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Old 23rd February 2008, 04:12 PM   #1
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Default Independent Bias Adjust for Each Tube?

Aside from the obvious....asymmetrical current draw (especially with aging tubes), what other benefits from independent output tube biasing adjustments are there?

Thanks,

John.
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Old 23rd February 2008, 04:54 PM   #2
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Doh! I forgot to mention that this thread applies to push-pull amps.
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Old 23rd February 2008, 05:51 PM   #3
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That's about it: balance the currents on both sides of the center tap, minimize residual DC core magnetization. It's especially important when using toroidals, since these have no gap whatsoever and are highly prone to core saturation.

Without that DC mag getting in the way, the bass performance of the OPT improves since the core isn't so easily saturated.
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Old 24th February 2008, 12:47 PM   #4
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With separate bias controls you don't need to have the output tubes perfectly matched. Most guitar amps use cathode bias so matched valves are mandatory. I was told by a veteran tube man, now passed over, that the trick was to set volume to minimum then adjust each bias control for the required tube current it controlled. After this, tweak one of them very slightly for minimum 120Hz (100 Hz in UK) output on the speaker terminals to achieve current balance. That's the core saturation bit as described in the previous post
Les
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Old 24th February 2008, 12:57 PM   #5
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'dual' bias has proved to be godsend when plugging in NOS/rare output tubes into my (modified) Dynaco Mark IIIs.
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