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Author Topic: Care and Feeding of HV Electrolytic Capacitors  (Read 22769 times)
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W7TFO
WTF-OVER in 7 land Dennis
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« Reply #25 on: July 31, 2011, 12:15:34 AM »

Capacitor - Solen Fast Metalized Polyprop, 8.2 µF @ 630 V  $5.25 per @ AES.

Not a bad deal for a cap that isn't an electrolytic or oil, seem to last forever in stuff I fix.  Just the thing for restuffing old can types.

Values go up to 47uF at that voltage.  They also have them rated at 1kV.

73DG
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KM1H
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« Reply #26 on: July 31, 2011, 10:10:47 AM »

Quote
When repairing an old radio, I often use twice or more filter capacitance than the original.
 

Be careful about doing that for the input cap. It will raise the B+ causing higher current in the tube circuits as well as possibly be too high for the rectifier tube causing an arc. The max C is usually in the tube spec sheet. I try and limit the input to close to original and use a lot more at the output to kill ripple.

I never use the Mouser or any other large distributor on line catalog, they are a PITA and you miss way too much.
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WD8KDG
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« Reply #27 on: July 31, 2011, 01:22:06 PM »

As to reforming etc: I'll stick to new electrolytic caps, not old stock. Still can't see how reforming will put the liquid, which has evaporated, back into the cap.

Mouser is the first place I look, still have a few of their paper catalogs. But looking online other caps are listed that might not be in the catalog. Why limit your choices??? And I keep an online account there, makes it easy to reorder if restoring same model RX or TX's

Craig,
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« Reply #28 on: July 31, 2011, 03:01:47 PM »

<<On an island>>

There was an earthquake. 

I do miss the days of getting a hardcopy paper catalog from Mouser and Digi-Key.
Sometimes the new is not always as good as the old. It made it a lot easier to thumb through the catalog and find what you wanted (and compare pricing at a glance) their on-site search utilities are not always your friend.

Slab you must have good eyes. 
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« Reply #29 on: July 31, 2011, 04:25:57 PM »

Slab you must have good eyes. 

Oy-vey! ! ! ! !  What, you never heard of a magnifying glass  Huh  Grin  Grin  Grin  Grin
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k4kyv
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« Reply #30 on: July 31, 2011, 09:14:18 PM »

Quote
When repairing an old radio, I often use twice or more filter capacitance than the original.
 

Be careful about doing that for the input cap. It will raise the B+ causing higher current in the tube circuits as well as possibly be too high for the rectifier tube causing an arc. The max C is usually in the tube spec sheet. I try and limit the input to close to original and use a lot more at the output to kill ripple.

The max C as specified in the tube spec sheet is usually well over twice what was actually used in an original pre-WWII power supply, even as referenced in the RCA manual of that same era. C was simply too expensive and bulky back then, so they used as little as they could get away with or fit into the space available. Many of the older BC sets used 4 to 8 mfd for the input cap with the type 80 rectifier (sometimes as little as 2 mfd), while post-War sets typically used 16 to 20 mfd for the first cap with the type 5Y3.  Exact same tube, just different base socket. Where you might get into trouble would be to try using 50 or 100 mfd for the input cap, although some of the tube type B&W TVs that I remember salvaging for parts back in the 50s did just that.

Excessive C at the input cap shouldn't boost the voltage significantly.  A capacitor input filter delivers DC voltage at near the peak voltage of the a.c. cycle, 1.414 X Vrms.  Once you have enough capacitance at the input to make the supply filter act like capacitor input, additional capacitance won't affect the voltage once the capacitor is fully charged. What strains the rectifier tube with too much input capacitance is the excessive current the capacitor pulls through the tube to initially charge it to full operating voltage. A large capacitor may act like a dead short for as long as a second or more, while the proper size cap acts like a dead short only for milliseconds. In every case, a capacitor input filter presents a dead short across the rectifier the instant a.c. voltage is first applied.

Tubes are intolerant of excessive peak current, but solid state rectifiers can handle it OK to some degree.  OTOH, tubes are much more forgiving than SS devices when the maximum p.i.v. is even slightly exceeded.
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Don, K4KYV                                       AMI#5
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« Reply #31 on: August 02, 2011, 02:56:45 PM »

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<<On an island>>

There was an earthquake.

The other one hasnt been invaded yet so quality is still good Grin

Quote
The max C as specified in the tube spec sheet is usually well over twice what was actually used in an original pre-WWII power supply, even as referenced in the RCA manual of that same era.


Not really and the world didnt revolve around RCA.


Quote
C was simply too expensive and bulky back then, so they used as little as they could get away with or fit into the space available. Many of the older BC sets used 4 to 8 mfd for the input cap with the type 80 rectifier (sometimes as little as 2 mfd), while post-War sets typically used 16 to 20 mfd for the first cap with the type 5Y3.

The 80 started off with a 4uF rating at full voltage; over the years and also by manufacturers the spec was raised to 10uF.

The 5Y3 series varies all over the place from 10 to 40uF starting with the 5Y3G which is the only true 80 replacement to the 5Y3GB. The GT and GA versions also have varying ratings dependent upon manufacturer.

It really pays to read all the specs.

 
Quote
Exact same tube, just different base socket. Where you might get into trouble would be to try using 50 or 100 mfd for the input cap, although some of the tube type B&W TVs that I remember salvaging for parts back in the 50s did just that.

Different tube, different spec and also possibly a lower RMS. The minimum plate supply impedance also played an important role. Most B&W TV's Im familiar with used a 5U4 of some kind.

Quote
Excessive C at the input cap shouldn't boost the voltage significantly.  A capacitor input filter delivers DC voltage at near the peak voltage of the a.c. cycle, 1.414 X Vrms.  Once you have enough capacitance at the input to make the supply filter act like capacitor input, additional capacitance won't affect the voltage once the capacitor is fully charged.



Supply impedance and the radios current draw both come into play when a widely varying DC load is involved. While a 4uF cap has little energy storage and the regulation is poor a 40uF will have much closer to the 1.414 of theory. This can be substantial if the radio is already using components at the edge of their specs. A higher voltage equates to higher current and hotter running power and audio transformers, especially in Hallidrifters with PP audio and failure prone iron, many consumer radios have similar problems.


Quote
What strains the rectifier tube with too much input capacitance is the excessive current the capacitor pulls through the tube to initially charge it to full operating voltage. A large capacitor may act like a dead short for as long as a second or more, while the proper size cap acts like a dead short only for milliseconds. In every case, a capacitor input filter presents a dead short across the rectifier the instant a.c. voltage is first applied.

Unless it has a controlled warmup characteristic.

Quote
Tubes are intolerant of excessive peak current, but solid state rectifiers can handle it OK to some degree.


Ask anyone who has used 866's. SS is rated at peaks per cycle and modern diodes such as the 1N5408 and 6A10 can handle substantial peaks.


 
Quote
OTOH, tubes are much more forgiving than SS devices when the maximum p.i.v. is even slightly exceeded.


Actually tubes such as the 80/5Y3 are not very tolerant of overvoltage and indirectly heated ones are even less so.
Ive measured 1N5408's and 6A10's as having a PIV of 300-500V above the spec without failure.

Carl
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k4kyv
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« Reply #32 on: August 02, 2011, 05:41:34 PM »

I don't believe there is any significant difference between the ratings of the consumer-grade 5Y3G and 5Y3GT, nor between different reputable manufacturers of the same tubes. I  have several books besides the RCA manual, including one by G-E and a couple of generic ones, and they all give approximately the same data.

The RCA book says that with high vacuum rectifiers, if the 1st capacitor is above 40 mfd, care should be taken to assure that the source impedance exceeds a certain minimum number of ohms (don't have the book right here with me for the exact figure, which I believe varies with tube type), but that shouldn't be a problem with those old broadcast receiver power transformers. Most of them were pretty poor in terms of voltage regulation, due to high leakage inductance resulting from their cheap design.

That's one of the reasons modern day HV power supplies using solid state diodes have as good regulation as they do (capacitive input supplies have had a reputation for notoriously poor voltage regulation); modern transformers are designed for low leakage inductance and pose a very low source impedance.  Most "vintage" transformers, designed for choke input filters, indeed have poor regulation when a capacitive input filter is used, because of the high leakage inductance due to physical construction and core material which results in a higher source impedance than a modern  transformer designed for the best possible regulation..

Maybe newer diodes that don't require the external equalising resistors and surge suppressing capacitors across each diode can take a certain amount of voltage beyond the PIV rating, but I never was able to keep a string of diodes (the older types using the caps and resistors) from popping like a string of fire-crackers even with the recommended resistors and capacitors, running at substantially less than their maximum rated voltage. This was particularly true when I tried to substitute SS diodes for tube type rectifiers in an existing choke input filter. One of the reasons is that the input capacitor presents a low impedance to the spikes as they exit the rectifier and takes them directly to ground; with choke input, the inductance of the filter choke presents a high impedance that allows a spike to build up to high enough PIV with the inductive kick to break down the diodes. Since I have plenty of tube type rectifiers on hand and filament transformers to run them, I gave up on the diode strings and just stick with hollow-state  rectifiers.  However, my BC1-T has used a couple of sets of plug-in SS replacements for the 8008 and 866A rectifiers working into choke input filters for almost a decade now without incident, so the designers of those direct-replacement modules must know something that I don't.

Choke input tends to work best with tube type rectifiers including MV and Xenon gas types, while capacitor input tends to work best with solid state. Capacitor input is not recommended for MV tubes because the peak current rating is easily exceeded, which according to the manufacturers significantly shortens the life of the tubes. It works OK with high vacuum rectifiers like the 5Y3/80 or even the transmitting type 836, because the higher internal resistance of the vacuum diode serves adequately as a current limiting resistor, so long as the input capacitance is not too high.
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Don, K4KYV                                       AMI#5
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« Reply #33 on: August 02, 2011, 08:54:25 PM »

There was a similar thread on here 2 months ago started by Stu AB2EZ on re-forming electrolytic capacitors in which I made a post on the availability of high-capacitance film capacitors available today called DC link capacitors:

http://amfone.net/Amforum/index.php?topic=27929.0
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k4kyv
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« Reply #34 on: August 03, 2011, 12:42:57 AM »

The best way to avoid burning out a transformer or rectifier due to a shorted electrolytic (or other component) is to wire a fuse into the a.c. power circuit; there should be one there anyway. I have re-formed old caps simply because I  had them on hand and needed one right away, not after waiting several days for an order to arrive. Sometimes, I have re-formed old ones with the firm intention of ordering new ones as a permanent replacement once I got the equipment running or the prototype developed, then procrastinating or forgetting about it until the cap goes, or as in some cases, the re-formed cap never crapped out.

I have had electrolytics to open up about as often as I have had them to short out. I have also had them explode and shoot stinky fibrous crud all over the place or totally pollute the under side of the chassis with gunk.
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« Reply #35 on: August 03, 2011, 02:13:46 AM »

I have had electrolytics to open up about as often as I have had them to short out. I have also had them explode and shoot stinky fibrous crud all over the place or totally pollute the under side of the chassis with gunk.

Not a bad reason to replace them with something better but close in Farad value.

73DG
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« Reply #36 on: August 03, 2011, 07:59:29 AM »

I have also had them explode and shoot stinky fibrous crud all over the place or totally pollute the under side of the chassis with gunk.


Ah Yes................. Capacitor cheese  Grin  Grin  Grin
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« Reply #37 on: August 03, 2011, 11:15:00 AM »

Quote
I don't believe there is any significant difference between the ratings of the consumer-grade 5Y3G and 5Y3GT, nor between different reputable manufacturers of the same tubes. I  have several books besides the RCA manual, including one by G-E and a couple of generic ones, and they all give approximately the same data.

Believe whatever you want, I was reading the original full spec sheets published over a few decades.

Quote
The RCA book says that with high vacuum rectifiers, if the 1st capacitor is above 40 mfd, care should be taken to assure that the source impedance exceeds a certain minimum number of ohms (don't have the book right here with me for the exact figure, which I believe varies with tube type), but that shouldn't be a problem with those old broadcast receiver power transformers. Most of them were pretty poor in terms of voltage regulation, due to high leakage inductance resulting from their cheap design.

I touched on the source impedance yesterday, maybe you missed it. All full spec sheets include that and an engineer understands what it means. However techs or complete newbies working on grandpas heirloom often/never dont pay attention to details and just think bigger is better....and bang.
Many if not most ham receivers and transmitters used cheap transformers also. Some consumer sets had outstanding quality in their top of the line products.

Quote
I have had electrolytics to open up about as often as I have had them to short out. I have also had them explode and shoot stinky fibrous crud all over the place or totally pollute the under side of the chassis with gunk.


Not a bad reason to replace them with something better but close in Farad value.

73DG

Not a bad reason to replace them before even turning a set on. More transformers are destroyed by fools who find some old set and just plug it in without a clue in the world.  Many others claim since its been working fine for them for XX years it dont need no stinkin capacitors. Then sometime later they are begging for a replacement transformer or crying about the cost of a rewind. They get no sympathy from me.

Carl
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k4kyv
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« Reply #38 on: August 03, 2011, 12:49:17 PM »

Hence the need for the a.c. primary fuse.  Most "consumer" electronics stuff in the day omitted the primary fuse, just as they omit RFI filtering to-day, to save a minuscule percentage on the total per-unit manufacturing cost, and with no fuse, for the hope that the set would burn out after a few years and the customer would buy a new one.

Maybe that's the origin of the term "consumer" electronics.  They are supposed to self-destruct after a certain period of time to keep the industry flourishing. I don't "consume" electronic and radio equipment.  About the only retail products I "consume" are food, drink, clothing and fuel. Everything else I try to keep using as long as I possibly can, therefore admittedly, I am a piss-poor "consumer". If the majority of the public had my buying habits, the economy would have tanked decades before it did, since the whole system largely runs on wastefulness.

Where can I find copies of those data sheets that show significant electrical differences between the 80, 5Y3G and 5Y3GT? (Add to the list the 5Y4G, which is nothing but a 5Y3G with different pin arrangements on the octal socket). By "original spec sheets" for the 80 are you talking about the ones that came with the original round-top versions like the UX-280, CX-380, etc? They did upgrade many tube specs in the mid-30s when they changed the bulb style from the globe-shaped originals, but with the upgrade came full-speed mass production, so 99% of the tubes found to-day are the more modern types with the ST envelope or the smaller GT bulb.
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« Reply #39 on: August 07, 2011, 05:47:16 PM »

Here's an example of a place with what appear to be decent rx filter caps:


http://cgi.ebay.com/Illinios-22uf-22-uf-350v-Axial-Capacitor-10-pcs-/190561144262
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« Reply #40 on: August 07, 2011, 09:43:49 PM »

Here's an example of a place with what appear to be decent rx filter caps:


http://cgi.ebay.com/Illinios-22uf-22-uf-350v-Axial-Capacitor-10-pcs-/190561144262

Says "new old stock" but doesn't say how old (5, 10, 20+ years)

You can go to Mouser and, most likely, get "fresh" stock of the same value/voltage/axial for $1.40 or $1.89 and even cheaper by the bucket.
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