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Old 17th May 2008, 12:07 PM   #1
bigwill is offline bigwill  United Kingdom
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Default I'm running my leak stereo 20 single ended. WHAT???

I took one of the EL84s out in each channel of my stereo 20. It goes just as loud and actually sounds much BETTER, especially in the way it clips. WHAT?


WHAT??!
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Old 17th May 2008, 10:56 PM   #2
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Perhaps you just changed the distortion from symmetrical (odd order) to non- symmetrical (even order). This is largely the difference between single ended and double ended(push-pull) output stages, although probably to an even greater degree in your instance since the output transformer was not designed for this.

Since each valve has his own cathode resistor, no harm will come from running the amp this way. If the cathode resistor were shared by both valves the current though your lone 6BQ5 would be too high and would probably result in an angry red plate.

Wade
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Old 18th May 2008, 12:50 PM   #3
bigwill is offline bigwill  United Kingdom
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It still sounds clean, and the bass is still present in spite of the magnetic bias in the OPT. When I tried it I just did it for a laugh to see what it sounded like but I was surprised that it actually sounded pretty good! I did check the schematic before pulling out one half of the output stage to ensure nothing harmful would happen and as you said, it is safe to run it like this and quite frankly I enjoy the sound!
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Old 18th May 2008, 05:38 PM   #4
dnsey is offline dnsey  United Kingdom
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Be careful - the ST20 output transformer is fairly marginal for the original design, and often fails. Putting a DC offset through it isn't going to help
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Old 18th May 2008, 08:09 PM   #5
bigwill is offline bigwill  United Kingdom
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Could you elaborate? I'm having trouble understanding how it could potentially damage the OPTs. Does having a magnetic bias present a problem? (Other than possibly magnetising the core)
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Old 18th May 2008, 09:45 PM   #6
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I'd be surprised if it did harm to the OPT. On the other hand, it will relieve the power transformer of some of its load, which might not be a bad thing.
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Old 19th May 2008, 06:44 PM   #7
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally posted by bigwill
Could you elaborate? I'm having trouble understanding how it could potentially damage the OPTs. Does having a magnetic bias present a problem? (Other than possibly magnetising the core)
That's exactly it- by having a DC current flowing in one half only you permentantly magnetise the core. This means that the transformer is then pushed heavily towards core saturation, and therefore LF distortion and even overheating.
I'm not sure how you degauss a transformer. Maybe you can just run it normally, but at low levels for several hours?
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Old 19th May 2008, 06:52 PM   #8
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You could just buy some relatively inexpensive SE transformers and plunk them in if you plan on making the switch permanent... If you've decided you like the sound, you should do that instead of messing with your PP transformers.
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