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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Re: importance of biasing tubes posted by LR-dan on April 11, 2007 at 10:27:40:
If your manual has pictures and schematics you can figure out if your bias is measured as a voltage drop across a resistor. The bias current is equal to the voltage you measure divided by the resistor value. If you have two sets of bias adjustment pots then you most likely have bias adjustment for each EL34 and that is good. Set each voltage reading to the same number for the bias pots if you have two. If the bias current is too high then you will overheat the tubes and shorten their life. Really depends on the amp design but at least keep them lower than the top range for EL34, go look up the data sheet for your EL34 and get the safe operating range numbers for it. Problem is that you need to know what your plate voltages are in order to know the power through your tubes. The power, voltage x current, is what will heat them up real fast.
In some vintage amps like my Dynaco Mark 3, the power supply has dropping resistors that depend on a certain amount of bias current in the output tubes to keep the power supply caps below their maximum operating voltage, the workaround for that is a redo of the caps in the power supply to handle higher voltages, so biasing SOME amps too low to protect the tubes and you could run into trouble. So much depends on your amp circuit design to know how it was designed.
This is about as far as I can take you without seeing the schematic. There are others here that know much more and may have experience with your amp.
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- Re: importance of biasing tubes - LR-dan 11:10:03 04/11/07 (0)
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