Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uwm.edu!csd4.csd.uwm.edu!info-high-audio-request
From: alp@techbook.com (Alan Peterman)
Newsgroups: rec.audio.high-end
Subject: Re: Destructive KT88
Message-ID: <6922@uwm.edu>
Date: 12 Oct 90 14:03:55 GMT
Sender: news@uwm.edu
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Approved: tjk@csd4.csd.uwm.edu

In article <6866@uwm.edu> KLUDGE@AGCB1.LARC.NASA.GOV writes:
>That's it.  I've had enough of crappy Chinese KT88's which suddenly decide
>to pull tremendous amounts of current, opening up the cathode resistor and
>glowing red hot.  This has happened three times on my Citation, with three
>different tubes.  Never happened at all with the old Mullard tubes.  What's
>the problem here?  It's not as if the driver stages were going into
>oscillation or anything catastrophic, but it happens repeatedly.
>--scott

Also check the blocking capacitors to the grids of the KT-88's.  They
block the high voltage from the plate of the drivers from the negative
bias on the grid.  When they start to leak, the grid becomes less
negative increasing the current - until meltdown.  They tend to leak
more when warm, so the amp runs OK for a half an hour (or two hours)
and then starts to self destruct.  If they haven't been replaced, they
are due to fail - replace all four.  They're lots cheaper than the tubes!


-- 
Alan Peterman                        (503) 684-1984 (hm)