Best plate voltage for 6dj8 at +-6ma - diyAudio
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Old 15th January 2009, 04:34 PM   #1
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Default Best plate voltage for 6dj8 at +-6ma

Hi, somebody can help me to find the best plate voltage for 6dj8 runing at +-6ma with plate CCS (1st input stage preamp class A) .

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Old 15th January 2009, 05:05 PM   #2
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Before somebody else jumps in and tells you exactly where you should run your tube I'd like to encourage you to experiment a bit. It's easy to try different operating points and ultimately, it's the only way to know for sure - and especially simple if you already know the current you want to run.
Put a constant current source on the plate of the tube set for your desired 6mA. Then, just by changing the cathode resistor so that IR=Vbias wherever you want to check out on the plate curve chart at 6mA you can get different corresponding plate voltages. It frees you up from depending on people's advice that may not actually work as well for your application as it did for theirs.
It's also a good part of the fun.
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Old 15th January 2009, 05:10 PM   #3
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Quote:
Originally posted by Hearinspace
It frees you up from depending on advice that may not actually work
IMHO that ultimate freedom lies in a datasheet, ruler, and pen.
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Old 16th January 2009, 09:10 AM   #4
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To operate 100v\6ma, in the cathode I have 270r. In parallel with a capacitor of 1,5uf, to improve the medium frequencies significantly. The tube that I have (Tronal), has excellent basses, and high, but it asks for a little in the medium, from there the reason of the capacitor.

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npr
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Old 16th January 2009, 09:16 AM   #5
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With that value of cathode bypass, you have a preamp with a fixed tone control with a 400-500Hz turnover. Is that intentional?
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Old 16th January 2009, 09:52 AM   #6
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Hi Sy. Yes. In the calculations that I made I got 2500hz (I can be wrong).
Of the few tubes (preamp) that until today I tested, 6dj8 is distinguished for the basses, but it loses a little in the remaining frequencies (my opinion).
I like to place the control of volume between the 1 and the 2 stage, with 10k or 4k7 pot, and for this requirement of current I think that 6dj8 is a good candidate.
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Old 16th January 2009, 10:19 AM   #7
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Sy, your calculations are correct.
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Old 16th January 2009, 11:43 AM   #8
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Well, maybe I'm not correct. If you're following the CCS'd 6DJ8 with a 4k7 pot, that pot is effectively the plate load. This means that the tube's impedance shunts the 270R with 4k7/33 = 142R. So the effective resistance is 142R in parallel with 270R = 93R. That puts the turnover at 1140Hz.
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Old 16th January 2009, 01:18 PM   #9
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If you connect the pot after a coupling capacitor then your initial calculations remain valid?
(1\(2*pi*270*0,0000015)=393hz
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Old 16th January 2009, 01:25 PM   #10
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Assuming that the f3 from the coupling capacitor is well below the turnover, then the pot is effectively the plate load for purposes of the calculation (and for purposes of load lines).
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