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Old 24th May 2011, 02:14 PM   #1
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Default This seems wrong?

I was looking around the bay and found a ST-70, This guy shows a pic of it lite up and it looks like it is going to blow up! The tubes are glowing very bright?

Is this right! Or something is causing these tubes to really glow. None of the 18 I have do this?


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Old 24th May 2011, 02:16 PM   #2
SY is offline SY  United States
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Combination of photographic exposure and some red plating of the output tubes. I'd only take that amp if I could get a lowball price.
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Old 24th May 2011, 02:19 PM   #3
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seems dodgy, plus the fact that it is sold as is is extremely suspect. All he needs to do is connect a source and a pair of cheap speakers to test the thing.
Also the pictures show that he has powered up the amp with no load attached to the secondaries.. one of the best ways to blow the OPT if there is any spurious signals...
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Old 24th May 2011, 02:31 PM   #4
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Thanks

I thought it looked funny. If mine did that I would look for a problem.

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Old 25th May 2011, 12:51 PM   #5
bigwill is offline bigwill  United Kingdom
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The guy said he just plugged in the tubes and turned it on, the bias is probably way off, hence the red plates...
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Old 25th May 2011, 01:39 PM   #6
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by quikie22 View Post
Also the pictures show that he has powered up the amp with no load attached to the
secondaries..one of the best ways to blow the OPT if there is any spurious signals...
Hi, this is a complete myth ... what alleged mechanism operates ? , rgds, sreten.
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Old 25th May 2011, 01:40 PM   #7
Doz is offline Doz  United Kingdom
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... or it's a 120V amp on 220v
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Old 25th May 2011, 01:42 PM   #8
Doz is offline Doz  United Kingdom
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Originally Posted by sreten View Post
Hi, this is a complete myth ... what alleged mechanism operates ? , rgds, sreten.
Output tubes push current into the OPT, which is now just an inductor in effect... the reflected voltage is large enough to damage the opt insulation. Whilst I've never done it myself, I have certainly seen a few output tubes destroyed because if it.
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Old 25th May 2011, 01:51 PM   #9
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@sreten: I have not tried it but was advised never to do it. Feel free to try/do it anytime and let us know. Anyway to quote what someone once kindly explained to me...

"Unloaded means basically reflecting an infinite impedance to the output stage. This will make the output signal much higher than normal, potentially causing output tube and socket arcing, and worse, perhaps arcing over of the wires inside the OT secondary, causing OT failure. "
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Old 25th May 2011, 02:08 PM   #10
Loren42 is offline Loren42  United States
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Originally Posted by SY View Post
Combination of photographic exposure and some red plating of the output tubes. I'd only take that amp if I could get a lowball price.
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