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PP KT88, Need help!
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Old 1st December 2012, 12:38 AM   #41
12E1 is offline 12E1  United Kingdom
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Location: Near Huntingdon, Cambridgeshire, UK
Originally Posted by EEApprentice View Post
I haven't been able to check anything when the feedback is connected for that channel. It is so loud I have to turn it off and I don't have a resistor box handy to load the output.
I wouldn't worry about a resistor box as such - many resistor boxes are not designed to handle significant power. I would find a pair of 8 ohm or similar wire-wound resistors rated to 25 or 50 watts. They can be ceramic, but I like using the aluminium clad ones since they can easily be bolted onto a more substantial piece of metal. Here's some.
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Old 1st December 2012, 12:55 AM   #42
scott17 is offline scott17
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It does seem very possible that your left channel output transformer primary is wired in reverse compared to the right channel. That would give you positive feedback.

Right channel could be blocking distortion. http://www.aikenamps.com/BlockingDistortion.html

Last edited by scott17; 1st December 2012 at 12:57 AM.
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Old 1st December 2012, 07:49 PM   #43
EEApprentice is offline EEApprentice  United States
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Join Date: Sep 2012
Scott17 I will forever love you.

THE AMP IS FIXED! Not only that, it COMPLETELY exceeded my expectations. I cannot believe I've spent weeks and weeks trying to fix something that was just the primary of the left OPT wired in reverse. It was positive feedback all along!

I also added 1.7k grid stoppers to all my KT88s and replaced the capacitors for the fluttering channel.

THANK YOU EVERYONE! I was so disappointed when this thing didn't work but thanks to everyone throwing their ideas, it is finally fixed... And it sounds AMAZING.

Thank you again!
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Old 1st December 2012, 07:54 PM   #44
DF96 is offline DF96  England
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Join Date: May 2007
Getting the feedback phase wrong is a very common mistake in a new DIY amplifier. Occasionally the OPT secondary will be mislabelled. That is why we asked you to check it.
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Old 1st December 2012, 10:17 PM   #45
scott17 is offline scott17
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I glad you've got it working and that you are happy. Fantastic.

I guess it's burned into you now to check that next time around.

Last edited by scott17; 1st December 2012 at 10:22 PM.
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