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    fried Weber copper cap

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    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    fried Weber copper cap

    Post by baddog1946 on Fri Aug 20, 2010 8:50 pm

    Hi Bob:
    I just had a problem with my ST-120 that needs your immediate advice before I go any further with troubleshooting.
    I was listening to it when I noticed that the rear left side KT-88 (Gold Lion) next to the Weber copper cap rectifier was glowing very brightly. I thought I had burned a tube or was about to burn a tube. The amp was still playing away when I went to the amp and I could also smell something burning. I shut down the amp right away and let it cool off.

    I removed the tube that was overheating and replaced it with another Gold Lion KT-88 I had that had very low hours on it.
    When I fired up the amp again all the tubes seemed to be glowing normally and I tried to set the bias on the output tubes and there was no reading on my meter or should I say it read.002 milliamps no matter what I did with the pot adjuster.

    This was the same on all 4 KT-88's at that point I thought it might be the rectifier and sure enough it was.

    When I removed the Weber copper cap as it was discolored and smelled bad. I eased the top off it and the insides were completely fried and the two large ceramic resistors were both cracked and one literally fell apart. It was burnt to a crisp inside. Most of the diodes etc. were also showing signs of major overheating.

    My question is not only what happened if you can guess but should I put a tube rectifier in there to see if it was only the rectifier or should I check something else first?

    I live in Costa Rica and it is a long way to get another Weber. I have three tube rectifiers but I do not want to fry them if there is another common problem.

    I'm hoping it is not a major problem with the amp and just a faulty rectifier. I haven't heard of a copper cap going like that but anything is possible.
    I have built a couple of DIY projects and am capable of doing some diagnostics but I need to know what to check and where the problem might be.
    Looking for a little advice here from the guy who knows this thing inside out before I sacrifice any more parts.
    Thanks
    baddog1946

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: fried Weber copper cap

    Post by Bob Latino on Sat Aug 21, 2010 9:15 am

    Hi Baddog,

    It appears from your description that a faulty KT88 took out the Weber Copper Cap. From past experiences with this sort of problem, usually the amp is OK. Try a new rectifier tube in there and you should be OK. The fact that you are getting no bias on all the tubes tells me absolutely that the Weber Copper Cap is now bad. Most tubes (and the Weber Copper cap) have a 90 day warranty. If you still have a dated sales slip you may be able to get another Weber Cap or another Gold Lion KT88. You will probably have to send back the bad tube/Copper cap.

    Bob

    GP49

    Posts : 718
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: fried Weber copper cap

    Post by GP49 on Sat Aug 21, 2010 12:35 pm

    That one red-glowing KT88 is the key symptom. Only one tube was running away. Had the bias circuit gone bad, ALL the tubes would have been glowing red and burning up.

    One tube running away SHOULD have caused an audible increase in hum as that tube drew down on the storage in the filter capacitors. It may not have been enough to blow the line fuse, though. Or that Copper Cap blew up quickly.

    Before installing a new rectifier, there are some other things to check. With power on and with NO rectifier, verify that you have correct negative voltage at the grid pin of that tube socket. You should have that negative bias voltage there even without the rectifier tube installed (may as well check it on all of the tubes for good measure). Then turn off the amplifier and install the rectifier tube, then turn it on WHILE MONITORING THE VOLTAGE ON THAT GRID PIN and the appearance of the tube. If the voltage stays negative, you're fine. If it goes positive or substantially less negative than the normal spec, or if the tube begins to glow any more than a very dull red, SHUT DOWN THE AMPLIFIER. You then need to check the coupling capacitor between the inverter stage and the output tube grid. If it is leaky or shorted, it will cause the tube to draw excess current, glow red, and eventually run away as its elements overheat and shift position.

    Check the cathode resistor on that tube, too. It was handling all the current from the runaway tube and may have gotten hot enough to go open or drift off in value. It was not the CAUSE of the failure, however may have been damaged in the incident.

    You should probably reset the bias before returning the amplifier to service.

    baddog19
    Guest

    fried rectifier

    Post by baddog19 on Sat Aug 21, 2010 7:22 pm

    GP49:
    Good tips to check out thanks for the advice. You are right it started to hum loudly and as I was right there I was able to shut it down quickly.
    Although I have done these tests you mention here yet I replaced the copper cap with a GX34 tube rectifier and reset the bias on all the output tubes and it is now set properly at .500 milliamps as recommended on all four.Seems stable so far.

    Interesting point here though, the bias is something I keep an eye on regularly and it was wandering on that tube a bit for the last few days. I usually checkit once or twice a week.

    I thought maybe it was because I had replaced a pair of the KT-88 tubes some time ago and lost track of which was which and could have mixed the matched pairs up. One of the pairs had slightly lower ratings by a
    This may not have been the cause but remains a possibility.
    At any rate it is now operating normally but I will definitely check out the coupling cap and the resistor and do the voltage test without the rectifier in there to see that all is normal.

    When I fired it up again the tubes are all lit up with the normal dull red and after an hour or so are still fine. I will do the checks though as I do not need a problem out here in the jungle with no parts stores for miles.
    cheers
    Baddog1946

    merlot

    Posts : 22
    Join date : 2008-12-22

    Re: fried Weber copper cap

    Post by merlot on Mon Aug 23, 2010 4:16 pm

    I own two of Bob's ST-120s and one ST-70. All had been using Copper Caps. Early on the ST-70 one was replaced under warranty and continues to work fine. About four months ago within a short period both Copper Caps in the ST-120s went bad - just like yours. The smell is terrible and the resistors in one literally crumbled. The copper discoloured from the heat. I disassembled the amps and measured all the voltages - everything was fine. Bad Copper Caps both of would have been from the same batch. Still using the same output tubes - they are fine.

    Right now I'm using a GZ34 and 5AR4 in their place. I bought new Copper Caps but have yet to use them. A tech at Weber did mention to me that they had made a change to the design.

    I was thinking of repairing/modifying the old ones. Replacing the diodes,resistors and mounting a small standby switch on the top or side somehow.

    Merlot

    baddog
    Guest

    fried Weber copper cap

    Post by baddog on Mon Aug 23, 2010 5:48 pm

    Merlot:
    Thanks for the feedback I think there might be a quality control problem with some of those copper caps. Mine got discolored as well.
    I ordered a new one also and hope I get the better design model if that's how it works. I am going to drop them a note about that.
    I am also contemplating re-building my old one they are absurdly simple and it would really only be a matter of putting some good quality parts in there.
    I'm looking around right now any recommendations on parts?
    I have also gone to a JJ electronics GZ-34 tube for the time being and all seems well.
    Do you think the cap blew the tube or the tube blew the cap?
    I had to replace a Genelex KT-88 at the same time which is supposed to be a quality tube.
    I hope it was the cap blew the tube because those suckers are pricey.
    baddog1946

    Bob Latino
    Admin

    Posts : 2378
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    Location : Massachusetts

    Re: fried Weber copper cap

    Post by Bob Latino on Mon Aug 23, 2010 6:13 pm

    Hi,

    Every now and then a company has a bad lot of products that for some reason fail early. It is possible that Weber had a situation like this. Personally, I have had a Weber WZ68 Copper Cap rectifier in my own ST-120 for just over 2 years and maybe a couple of thousand hours with no issues ..

    Bob


    Last edited by Bob Latino on Sat Aug 28, 2010 7:26 am; edited 1 time in total

    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    Re: fried Weber copper cap

    Post by baddog1946 on Mon Aug 23, 2010 7:17 pm

    Hi Bob:
    I know I really liked it. I guess it was the luck of the draw to get a bad one. That's why I was thinking of re-building it. Looks pretty easy. The base was not burned and the parts are cheap.
    I'm more worried about those expensive Gold Lions going up in flames.
    Baddog1946

    merlot

    Posts : 22
    Join date : 2008-12-22

    Re: fried Weber copper cap

    Post by merlot on Mon Aug 23, 2010 8:43 pm

    Baddog1946,

    I never lost any output tubes. I would therefore assume it was bad Copper Caps. Everything else checked out fine. I have been operating the amps for a few months subsequent and all is fine. I bought 5 new Copper Caps but haven't tried them yet.

    They seem quite straight forward. I was going to use 1N4007 diodes with ceramic resistors so that I get a lower voltage. The Copper Caps have a thermistor (I think) NTC 1000-11. Haven't looked into that yet but with a standby switch I was going to drop the thermistor. Any thoughts?

    Merlot

    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    Re: fried Weber copper cap

    Post by baddog1946 on Mon Aug 23, 2010 11:47 pm

    Hi Merlot:
    You are going to need a very heat resistant switch if you are thinking of mounting it on the cap itself somehow as those caps get hot normally. Weber says there is a built in delay on the originals but can't I figure how it would work when I look at the guts its pretty filled up in there. Or did you mean a delay switch elsewhere on the amp?

    merlot

    Posts : 22
    Join date : 2008-12-22

    Re: fried Weber copper cap

    Post by merlot on Tue Aug 24, 2010 8:46 pm

    Baddog,

    A very valid observation. I'll have to see how hot the resistors get. I understand the Weber delay is quite short compared to a 5AR4 or GZ34. The other challenge is the high voltage. I bought some micro 250v switches to try. This will be a fall project....

    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    Re: fried Weber copper cap

    Post by baddog1946 on Thu Aug 26, 2010 10:28 am

    Merlot:
    Keep me posted on that switch idea.
    Bd

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: fried Weber copper cap

    Post by tubes4hifi on Sat Aug 28, 2010 12:31 am

    there is also a possibility the 10 ohm cathode resistor on that output tube may have burned out,
    ( actually a good thing, as it may have saved the output tube )
    so I'd check that and make sure it isn't burnt, I usually use 1 watt metal oxide resistors on those

    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    Re: fried Weber copper cap

    Post by baddog1946 on Sat Aug 28, 2010 4:52 pm

    Thanks for the tip
    I just put it on my list of things to check when I pop the bottom off for a look next week.
    Right now the amp seems unhurt but I will be looking for any cooked/damaged parts very soon. They may not be dead but could be out of spec a little if they are partially cooked.

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