JJ ez81 question

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neon333
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JJ ez81 question

Postby neon333 » Fri 01/07/11 5:37 pm

I'm stumped with this one. JJ ez81 where the middle third of the plate is much darker than the rest of the plate. It is not a gradual darkening but a very neat, defined area that is almost black. What would cause this? It's been in the amp about 4 years now. The amp has felt a little too saggy lately even with some great stuff from Terry that I was trying out.

Thanks
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neon333
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Postby neon333 » Thu 01/20/11 8:13 pm

anybody?
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dotfret
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Postby dotfret » Mon 01/24/11 9:33 pm

Nobody talking to ya? OO, hard question y asking!

Diagnosing dud rectifiers is not in most books, and if you get that plate stripe it only says that the working area is less than most of the obvious area.

If you can hear the sound is getting saggy, perhaps it is time for a new rectifier.
So fit a new valve, then get one of those digital thermometers for measuting indoor/outdoor temperatures from a supermarket of garden centre. Tape that to your PT core, and check the temp after you've been working the amp for a half hour, and an hour. Y learn what should be, you know what hot is when you look at your PT. The rectifier chugs on for years. When it does die, the PT runs hot. Check PT temp every few months, then you know what is normal. Wax coming out between the laminations and a potential for cooking bacon is not normal.

If it needs a new rectifier, the working temp of the PT should drop by about 5-10 degF when you change the valve. If that doesn't happen, look for a dodgy capacitor.
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