Almost EVERY SINGLE CAP was EXTREMELY leaky! I mean, almost total leakage of DC. Very few (2 or 3) out of ~~75 were usable.
(http://t3.gstatic.com/images?q=tbn:ANd9GcR4IMjTrvuumrF4LzmMMabgcvoGLoECmVx2WHegPC4JS7hRtKA1)
*Sprague "Black Beauties" (black body w/ red writing) > Almost EVERY SINGLE CAP was totally usable, with almost zero leakage! I threw away maybe 1 or 2 out of 100.
(http://t3.gstatic.com/images?q=tbn:ANd9GcRM_xGWQkcgy1Akd3sJc0MeSksiX-OBRb4llGg_zgTApCvqww-aRA)
I found it interesting, that products from the same mfg were so very different in long term stability!
*Blue Molded caps (like are found in old Fenders) > Again, almost every one was good.
(http://t0.gstatic.com/images?q=tbn:ANd9GcQLOPvu6I9aXMGltp3dKd3MLVBNWmhRBd8oSE7vMclPAIxb9mkx)
*Mica molded "Domino" caps > Again, almost every one was good. (these are very under-rated caps for "tone"....very smooth response, if you can find places to use them, as they are usually very small in capacitance, but very large in size)
(http://t1.gstatic.com/images?q=tbn:ANd9GcQgz0Jejc5QwdjsLL9B0ZL2byWurOuDh_14gVuJ9oud7mMr1LU8FQ)
*various "Ajax" & other waxey covered caps.....all were bad.
So, I cleaned out a bunch of useless crap, & have some very nice caps left that I have fairly good confidence in to last a very long time.
Anyone else have similar (or differing) experiences?
Geezer
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I have an old cap tester that uses a magic eye tube to indicate if a cap is leaky. I picked it up at a flea market years ago because I thought it was neat. I have never even tried it out.
I too have a huge stash of old caps, so you may have just motivated me to pull out that old tester to see if it works.
Dave
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Geezer, just curious. How did you test and what did you use to test those old caps for DC leakage? Punky
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Geezer, I sent you a PM
Dave
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Geezer, I have come up with the same results through the years.
Check eBay for those Black Beauties. They go for a pretty penny.
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Can leaky caps be used for a guitar's tone cap?
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Geezer, just curious. How did you test and what did you use to test those old caps for DC leakage? Punky
I clipped one end to B+ (V1 plates of an amp) & measured the DC present on the other end.
Can leaky caps be used for a guitar's tone cap?
That's a good question.....if the capacitance is within spec, does the caps ability to block DC matter in the case of bleeder-type cap circuits?
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Can leaky caps be used for a guitar's tone cap?
Sure they can: no DC, no leaks.
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Can leaky caps be used for a guitar's tone cap?
Sure they can: no DC, no leaks.
Well then, I guess I'll be digging out some of the caps to see if the .02's & .047's are within spec & puttin'em on ebay for the cork sniffers..... :angel
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Very interesting stuff. I think I understand how to test for leakage but how would I test for capacitance? Can a normal DMM do this or is a specific tester needed?
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My normal Fluke 87V can.
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Yeah, some models have capacitance testing on them, but others don't.
I have one meter that does, another that doesn't....I bought a cheapo cap tester for ~~$35usd & it works great.
Every time I use it, I always "calibrate" it (hah) for accuracy by testing a few new "known good" caps before starting on any old "unknowns".
Did a quick search.....this is the one I have>
http://www.mcmelectronics.com/product/72-8150&CAWELAID=220586396
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Well, I only have 2x .022's, but they have fairly long leads, are in good cosmetic shape & both test the same @.022 on the tester.....looks like there're good for at least $50 for the pair on ebay! :laugh:
G
edit: Well crap.....LP's use 400v rating (yellow band) and mine are 600v (blue band)....so I guess they worthless. :sad:
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Thanks Geezer, Here is another question. If you have a cap that is rated say 400 or 600vdc and it tests good for leakage at 200vdc, is it possible for the cap to leak at a higher voltage maybe 350vdc or higher? Punky
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Okay, so we're talking about testing the capacitance outside of a circuit then - not "under a load" right?
The cheap Harbor Freight meter pictured below is all I have at the moment but I think I'll need to get a better one soon.
Speaking of the leads, I have a lot of used capacitors and resistors that I think are usable but some have short leads since they were snipped out of different circuits. How do you guys feel about soldering new wire to them using them again? I know some would say it's not worth it for parts that are relatively cheap but sometimes it's just a matter of using what's around rather than wait for an order to arrive in the mail.
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How do you guys feel about soldering new wire to them using them again?
Personally, I wouldn't have a problem with that & have been know to do it!
If you have a cap that is rated say 400 or 600vdc and it tests good for leakage at 200vdc, is it possible for the cap to leak at a higher voltage maybe 350vdc or higher?
Others will have to answer that, but I tested all of these caps @ ~~340v.
G
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edit: Well crap.....LP's use 400v rating (yellow band) and mine are 600v (blue band)....so I guess they worthless. :sad:
I have a buddy that goes to ham fests and he was buying 1000v caps to resell on e-bay. Tone Quest Mag. also has talked about using 1000v caps for guitar tone controls. I think as long as they'll fit in the route cavity guys will buy them.
Brad :smiley:
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If you have a cap that is rated say 400 or 600vdc and it tests good for leakage at 200vdc, is it possible for the cap to leak at a higher voltage maybe 350vdc or higher?
Yes. Caps should be tested at nominal voltages.
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Thank you FYL.
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Well, I only have 2x .022's, but they have fairly long leads, are in good cosmetic shape & both test the same @.022 on the tester.....looks like there're good for at least $50 for the pair on ebay! :laugh:
G
edit: Well crap.....LP's use 400v rating (yellow band) and mine are 600v (blue band)....so I guess they worthless. :sad:
Yeah you poke fun, but I sold a bunch of bumble bees for an outrageous amount of money on ebay.
I would take a pic and give it a post there!
heck I sold a box of 6 NOS black beauties (red print)(found in the bottom of a tube caddy) for $80 and it sold faster than I could check the listing. My guess it was up for less than 45 seconds.
Ray
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I bought the more expensive Harbor Freight DMM ($19.99, but I got 20% off w/coupon). It's a pretty good meter & even measures temperature which is pretty cool. It does capacitance but apparently only up to 20mfd. It seemed very accurate up to 20mfd but I get no reading at all above. I guess I should have realized this since the highest setting says 20, I just thought it might measure higher. Now I'm trying to figure out if I should keep it or not because a lot of the caps I want to measure are >20mfd.
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I bought the more expensive Harbor Freight DMM ($19.99, but I got 20% off w/coupon). It's a pretty good meter & even measures temperature which is pretty cool. It does capacitance but apparently only up to 20mfd. It seemed very accurate up to 20mfd but I get no reading at all above. I guess I should have realized this since the highest setting says 20, I just thought it might measure higher. Now I'm trying to figure out if I should keep it or not because a lot of the caps I want to measure are >20mfd.
I've got the same one and use it only for it's capacitive function.
I seldom use caps higher than 20 uf. If it goes out of range and its rated higher than 20 I figgure it good.
I use it mostly for those pesky tantalum and mica's that is too small to read. or those dang O drops. more than once I've not moved over 6 places.
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I have no problem using a NOS cap or resistor in a build if it test good and I come up short on new parts to complete the build.
On used parts---my brain don't compute!!! When I finish a build I want to have a good warm fuzzy that if all the parts are up to par that the amp can probably be used for years before it will need service other than tubes. Used parts don't allow me to have a warm fuzzy---rather it's un-justified Illusion or not--when I finished my tweaking and close the chassis I wan't it to stay closed for a while and the so called old parts mojo could quickly end in blowjo. Platefire
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Personally, I would not test for leakage with a meter.
I would feed some high voltage DC to a cap on a breadboard and see how much DC leaks through the cap.
Just like in a real amp, except leave the non high voltage end hanging in the breeze so there is no drain off and you can get a good measurement.
Remember, voltage can be compared to water pressure.
Just a wild ass assumption on my part, but just like in water connections, the higher the pressure, the more a connection is bound to leak.
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That's how I test for leakage.
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blowjo
my new favorite word
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Personally, I would not test for leakage with a meter.
I would feed some high voltage DC to a cap on a breadboard and see how much DC leaks through the cap.
Just like in a real amp, except leave the non high voltage end hanging in the breeze so there is no drain off and you can get a good measurement.
Remember, voltage can be compared to water pressure.
Just a wild ass assumption on my part, but just like in water connections, the higher the pressure, the more a connection is bound to leak.
Yes, that's also how I check for leakage (as stated in an above post).....I only use a cap meter to test to make sure the capcitance is still within spec, or usable as another value if it has drifted from the original value. For leakage, I tap into the B+ & measure for DC on the other end of the cap.
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Sounds like your test caps are electrolytic? This is not a good test for electrolytics. Try it on small coupling caps.
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EDIT: I THINK SLUCKEY'S POST ABOVE WAS IN RESPONSE TO THIS POST WHICH I DELETED AND AM NOW RE-POSTING. I REALIZED CONNECTING TO CHASSIS GROUND IS NOT THE RIGHT WAY TO DO THIS. I THINK THE TEST PROBES NEED TO BE ON OPPOSITE SIDES OF THE CAPS BEING TESTED. DOING THIS SHOWS NO LEAKAGE ON THE CAPS. THESE ARE LARGE ELECTROLYTICS.
I must be doing something wrong. I measured 6 used caps and every one of them leaks DC. I know they're used but this is strange, right?
Here's how I tested:
The amp pictured below is my Fender Pro Jr. The point labeled as "CP1" where the blue wire from the OT connects gave me a reading of 324 VDC with one lead of my meter connected to it and one lead connected to chassis ground.
From this point I clipped a lead to the positive side of a capacitor. I connected the other leg of the capacitor to the red test probe of the meter. The black test probe of the meter was connected to chassis ground. The meter was in the 1000 DCV setting.
Each capacitor read close to 318 VDC initially then moved slowly down. I let one of them go all the way down to around 60 VDC but it took about 10-15 minutes.
What am I doing wrong here, or all they really all leaky?
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THIS ARE LARGE ELECTROLYTICS
Your procedure is fine, but it doesn't work for electrolytics. Use that test for small coupling caps only.
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THIS ARE LARGE ELECTROLYTICS
Your procedure is fine, but it doesn't work for electrolytics. Use that test for small coupling caps only.
Okay, but the other way (test probe on either side of the cap) does work for electrolytics right? With this method 3 of my 6 electrolytics tested bad. They had readings from 2-7 VDC. One of them started at around .5 then kept going up until I took it out.
The good ones were in the .2-.5 VDC range. This is fine right?
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A good procedure for electrolytics would be to put an amp meter in series with the e-cap. Then connect the negative meter lead to the negative side of a voltage source (chassis ground in your case). Connect the positive meter lead to the negative e-cap lead. Connect the positive e-cap lead to the positive voltage source (CP1). CAUTION! BE SURE THE VOLTAGE SOURCE DOES NOT EXCEED THE E-CAP VOLTAGE RATING. An e-cap will show current flow which will decrease as the cap charges. A perfect e-cap will show zero current when the cap becomes fully charged. I don't think there are any perfect e-caps, so you will always have some minimum current reading even after the cap has fully charged. This is leakage current. Low values are acceptable. High values are not.
Just how much leakage is acceptable depends on the application. For guitar amps, I would not be concerned with any particular number. I'd look at two things...
1. Is the cap body warmer than the ambient air temp, ie, warm to the touch? Shouldn't be. Warm indicates leakage. Replace it.
2. Is the B+ voltage low at the cap and there's no other apparent load on the cap causing the voltage to be low? The cap is probably leaking too much. It may even be warm. Replace it.
You can use an analog ohmmeter to find many leaky e-caps, but this method is not fool proof and requires some hands on experience to develop a 'feel' for it. Each ohmmeter will behave slightly different too. First, discharge the test cap. Then set your meter on a high resistance scale and connect the meter leads across a cap, observing polarity. The needle will quickly move toward zero ohms on the scale and then slowly move back toward infinity. (Big e-caps rarely make it all the way back to infinity) The needle will usually stop moving at some high resistance reading for an acceptable e-cap. Continue watching the needle. If it begins creeping back toward zero, the cap leakage is increasing and the cap should be trashed.
I don't keep any used e-caps. I don't even fully trust brand new e-caps after they have been stored for a few years.
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Thanks for all the info. I don't get it yet but I'm working on it.
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The guys were originally talking about testing small caps for leakage. You know, the kind that might be used as a coupling cap from the plate of one tube to the grid of another tube. In the case of coupling caps, zero tolerance is desirable for leakage because even a small voltage 'leaking' thru a coupling cap can mess up the bias voltage on the grid of the next stage. 10 volts leakage would probably give a very distorted sound in a preamp tube and might even melt down some expensive power tubes.
However, 10 volts 'leaking' thru a B+ rail filter cap may not exhibit any ill side effects. It could be very normal.
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Yes, as Steve said.
I was talking about coupling caps, not electrolytics.
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Okay, I will test my coupling caps that way. What I don't get is the electrolytic thing. Probably not the smartest thing to do but I think I'll just try them in the amp.
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If a coupling cap is in circuit (say from B+ of a plate to grid of the next stage) what happens that prevents a person from measuring the cap for leakage. If a cap is leaking 2VDC, won't you be able to see that on the grid.?
Thank You
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If a cap is leaking 2VDC, won't you be able to see that on the grid.?
Maybe. Depends on the particular grid circuit. If the grid should be normally running with zero volts and you measure 2 volts, then a leaky coupling cap or a bad tube would be prime suspects. So, you pull the tube but the voltage is still there. The next step would be to disconnect the cap lead that connects to the grid and measure the voltage on the dangling cap lead. If got voltage, replace the cap. If not, look elsewhere.
But, what if the cap is coupling to a tube grid that operates with a voltage on it? Maybe a 6L6 that normally has -50vdc on the grid. But the cap is leaking positive 2 vdc. So, your meter reads -48vdc. Would you suspect a problem? It's not so obvious now. Or what if the coupling cap feeds a LTP PI grid. That grid operates with a fairly big positive voltage, but because of the very high circuit impedance due to the bootstrap bias arrangement, you can't accurately measure the grid voltage because your meter will load the grid circuit. How would you ever know that the cap was leaking?
The only sure fire way to know is to leave the cap connected to the plate but disconnect the cap from the grid and measure the voltage on the flying cap lead.
I've been talking as if a cap will leak a certain amount of voltage, but the truth is that a cap may 'leak' 2 volts in one circuit but will 'leak' 10 volts in another circuit. The cap actually becomes resistive and the voltage 'leaked' on thru to the other side will depend on the resistances in the grid circuit.
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sluckey.....
Well said.
Thank you, I appreciate the explanation.
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Some time ago I got a big lot of old capacitors. I tested all of them with my heathkit it-11 and got similar results as you did Geezer.
However I wanted to say something about black beauties.
I got lots of these capacitors, some of them like the one in your pictures, and some other with some kind of small metal ball at the beginning of one of its leads (probably indicating the outer foil). The first ones were all good (no leakage at his max voltage rating), while the latter were all leaky even at low voltages.
Maybe some of you find this useful. I would avoid using these capacitors (the ones with the metal ball) if not able to test for leakage them first.
Hope you are all good!!
Regards,
Francisco Pérez
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If a cap is leaking 2VDC, won't you be able to see that on the grid.?
My experience has shown that if you are reading 2vdc in a common grid circuit, the voltage is much higher than that in real life.
The grid is tied to ground through a resistor, so you won't get a true reading.
I have info about this somewhere on the library page
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My experience has shown that if you are reading 2vdc in a common grid circuit, the voltage is much higher than that in real life.
The grid is tied to ground through a resistor, so you won't get a true reading.
Same experience here. Once you lift the cap lead from the grid circuit, the voltage measured on the flying cap lead is almost always much more when using a modern high impedance meter. That's because now rather than a typical 500K or 1Meg grid resistor, all you have is the very high resistance of the meter input.