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Old 15th February 2012, 06:47 PM   #1
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Default Question about Shishido 2A3

Need some help understanding the purpose of the 22k 3W resistor between the cathode of the 2A3 and the 150K plate load/voltage dropper for the 12AX7.

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Old 15th February 2012, 07:14 PM   #2
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By applying some positive voltage to the cathode, the correct voltage relationship between the 2A3's grid and cathode is maintained. As the tube ages, less cathode current would be drawn resulting in less negative voltage appearing on the grid. (with respect to the cathode) This is especially important because the tube's grid is directly coupled to the previous stage.
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Old 16th February 2012, 02:39 AM   #3
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It has to do with the fact that 195V across 3K is 65mA, so the 22K resistor provides the other 5mA so that the 2A3 doesn't have to.

The 22K resistor could be omitted if a 3250 ohm cathode resistor was used instead, in fact 3.3K is probably more than close enough. You would have to adjust the value of that 25K resistor to compensate for the greatly reduced current however. (Change to about 120K)

You might also consider a 6SL7 in the first stage, you'll only need to tweak its cathode resistor value to get the right plate current in the first stage. (I'd probably run half a 6SL7 per channel)
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