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Old 29th April 2004, 11:26 AM   #1
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Default cf topology lessons... pls help

Hi guys!

I built the circuit below using the part values indicated in the schematic from an old publication. First of all it works, I measure a B+ of 148.8VDC at the plate to ground of the tube and 4.2VDC from cathode to ground (across 22K).

Can anyone give me the mathematical formula for computing the gain of this circuit?

How do I choose the value of the cathode resistor if I will use different tubes? I would like to learn how to effectively use the graphs in tube datasheets for this particular circuit.

Thanks in advance!

JojoD

PS, I know the supply is a bit low but this is an experiment and I don't want to blow things up especially my tubes.
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Old 29th April 2004, 11:38 AM   #2
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Default CF

Generally speaking, Cathode Followers have no, or a slight reduction in gain, What they do have is the ability to match up a high impedence source to a lower imp load.

If you need gain, you could precede this stage with a common cathode gain stage using the other half of the 6N1p
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Old 29th April 2004, 11:56 AM   #3
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Default Re: CF

Quote:
Originally posted by Alastair E
Generally speaking, Cathode Followers have no, or a slight reduction in gain, What they do have is the ability to match up a high impedence source to a lower imp load.

If you need gain, you could precede this stage with a common cathode gain stage using the other half of the 6N1p

But how do I choose the cathode resistor for optimum tube performance?
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Old 29th April 2004, 01:18 PM   #4
s2kov is offline s2kov  Canada
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JojoD,

You have to determine first your operating point of the tube you will use. By then, you can compute for the value of cathode resistor.

Of course, Ohm's Law will always be here....
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Old 29th April 2004, 02:00 PM   #5
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Quote:
Originally posted by s2kov
JojoD,

You have to determine first your operating point of the tube you will use. By then, you can compute for the value of cathode resistor.

Of course, Ohm's Law will always be here....

Please elaborate. Let's use the 6N1P tube as an example for this exercise.
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Old 29th April 2004, 02:42 PM   #6
s2kov is offline s2kov  Canada
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I don't have datasheet for 6N1P.... But assuming you're using Plate Voltage and Current of 148V and 15mA respectively. Corresponding bias voltage is, say, -6V then you can compute for Rk using this formula:

Rk = Vg/Ia

Where:

Rk = Cathode Resistor
Vg = Grid Voltage
Ia = Plate Current

Please note that above figure is just an assumption, just focus on the above formula base on your operating point that will be used.

Tube Gurus,

Please correct me if i'm wrong....


Quote:
Originally posted by JojoD818



Please elaborate. Let's use the 6N1P tube as an example for this exercise.
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Old 29th April 2004, 04:17 PM   #7
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I hope this can help us in this exercise...
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Old 29th April 2004, 04:44 PM   #8
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Something like this?

Given:
300V on plate
bias 15mA

4V / 15mA

Rk = 266R

Try to search for grounded cathode amplifier at www.tubecad.com.
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Old 29th April 2004, 06:04 PM   #9
s2kov is offline s2kov  Canada
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Another one:

Plate Voltage = 200V
Plate Current = 10mA
Grid Voltage = -2V

Rk = 2/0.01 = 200R


Hope this help....
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Old 29th April 2004, 07:01 PM   #10
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Quote:
Originally posted by s2kov
Another one:

Plate Voltage = 200V
Plate Current = 10mA
Grid Voltage = -2V

Rk = 2/0.01 = 200R


Hope this help....
This settings would be better, as the maximum anode voltage is 250VDC (from the datasheet)
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