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Paper In Oil Caps: Replace On Sight Or Keep?

FlaCharlie said:
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!
Click to expand...

You're welcome!

Yeah, I was thinking of upgrading to a Sprague Tel-Ohmike tester with a analog meter that would give a numeric reading so I could tell with a little more accuracy how bad the leakage was - then thought - Why do that when I could just connect a meter in series and get a digital reading?
 
maxhifi said:
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.
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Yes - and to clarify, I am not a commercial technician - just working on this amp for a friend. ^_^

Normally I wouldn't give it a second thought - but after I told him that a lot of the PIO caps were leaky and that my plan was to replace them, he talked to a tech at a music store who told him that his amp would probably lose its mojo if he let me replace the leaky caps. So my friend got worried about it and was thinking of taking it to the tech at the music store instead (who had already given up on it a few months prior) - so I posted here to get a pulse on how other techs handle leaky PIO caps.

I like your idea about saving the caps and giving them to him in a ziplock bag "just in case".

Thanks all for the advice!
 
One final point: you can get old stock USSR made paper in oil capacitors which aren't leaky, and also newly made Jensen, audio note, etc.

So it IS possible to test them all and replace the old ones with good PIO capacitors if the owner really wants it.

Also, a champ is as simple as a tube amp gets. I hate to say it but I wouldn't have much use for the opinion of a tech who can't fix such a simple amp on technical matters. (The music store guy)
 
My tester has a neon lamp for leakage. Over some amount it comes on, though I don't know the threshold off the top of my head. For accurate measures, a 1K or 10K in series with the 500v bench supply will turn voltage across the resistor into leakage very easily. My DVM doesn't have quite enough resolution for that kind of thing.
 
cecilk said:
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.
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The original vitaminQ pio caps are constructed of a brass body with glass end seal. That construction insures that no air or moisture can migrate to the inside elements and start breaking down the insulation between the conducting foil. Other than physical damage the cap or an over voltage incident/s this cap is designed to last for many , many years as in industrial and military applications. If your japanese caps are made this way i would tend to check for leakage via voltage or tester and do further checks before replacing them.
 
When a piece has a collectors value that maybe worth more than the price of a working unit with non original parts there are 2 different points to consider. I am not familiar with guitar amp values and the collectors value. So the pros and cons of that issue should be discussed with the owner. If you discuss this with him then whatever he says to do , it will be the owners decision and so out of your hands.
 
cecilk said:
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:

View attachment 962526

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?
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At what input voltage did the output wave look like the photo?
 
triode17 said:
At what input voltage did the output wave look like the photo?
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Here is signal at grid of first half of V1 (a 12AX7):
01 V1 Grid.jpg
And after the first coupling cap after the plate of V1:
02 V1 Plate after cap.jpg

So far, so good, right?

Then there is a weird little tone circuit involving a few of the PIO caps and two pots leading to the second half of V1.

Here's the grid of the second half of V1:
03 V1 Grid2.jpg

It's starting to really clip and distort here - and the volume for these photos was only at 1 or 2!

Then after V1 it heads through a coupling cap toward the first half of V2 (a 12DW7) which is the start of the vibrato circuit.

Here's the grid of the first half of V2:
05 V2 Grid1.jpg

Yikes! Looks like some noise in there as well as distortion. Vibrato circuit does not really work, either.

After the 10 PIO caps in the vibrato circuit - by the time it gets to the power tube it's totally wack:

07 Output.jpg

My plan is to remove each PIO cap starting at the input stage, test it for capacitance and leakage. If it is leaking, it gets replaced. Then I'll run the signal trace again and take a gander at the waves. I'll give the owner the bag of caps at the end and he can decide what to do with them. Owner says he is good with that - I'll let you know how it goes!
 
cecilk said:
Here is signal at grid of first half of V1 (a 12AX7):
View attachment 964415
And after the first coupling cap after the plate of V1:
View attachment 964416

So far, so good, right?

Then there is a weird little tone circuit involving a few of the PIO caps and two pots leading to the second half of V1.

Here's the grid of the second half of V1:
View attachment 964418

It's starting to really clip and distort here - and the volume for these photos was only at 1 or 2!

Then after V1 it heads through a coupling cap toward the first half of V2 (a 12DW7) which is the start of the vibrato circuit.

Here's the grid of the first half of V2:
View attachment 964419

Yikes! Looks like some noise in there as well as distortion. Vibrato circuit does not really work, either.

After the 10 PIO caps in the vibrato circuit - by the time it gets to the power tube it's totally wack:

View attachment 964420

My plan is to remove each PIO cap starting at the input stage, test it for capacitance and leakage. If it is leaking, it gets replaced. Then I'll run the signal trace again and take a gander at the waves. I'll give the owner the bag of caps at the end and he can decide what to do with them. Owner says he is good with that - I'll let you know how it goes!
Click to expand...

Are you totally sure the amp isn't just clipping? What's the highest rms voltage of a clean sine wave you can get on the 6V6 grid.
 
Try it as low as you can get it. Guitars don't have much output. Under about 0.1 volt will probably be sufficient.
 
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