Hi all, I've made a valve output stage for my DAC but don't feel entirely confident I've done the calculations correctly. It is based on a parallel single ended, grounded cathode design borrowed from Andrea Ciuffolis audiodesignguide website - DAC ended - cheap version but using 6n6p. Using formula from valve wizard site I have B+ of 270v I've used 3750k anode resistor, this then connects to the anode of each triode half of 6n6p, and 170r on each separate cathode to ground with 470uf oscon cathode bypass (just because I had that value handy) I was aiming for 20ma operating point. Can anyone advise if this looks about right please? Cheers T
> 3750k anode resistor
Over 3 megs?? That can't be right. Especially for 20mA(!!) current.
Surely more like a few K-Ohms?
270V at 20mA is 13,500 Ohms or 13.5K. To allow 20mA to flow, your plate resistor must be much less.
Taking real-simple concept, plate should sit at ""about"" half of B+. Half of 270V is 135V. If you truly want 20mA, 135V/20mA is 6,750 or 6.7K.
Is 20mA at 135V even possible with this tube? Assuming this datasheet it is. Bias for my half-voltage dart-toss is 4V, so 200 Ohms Rk.
> 3750k
3.7K at 20mA is a 74V drop from 270V, leaving plate at 196V. This is super-close to the Pdiss melt-down limit. I would not run a tube that hot if I could avoid it. But if you have a whole box of these tubes, you could. (I did, in the 1970s when TV Tuner tubes were being thrown-out.) Looks like 8V bias, and 8V/20mA is 400 Ohms, so something does not add-up.
Over 3 megs?? That can't be right. Especially for 20mA(!!) current.
Surely more like a few K-Ohms?
270V at 20mA is 13,500 Ohms or 13.5K. To allow 20mA to flow, your plate resistor must be much less.
Taking real-simple concept, plate should sit at ""about"" half of B+. Half of 270V is 135V. If you truly want 20mA, 135V/20mA is 6,750 or 6.7K.
Is 20mA at 135V even possible with this tube? Assuming this datasheet it is. Bias for my half-voltage dart-toss is 4V, so 200 Ohms Rk.
> 3750k
3.7K at 20mA is a 74V drop from 270V, leaving plate at 196V. This is super-close to the Pdiss melt-down limit. I would not run a tube that hot if I could avoid it. But if you have a whole box of these tubes, you could. (I did, in the 1970s when TV Tuner tubes were being thrown-out.) Looks like 8V bias, and 8V/20mA is 400 Ohms, so something does not add-up.
DAC End 2 - the AD1865N-K with single ended vacuum output stage
This was basic topology I followed, in my stage, using 6n6p R1-R4 have been replaced by 4 x 15k giving 3k750 and R5/R6 are 170R
This was basic topology I followed, in my stage, using 6n6p R1-R4 have been replaced by 4 x 15k giving 3k750 and R5/R6 are 170R
I should add that I am very pleased with the sound I have. My concern is that the unit has now been running for about 4 weeks (not constantly of course) and it seems the output level has reduced a little over this time. My concern is that I have set a bad operating point and that valves are being stressed/damaged.... Cheers
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