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Old 29th February 2004, 09:02 AM   #1
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Default 6N1P preamp too much gain...

Hi guys!

I built this preamp with a 6N1P from a Svetlana Technote. It works well and sounds great. However, it seems that there is too much gain that I can't open my 100K pot. The most is up to 8'oclock only.

1. What formula can I use to determine the gain?

2. Must I increase/decrease the resistance of the plate resistor?

3. What Happens if I increase/decrease the value of the 100uf capacitor?

Thanks for the help!

JojoD
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Old 29th February 2004, 09:19 AM   #2
mig-ru is offline mig-ru  Russian Federation
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Get rid of the 100uF cap, this will lower distortion, and gain. Also a larger value pot could be used, again lower distortion, and more signal impedence. Might try lowering B+ voltage.
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Old 29th February 2004, 09:52 AM   #3
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Ok, so is it true then that when I increase the 100uf cap, gain also increases? And when I decrease it's value (or remove it), gain also decreases?
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Old 29th February 2004, 10:05 AM   #4
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Ok, so is it true then that when I increase the 100uf cap, gain also increases? And when I decrease it's value (or remove it), gain also decreases?
Not true. Changing the value changes the gain only for low frequencies.
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Old 29th February 2004, 10:11 AM   #5
EC8010 is offline EC8010  United Kingdom
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Changing the value of the 100u capacitor doesn't change the gain but the frequency at which it takes effect. The capacitor is a short circuit at AC and prevents an AC signal appearing at the cathode that would cause negative feedback (reducing gain). You can calculate the gain of stage (without feedback) using:

G = uRL/(RL+ra)

Where:

u = the amplification factor of the valve
RL= the anode load resistor
rl = the anode resistance of the valve at the operating point

I don't have data for the 6N1P to hand, but it should be on your technical note.

Removing the 100u capacitor will reduce gain, but greatly increase output capacitance, making the stage incapable of driving cable capacitance. By the sound of things, you have so much gain that you could afford to convert your stage into a cathode follower. This is easily done.

Leave the cathode resistor where it is, but lift the lower end away from 0V. Remove the anode resistor entirely, and connect it between the lower end of the cathode resistor and 0V. Remove all connections to the grid, and connect a 10k resistor as close as possible. Connect a 1M resistor to the other end of the 10k resistor and between the junction of the two resistors coming from the cathode. Connect the anode directly to HT. You now have a self-biased cathode follower, but need to make signal connections. Add a 10n 400V capacitor between the wiper of your volume control and the junction between the 10k and 1M resistors. Move your output coupling capacitor to connect to the cathode. Enjoy the results...
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Old 29th February 2004, 11:58 AM   #6
arnoldc is offline arnoldc  Philippines
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the gain of a stage with unbypassed cathode resistor is:

gain = u/(1 + [(rp + Rk x u) / Rl])

a cathode follower will result in unity gain
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Old 29th February 2004, 12:32 PM   #7
EC8010 is offline EC8010  United Kingdom
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Hello, arnoldc. Your equation isn't quite right because the inversion of the valve means that you see Rk multiplied by a factor of (u+1). That's why you see the (u+1) factor is so many equations such as for Miller capacitance. Thus, your equation should be:

gain = u/(1 + [(rp + Rk(u+1)) / Rl])

Which is perhaps not too elegant, so this might be better:

gain = uRL/[RL + ra + Rk(u+1)]

Either way, with typical values of u, neglecting the +1 factor only contributes a small eror.

As you say, a cathode follower has a gain of (almost) unity.

The diagram's rather nasty, but you can now see how a common cathode stage can be converted into a cathode follower.
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Old 29th February 2004, 04:05 PM   #8
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Quote:
Originally posted by EC8010
Hello, arnoldc. Your equation isn't quite right because the inversion of the valve means that you see Rk multiplied by a factor of (u+1). That's why you see the (u+1) factor is so many equations such as for Miller capacitance. Thus, your equation should be:

gain = u/(1 + [(rp + Rk(u+1)) / Rl])

Which is perhaps not too elegant, so this might be better:

gain = uRL/[RL + ra + Rk(u+1)]

Either way, with typical values of u, neglecting the +1 factor only contributes a small eror.

As you say, a cathode follower has a gain of (almost) unity.

The diagram's rather nasty, but you can now see how a common cathode stage can be converted into a cathode follower.

Thank you very much for the schematic! The 6N1P Datasheet says Anode resistance is 4400 ohms (nominal), and u = 33, when I use it to your formula I get gain of 20. Am I on the right path?

Thank you for your patience,
JojoD
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Old 29th February 2004, 04:34 PM   #9
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by JojoD818
The 6N1P Datasheet says Anode resistance is 4400 ohms (nominal), and u = 33, when I use it to your formula I get gain of 20.
That's right, removing the cathode resistor will drop the gain to 20. Actually, it will probably be a little less because valve manufacturers always quote ra at maximum current and a low anode voltage, which gives a lower figure than you usually obtain in practice. But if you are only able to turn your volume control to 8 o'clock, this drop in gain will be almost unnoticeable. I really think you ought to try configuring the stage as a cathode follower...
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Old 29th February 2004, 07:21 PM   #10
DougL is offline DougL  United States
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The 6N1P Datasheet says Anode resistance is 4400 ohms (nominal), and u = 33, when I use it to your formula I get gain of 20. Am I on the right path?
I added the tube to Tube Cad.

It got a gain of 17 unpypassed and a gain of 29 bypassed, or 24.6 dB vs 29.3 dB. The Cap adds about 5 dB to the gain.

The other thing that changed was the output Z.
It went from 4 k with the cathode bypacss cap to 15k unbypassed. Along with a calculated current of 1.9 mA, this circuit would not be a candidate for driving long or highly capative loads.

By the way, Tube cad recomended a 70 uF Bypass, so the 100 uF
seems realistic.

HTH

Doug
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