Paper In Oil Caps: Replace On Sight Or Keep?

[cecilk]

I'm working on a cool old tube guitar amp from the 50's that uses paper in oil caps (silver ones labeled "JCP-C-Oil" and yellow ones labeled "NCC").
(The circuit is similar to a Vibro Champ manufactured in Japan for the "Noble" brand)

The amp exhibits gnarly signal distortion when I run a clean sine wave through it - here is the signal at output on volume ~1 and tone at noon:



My initial thought was that this could be due to DC voltage leakage through the PIO coupling caps.

I tested one 0.05 μf PIO cap out of circuit and it measured dead on at 0.04975 μf at 0.3% Vloss.

However, I also measured it with an old RC tester and measured 9.8 μA of leakage at 400v (its stated rating). From what I've read, leakage on coupling caps is never good and any measurable leakage is grounds for replacement.

There are about 14-15 of these PIO caps in the circuit. I was planning to replace them all with orange drops - but owner's musician friends got him concerned that he will "lose the vintage tone" if he does.

What is your rule of thumb with PIO caps? How much leakage is too much?

 

[gadget73]

The Japan ones go in the trash. Old US ones with glass end seals are usually OK and deserve a test to confirm.

 

[thorpej]

One need not be concerned with losing the vintage tone. I've built vintage-clone guitar amps of various ilk using modern MPW polypropylene film caps and they sound just the way they're supposed to. I honestly don't get the obsession with vintage caps.

 

[FlaCharlie]

Any chance you could post pics of them? The gray PIO caps commonly found in Japanese gear have a reputation for being leaky and unreliable. Yours may be from a different manufacturer.

I would measure them and base my decision on that. I've never seen leakage measured in terms of amperage, only voltage. Perhaps your measurement is 9.8 micro volts?? Or 9.8 millivolts?? They probably measure even better at their actual working voltage.

Frankly, I'd be surprised if your prospective orange drop replacement measured better. I have a vintage Eico cap checker but it doesn't give an numerical measurement of the amount of leakage. I've never seen a coupling cap (even new ones) that doesn't measure at least a tiny bit of leakage when measured in circuit using a multimeter.

 

[maxhifi]

I would say repair the amp first, then start changing caps. You should be able to track down the source of the distortion using a scope and signal generator, plus a multi meter.

 

[thorpej]

Any chance you could post pics of them? The gray PIO caps commonly found in Japanese gear have a reputation for being leaky and unreliable. Yours may be from a different manufacturer.

I would measure them and base my decision on that. Frankly, 9.8 micro amps seems extremely low to me. I wouldn't even worry about 9.8 mA! They probably measure even better at their actual working voltage. Frankly, I'd be surprised if your prospective orange drop replacement measured better. I have a vintage Eico cap checker but it doesn't give an actual measurement of the amount of leakage. I've never seen a coupling cap (even new ones) that doesn't measure at least a tiny bit of leakage when measured in circuit using a multimeter.

Um, I'd worry about 9.8mA of leakage! And I imagine in a Fender-type circuit, those caps are, in fact, working close to their voltage rating.

I would say repair the amp first, then start changing caps. You should be able to track down the source of the distortion using a scope and signal generator, plus a multi meter.

Leaky coupling caps are going to mess with the operating points of the downstream stages, so changing out the caps for new ones is, in fact, part of repairing the amp, seems to me.

 

[FlaCharlie]

Um, I'd worry about 9.8mA of leakage! And I imagine in a Fender-type circuit, those caps are, in fact, working close to their voltage rating.

I edited my post because I'm confused about the measurement being in amperage, not voltage. I've always seen leakage measured in mV DC. So maybe amperage leakage is different?? However, even if it is, he claimed his reading was in micro amps, not milli-amps. Wouldn't 9.8 mA = 98 micro amps? Conversely, 9.8 micro amps would be .98 mA, right?

 

[thorpej]

I edited my post because I'm confused about the measurement being in amperage, not voltage. I've always seen leakage measured in mV DC. So maybe amperage leakage is different?? However, even if it is, he claimed his reading was in micro amps, not milli-amps. Wouldn't 9.8 mA = 98 micro amps? Conversely, 9.8 micro amps would be .98 mA, right?

In an RC coupling circuit, leakage current from the cap causes a voltage drop across the grid leak resistor of the subsequent stage. Because the grid leak pulls the grid down to ground (0V), any leakage current flowing across it creates positive voltage at the grid.

 

[maxhifi]

Leaky coupling caps are going to mess with the operating points of the downstream stages, so changing out the caps for new ones is, in fact, part of repairing the amp, seems to me.

Agree, but the specific leaky caps and/or other bad parts
could be identified with voltage checks And signal tracing, rather than shot gunning of replacement parts. Then a decision can be made to leave or replace the other ones which aren't affecting circuit operation. Every tech has his own approach to fixing things, I'm just sharing mine.

 

[FlaCharlie]

In an RC coupling circuit, leakage current from the cap causes a voltage drop across the grid leak resistor of the subsequent stage. Because the grid leak pulls the grid down to ground (0V), any leakage current flowing across it creates positive voltage at the grid.

Thanks for the explanation, my knowledge of theory is limited.

But in practical terms, while not desirable, would a leakage of 9.8 micro amps (less than 1 mA) be a concern? Is that unusually high? I've never tried to measure grid current like that. If the grid voltage, when measured from the cathode, is in spec, wouldn't it be OK?

 

[tube-a-lou]

Get a few Sprague oil caps I have a few in my Fisher amp and they sound really good, I know how you
feel about vintage guitar amps, the original the better which is great if the caps are good but when they
get old and leaky it doesn't do the amp good. The guitar world is different everything original stay's look
at this when you have a 1950's strat that has the original rubber tubing in it that holds the pickups and
someone takes it out and sell's it on ebay for $50.00 dollars that's a little extreme.

 

[maxhifi]

Thanks for the explanation, my knowledge of theory is limited.

But in practical terms, while not desirable, would a leakage of 9.8 micro amps (less than 1 mA) be a concern? Is that unusually high? I've never tried to measure grid current like that. If the grid voltage, when measured from the cathode, is in spec, wouldn't it be OK?

9.8uA would be enough to shift the bias 0.98V across a 100k resistor, and 4.6V across a 470k resistor. Although 9.8uA sounds tiny, it is enough leakage to totally throw off the operating points of an amplifier

 

[cecilk]

Any chance you could post pics of them?

Here are some shots of the silver caps and the yellow caps:

 

[cecilk]

One need not be concerned with losing the vintage tone.

I'm with you on that count - but it's not my amp so I must take the owner's perspective into consideration.

 

[gadget73]

Those don't look like the usual crappy Japan caps. The usual ones have grey shrink wrap and a rubber end seal. If those have a glass end seal, they may be OK. The leakage would concern me though, especially if its going to an output tube. In some circuit locations it may not matter very much though.

As a test, clip one end out and sub in a known good cap to see if the waveform goes back to normal. If it does, you have your answer.

I honestly wonder how much "vintage tone" is simply bad parts, and how much is how the thing sounded a half century ago. I suspect if you could go back in time and hear what one of these things sounded like when new, it wouldn't be what it is now with all original parts.

 

[cecilk]

I've never seen leakage measured in terms of amperage, only voltage. Perhaps your measurement is 9.8 micro volts?? Or 9.8 millivolts?? They probably measure even better at their actual working voltage.

I'm using a DMM in series with the capacitor under test connected to an old RC tester (Similar to a Conar Model 311) that I rebuilt which is capable of putting voltage (up to 500v) across the capacitor under test. I measure the leakage current via the DMM. The RC tester also has a magic eye tube that shows leakage - but I use the DMM to get a numeric reading.

Frankly, I'd be surprised if your prospective orange drop replacement measured better.

I just tested a brand new Sprague orange drop, Mallory 150, and even a vintage Sprague Vitamin Q PIO under this same setup. All showed less than 0.1μA of leakage current.

 

[cecilk]

repair the amp first, then start changing caps

Yes - I was just curious to hear everyone's general approach to PIO caps.

Some advocate changing any PIO or paper/wax caps out of hand from a device this old as a safeguard.
Others seem to advocate holding on to these old caps at all costs lest the magic mojo be lost.
Others say if it ain't broke don't fix it.

 

[maxhifi]

I'm with you on that count - but it's not my amp so I must take the owner's perspective into consideration.

I would kind of look at this in another way: If you don't fix it right, it's going to come back, and your reputation would suffer. I would take a risk with capacitors that old in my own equipment which will be used under my own supervision, but for a customer I would change them all. You measured one to be leaky, the other similar capacitors cannot be far behind. Give them to the customer in a zip-lock bag with his recapped amp. Either that or test them one by one, replace only the bad ones, and inform the customer that the amp cannot be considered reliable. If he's a collector that may not be important.. if he's a player it sure will be.

 

[FlaCharlie]

I'm using a DMM in series with the capacitor under test connected to an old RC tester (Similar to a Conar Model 311) that I rebuilt which is capable of putting voltage (up to 500v) across the capacitor under test. I measure the leakage current via the DMM. The RC tester also has a magic eye tube that shows leakage - but I use the DMM to get a numeric reading.

I just tested a brand new Sprague orange drop, Mallory 150, and even a vintage Sprague Vitamin Q PIO under this same setup. All showed less than 0.1μA of leakage current.

Sounds like your cap tester is functionally very similar to my Eico, which also uses a magic eye tube to indicate leakage and allows 500v across the cap. I never thought about connecting a meter in series to get a numeric reading. I'll have to try that sometime - thanks for the tip!

Given the results with the other caps, I would definitely replace the coupling caps to the output tubes at the very least. Some of the others may not be as critical but I'd probably replace them too unless the owner wants to keep in original.

 

[johnblue]

I have a little drawer full of paper in oil caps that I have changed out, I save them because they are supposed to have mojo, but in all honestly I never use them I pretty much always switch them out. I probably never will go back and use the ones I saved either... Life is too short, my time on the bench is limited, if i'm hunting gremlins, all caps get changed.

There are a few times I've kept them in, if the gear is in decent working order and hasn't been sitting dusty in a hot garage for 10+ years, then maybe those paper in oil couplers can be salvaged, if I am raising the dead, they are gonzo.

Your pics do show a clean belly, so I understand your conundrum.

I think someone raised a good point about if you are a paid tech you should probably switch them, sounds like a good practice to me. if I paid someone to resurrect a long dead piece of gear I would expect these caps to be changed, no matter what the mojo, they arent going to "improve" with age, they can only get worse.

I have a hard time finding any mojo with cap-types anyways, i do however get mojo from a trusty piece of equipment, something I can count on.

If you switch them, you get the satisfaction of saying "here ya go, she's as good as new"