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Old 12th February 2009, 07:46 PM   #11
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Location: Ashland,Ky U.S
I did some experiments with a circuit close to that using a 12au7. It sounded much better running the tube in A2 mode. Very difficult to get decent current or sound quality for that matter out of it at such low plate voltage. A low voltage tube would work much better. As stated above it will give some fairly high distortion since the tube will always be operating in the very nonlinear portion of the plate curves. A 1k pot in the cathode would be better than the 3.3k or 2.2k shown in the schematic. Based on the 6DJ8 datasheet plate curves you will need something like .6 to .7 volts to get 1 ma at around 15volts on the plate, that being if the tube still tests strong. A used tube may need even less to get any current out of it, which limits the amount of input and thus available gain before grid current even further. Using a high voltage b+ for the tube and then capacitor coupling to an output stage will no doubt give better results than the circuit above.
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