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Old 5th November 2011, 11:22 PM   #1
Trileru is offline Trileru  Romania
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Default global negative feedback in el84 push pull

Hello all.
I am trying to apply global negative feedback as described in the Bevois Alley amp by Morgan Jones and I can't figure out the Rl of the input stage and Rl of the concertina. How do I calculate each?
Another question is in the formula needed to find rk :

rk=(Rl+ra)/u+1 ... that u is the gain coefficient of the tube or the actual gain measured at one of the outputs of the concertina? The 6n1p-ev dictates a u of 35 while I measured 24.8 in my amp. Witch one should I use?
My soundcard outputs 1.2V measured at full volume. What is the full voltage needed for 12.5W output? So I can adjust the gain of the amp accordingly.
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Old 6th November 2011, 12:15 AM   #2
Trileru is offline Trileru  Romania
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here is my current setup
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Old 6th November 2011, 10:46 AM   #3
SY is offline SY  United States
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That rk is the "looking in" impedance at the cathode. For the purposes of feedback calculations, it's effectively in parallel with the cathode resistor. The u is the mu of the tube.

By Rl, do you mean the plate load resistors? Those are derived by playing with load lines on the tube's plate curves.
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Old 7th November 2011, 06:18 AM   #4
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What is the full voltage needed for 12.5W output? So I can adjust the gain of the amp accordingly.
The open loop gain of the schematic you showed is some 43 ( 32,7 dB)
assuming you have typical 8k to 8 ohm output transformer.
With 5k6 feedback resistor from the secondary to the cathode of 6N1P the GNFB is 11 dB and total gain is 21,7 dB (A=12.2). Thus the 12.5 W output (10 Vrms) requires some 0.82 Vrms input level. Feedback resistor should have small 47...100pF capacitor in parallel depending on the OPT you use.
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