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6N1 preamplifier with phono
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<blockquote data-quote="handsome" data-source="post: 709462" data-attributes="member: 772"><p>If you don't mind me commenting on your build sir, and if you do please ignore the following (or just tell me to STFU)</p><p></p><p>18mA is way too high for a 12AU7 you saved the resistor but you are going to cook the poor tube :flame: if your cathode resistor has 180V across it then the tube has only 120V across it and with 18mA of current that means it is biased positive and running extremely hot..... </p><p></p><p>I assume your Ht is ~300V so try this: choose a resistor value for the first tube's anode and the second tube's cathode - make it between 22k to 39k (good values for a 12AU7). Then adjust the value of the first tube's <em>cathode resistor</em> until you get the same current through each tube (between 5 and 10mA) OR aim to get approximately a third of the HT voltage across the second tube's cathode. </p><p></p><p>The CCDA is just a special case Cathode Follower direct coupled to a Common Cathode stage - special because we aim to balance the AC currents. In any direct coupled layout the anode voltage of the first tube sets the bias conditions for the second tube. So by choosing a reasonable load resistor value you can then adjust the first tube's cathode resistor to then set the conditions for both tubes. </p><p></p><p>You will have to ascertain the first tube's anode voltage from the datasheet curves as trying to measure the voltage on the cathode follower's grid is not possible as the multi meter's impedance will create an error.</p></blockquote><p></p>
[QUOTE="handsome, post: 709462, member: 772"] If you don't mind me commenting on your build sir, and if you do please ignore the following (or just tell me to STFU) 18mA is way too high for a 12AU7 you saved the resistor but you are going to cook the poor tube :flame: if your cathode resistor has 180V across it then the tube has only 120V across it and with 18mA of current that means it is biased positive and running extremely hot..... I assume your Ht is ~300V so try this: choose a resistor value for the first tube's anode and the second tube's cathode - make it between 22k to 39k (good values for a 12AU7). Then adjust the value of the first tube's [i]cathode resistor[/i] until you get the same current through each tube (between 5 and 10mA) OR aim to get approximately a third of the HT voltage across the second tube's cathode. The CCDA is just a special case Cathode Follower direct coupled to a Common Cathode stage - special because we aim to balance the AC currents. In any direct coupled layout the anode voltage of the first tube sets the bias conditions for the second tube. So by choosing a reasonable load resistor value you can then adjust the first tube's cathode resistor to then set the conditions for both tubes. You will have to ascertain the first tube's anode voltage from the datasheet curves as trying to measure the voltage on the cathode follower's grid is not possible as the multi meter's impedance will create an error. [/QUOTE]
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Commercial Members
Commercial Members - News & Discussion
6N1 preamplifier with phono
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