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Old 30th November 2008, 03:47 AM   #1
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Join Date: Apr 2008
Default Help! May have dropped 12sn7 in place of 6sn7

I bought what I thought were Philco branded "Bad boy" 6sn7's off ebay, not labeled 6sn7 or anything at all, and plugged them in my yaqin mc-100b. They lite up just fine, but one had no sound until I tapped it, then she went. Next day sounded like it was shorting out, distorting and such. On the next fire up she went perfect as drivers, but when I put them back into the gain stage the one didn't work, and next thing the other only worked when the amp was powering down and turned off.....weird! The end result was the left channel no longer works with any tubes. I think they may have been 12sn7's or other, because during this I would occasionally place "correct working" tubes to check nothing bad was happening, but of course something bad did happen, any ideas what? Does this sound like what would happen with the wrong tube?

Here's the schematics from DIY
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Old 30th November 2008, 03:45 PM   #2
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Join Date: Jun 2002
Location: Macedon NY
12SN7 won't light visibly on 6.3V. And you certainly can't hurt an amp by plugging a 12SN7 into a 6SN7 socket (might hurt your ears with distortion, depending on circuit...). But a SHORTED tube... that COULD zap something, depending on what's shorted and the circuit it's in.
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Old 1st December 2008, 02:47 AM   #3
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Join Date: Apr 2008
That could be, but I'm not 100% sure about the 12sn7, it could be anything that resembles a "bad boy" but I'll bet it's just shorted the way the one was acting, the other seemed OK and sounded good. I opened up the amp and didn't notice anything visual out of the ordinary, I'll check the power tubes next all the pre tubes are good ones now. Then attempt a diagnosis.
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