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 Post subject: Leaky Caps
PostPosted: October 7th, 2021, 2:22 pm 
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Joined: July 17th, 2016, 6:24 am
Posts: 1099
Couple of years ago, fuse in the Cary/AES Superamp was blowing. David Berning identified the problem to a shorted coupling cap. Original thread here
viewtopic.php?f=5&t=1449

Amp worked fine for a few months after replacing the bad Jenson Copper foil cap. I left it on and went away for a long weekend to find one of the output pentodes is glowing bright. Powered down the amp and let it cool. Within a few mins of powering it back on, the same tube was red plating again and burnt with a flash. At that time I thought it as a bad tube, so tried another tube, same result. Since then, this amp is sitting in the storage.

Now, I want start using the amp again so reading about the problem. Looks like another coupling cap is leaky. I guess Jenson caps have started failing. I can replace all four coupling caps.

My question is, how do I test the amp after replacing the caps without blowing the tubes? How do I test the leaky capacitors?


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 Post subject: Re: Leaky Caps
PostPosted: October 7th, 2021, 3:09 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 914
Shashi,

Replace all with good film capacitors, and you likely will not have a problem. The problem is that an oil cap will dry out, become electrically leaky and start to pass DC to the grid of the tube, which results in an increase in grid voltage and more current through the tube (red plating). So replacing these *should* address the root cause. What is unknown is whether anything else was damaged by the over current on the tube(s). The most critical thing is the output transformer, but it is likely OK.

Is this is fixed or cathode biased amp. If fixed bias, I would start by adjusting the bias to the most negative voltage setting. The tubes will be "cold", but if it works OK, then adjust the bias up to a normal setting. If cathode bias, then there is not much to do except hope for the best. I would definitely replace the tubes, do not use them until you have a chance to have them tested.

BTW, NEVER leave a tube amp on unattended for a long period of time. Waste of tube life.

David


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 Post subject: Re: Leaky Caps
PostPosted: October 7th, 2021, 3:18 pm 
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Joined: February 28th, 2013, 3:31 pm
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If you have a variable DC power supply you can connect the cap under test to the supply through a high value resistor in case it shorts out during the test. Put a DC milliamp meter in series with the cap and slowly bring up the power supply until reaching the rated voltage. Once the voltage has stabilized, if the cap is an electrolytic of relatively low capacitance the leakage should be under a milliamp. If it is a film or paper, the leakage should be almost unmeasurable. If you don't have a variable power supply but have a variac, a high voltage transformer, a suitable rectifier and filter capacitor you can make a supply that will work. The filter cap can be small because if the cap is good, current should be minimal. If you build your own supply, when you turn it off, make sure you have a resistor to discharge it.


Tom


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 Post subject: Re: Leaky Caps
PostPosted: October 7th, 2021, 3:29 pm 
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Joined: February 28th, 2013, 1:19 pm
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THROW THE LEAKY CAPS AWAY!!!!

The Jensen caps are notorious for failing. Based on your original post, the caps are likely 20 years old. The Jensen oil caps have end seals that are cardboard, either rubberized or oil impregnated as I recall. The have a rolled mechanical "seal" at the casing. Unlike Vitamin Q, Russian PIOs etc., they are not hermetically sealed. Therefore, they are subject to drying out over time and becoming electrically leaky. Since they are prone to failure, why even bother testing them for reuse. The output tube cost more than a decent quality replacement film cap. Try the CDE 942C caps in the higher voltage ratings (1000V). If you want to keep with an oil cap, then use a Russian PIO that is hermetically sealed and will not dry out.

David


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 Post subject: Re: Leaky Caps
PostPosted: October 7th, 2021, 3:54 pm 
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Joined: January 15th, 2015, 7:19 am
Posts: 1697
Location: Baltimore MD
If old paper and oil caps are still good how about dipping them epoxy


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 Post subject: Re: Leaky Caps
PostPosted: October 7th, 2021, 4:03 pm 
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How do you REALLY know they are good? If you hermetically seal an old cap, they might be good today when you test them, but if they have already dried out to some extent, they may become leaky later. It really amounts to a poor design of the endcap of the capacitor.

David


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 Post subject: Re: Leaky Caps
PostPosted: October 7th, 2021, 4:05 pm 
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Joined: January 15th, 2015, 7:19 am
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Location: Baltimore MD
If you can test capacity and leaking what else is there
Good quality oil and paper cap are not cheap


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 Post subject: Re: Leaky Caps
PostPosted: October 7th, 2021, 4:53 pm 
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Joined: July 17th, 2016, 6:24 am
Posts: 1099
David McGown wrote:
Shashi,

Replace all with good film capacitors, and you likely will not have a problem. The problem is that an oil cap will dry out, become electrically leaky and start to pass DC to the grid of the tube, which results in an increase in grid voltage and more current through the tube (red plating). So replacing these *should* address the root cause. What is unknown lis whether anything else was damaged by the over current on the tube(s). The most critical thing is the output transformer, but it is likely OK.

Is this is fixed or cathode biased amp. If fixed bias, I would start by adjusting the bias to the most negative voltage setting. The tubes will be "cold", but if it works OK, then adjust the bias up to a normal setting. If cathode bias, then there is not much to do except hope for the best. I would definitely replace the tubes, do not use them until you have a chance to have them tested.

BTW, NEVER leave a tube amp on unattended for a long period of time. Waste of tube life.

David


It should be fixed bias. It has a single biasing jack for all four tubes.


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 Post subject: Re: Leaky Caps
PostPosted: October 7th, 2021, 5:41 pm 
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Joined: July 17th, 2016, 6:24 am
Posts: 1099
David McGown wrote:
THROW THE LEAKY CAPS AWAY!!!!

The Jensen caps are notorious for failing. Based on your original post, the caps are likely 20 years old. The Jensen oil caps have end seals that are cardboard, either rubberized or oil impregnated as I recall. The have a rolled mechanical "seal" at the casing. Unlike Vitamin Q, Russian PIOs etc., they are not hermetically sealed. Therefore, they lent are subject to drying out over time and becoming electrically leaky. Since they are prone to failure, why even bother testing them for reuse. The output tube cost more than a decent quality replacement film cap. Try the CDE 942C caps in the higher voltage ratings (1000V). If you want to keep with an oil cap, then use a Russian PIO that is hermetically sealed and will not dry out.

David


Yes, I will throw away the Jensen caps.

For now, this amp will be powering your bass horns between 100-450Hz. So, I need caps that are excellent in midrange. HF characteristics should not matter as the amp never sees high frequencies. I have power and output transformers for a 300B amp. When 300B is done, I might use it for bass horns and relegate Superamp for powering your JBL LE5-2 midrange horn between 450-2200Hz.

Russian PIOs are very cheap on EBay. Found this review of Audiocap Theta interesting. Might try them out.

Quote:
AudioCap Theta is constructed with polypropylene film and tin foil with gold-plated OFHC leads, and it is very reasonably priced compared to AudioCap PCU, which is polypropylene film and Copper foil and priced accordingly. I have read AudioCap Thetas being described as lean and clinical in the past, and that's exactly how they sounded in the beginning. However, after proper break-in, these things became extraordinarily rich and warm in tone, without any wooly, syrupy sloppiness. AudioCap Thetas definitely had another notch of detail and resolution in the mid-midrange compared to even the best metallized polypropylene caps, resulting in sumptuously textured and detailed voices; however, the upper-midrange and treble also retained this rich smoothness, which in fact made them sound a touch less open and sparkling compared to metalized poly caps like WIMA.


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 Post subject: Re: Leaky Caps
PostPosted: October 7th, 2021, 10:26 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 487
I turned David on to the Cornell-Dubilier 942C series caps. *You will not find a better cap for the money.* Or possibly *any* money, for that matter, unless you want to spend $300 a piece or more. If you want a clean, punchy midrange with no phase issues, this is the cap for you. ;-) IMO, they best the old V-Cap TFTF, though not the new V-Cap CuTF, which is probably the best cap I've heard. But I use the 942's in all my amps with zero complaints. John Broskie started selling these for his Aikido kits and they are truly excellent.


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