12B4A Linestage & PSU - diyAudio
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Old 26th August 2003, 11:50 PM   #1
G is offline G  United States
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Default 12B4A Linestage & PSU

Here is my modest attempt at designing a linestage with the 12B4A tube. I think it will give good performance. The Rp is so low that there shouldn't be too much distortion at the output. The gain of this circuit is low ( 4? 5? ) but I don't need much if any. I decided on a anode follower because evidently CF don't have very good PSRR. So I think I will give this a shot a see how it sounds. I'll let you know.

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Old 27th August 2003, 01:02 AM   #2
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Hey-Hey!!!,
I will have to say that the 12B4 is a good linestage tube. I personally like the 12SN7 better, but that is not important at this time. In the experimenting I did with the 12B4, I found that 180-200 volts on the plate sounded best at 10 mA of current. LED biased to near 30 volts at the cathode( 14 red ones, one yellow). I never tried this one with a resistive load, and went straight to cascoded mosfets.
I also use the 12B4 as a driver stage in the PP amps, choke loaded and common cathode, diff-pair rigging( CT choke ). The tube exhibits less increase in plate resistance as voltage increases than most( actually, any that are comparable in performance )., and can stand 5-600 volts on its plate without worry. And the open anode structure lets those big cathodes glow for all to see. Toooo cool!!!
regards,
Douglas
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Old 27th August 2003, 02:20 AM   #3
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Not too bad at all. But contrary to Douglas' opinion, I fond the 12B4A in a linestage runs better with about 100Va-k and about 25 - 35mA. So my suggestion would be to adjust the bias resistor down to about 440R (vg = -11V) and adjust the dropper in the PSU to get it to 310V again. Gain will be around 5 and the output Z ~ 900R

Better still would be to ditch the last RC in the PS and up the value of the anode R to about 10k keeping the current around 25mA. You'll need a 25W resistor for that; 2 x 5k 12W Mills in series or 2 x 20k in // would be fine.
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