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Old 17th March 2014, 01:04 AM   #1
IdaCap is offline IdaCap  United States
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Default Possible bad tube?

First time tube amp builder and thanks to all for the guidancefrom this forum to get this done this far. Just finished up my SSE build and ran it through the checkout all voltages and resistors read good. Fired it up, good sound decent volume through my cheap shop speakers. Played for about 30 seconds after warming up and one tube started red plating. Got four or five good thumps and then the right channel went out. The plate/grid glowed brighter with each thump and then the sound stopped. I then shut off the amp and allowed it to cool and checked everything again. Same thing resistors were at the correct levels voltage was correct as well. Switched tubes to alternating socket and the problem followed. Bad tubes? Starting with basic JJ EL 34S as they were cheap. Basic design suggested by George for the first start up. Using big edcors CXSE 25-8-5K for the OPT and edcors XPWR 035 PT, triad choke, no motor run cap and no cfb or ul connected yet. Any thoughts, bad tube since the problem followed the tube? Thanks in advance for the help.
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Old 17th March 2014, 02:53 AM   #2
IdaCap is offline IdaCap  United States
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Default Pics of Tube

Tube on the left has the problem. Getter Looks fine.
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Old 17th March 2014, 03:03 AM   #3
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I would vote for a bad tube.
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Old 17th March 2014, 03:06 AM   #4
AJT is offline AJT  Philippines
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yes a ba tube most likely, but first check that you do not have a bad solder joint which shows up as the amp heats up....
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Old 17th March 2014, 03:23 AM   #5
IdaCap is offline IdaCap  United States
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Solder joint wouldn't follow the tube when I change sockets would it?
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Old 17th March 2014, 03:40 AM   #6
AJT is offline AJT  Philippines
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when i encounter a seemingly bad tube, i try to investigate first what could be the cause,

it could be that the bias on the tube is such that your plate dissipation is more than it should,

one easy way is to put that red-plating tube to a hole that has good tube and see...

or you can just simply replace it with a known good tube and see...
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Old 17th March 2014, 03:45 AM   #7
IdaCap is offline IdaCap  United States
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Done that and the tube was bad in the other channel as well. Thanks. Guess I will wait for my replacements to come.
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Old 21st March 2014, 04:15 AM   #8
IdaCap is offline IdaCap  United States
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Default New tubes worked

Got another pair today and the problem was a bad tube. All good with EH 6550's installed. Way better looking tubes too.
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Old 26th March 2014, 04:50 PM   #9
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Quote:
Originally Posted by IdaCap View Post
Solder joint wouldn't follow the tube when I change sockets would it?
No... but if a bad solder joint caused the tube to red plate and in so doing damaged the tube (an internal short for instance) and you moved it to the good channel, of course it "followed" the tube it would seem. The bad tube could then damage components in the good channel. When a tube does what you described, it is BAD, kaput, no more, off to meet it's maker... So there's no point in moving it.


And if there is a bad solder joint, it could damage the good tube, if you swapped them. Always fault check first.

Last edited by cogsncogs; 26th March 2014 at 05:02 PM. Reason: added damage good tube
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