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27th August 2010, 05:35 PM  #1 
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Calculating output impedance
6080 in common cathode mode: I'm making calculations for output impedance.
I understand the plate resistor should be ~x10 times the plate resistance. 6080: 280 ohms so I need around 2800 ohms resistor here. output imp: Zout: 280 x 2800/280 + 2800 = 254 ohms If I parallel 2 tubes this is halved? Also I wanted to use a CCS instead of a plate resistor but then these have a very high impedance no? This would significantly increase the output impedance?? 
27th August 2010, 05:38 PM  #2 
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Can I take a stab at this?
 two triodes in parallel = 1/2 output impedance  CCS in plate, output resistance = very close to tube's internal plate resistance How'd I do?  
27th August 2010, 05:46 PM  #3  
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27th August 2010, 05:49 PM  #4  
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Note two things: plate resistance is NOT a constant look at the graphs of plate resistance versus current on the datasheet to get an accurate value. Second, the effective plate resistance is equal to the plate resistance only if the cathode is at AC ground. If it's degenerated (e.g., unbypassed cathode resistor), the effective plate resistance will be the sum of the plate resistance and mu times the resistance in the cathode circuit.
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27th August 2010, 05:51 PM  #5  
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Quote:
2 x 6080's in parallel Zout would be ~140 ohms more or less with a good CCS. Cascode DN2540? 

27th August 2010, 06:04 PM  #6  
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So for a 5687 with a 200 ohm cathode resistor left unbypassed, rp (if bypassed) = 2500 mu = 16 Rk = 200 rp + mu = 2516 2516 * 200 = 503,200 503,200 ohms!? That can't be right. Where did I go wrong? Do you have to figure the cathode resistance as the tube's rk in parallel with the cathode resistor?  

27th August 2010, 06:17 PM  #7  
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same here with the 6080. The Zout ends up as a very high number is cathode resistor is unbypassed.
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Last edited by Brit01; 27th August 2010 at 06:19 PM. 

27th August 2010, 06:29 PM  #8  
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(mu + 1)Rk = 17*200 = 3400 ohms. rp = 2500 ohms Total effective plate resistance is then 3400 + 2500 = 5900 ohms.
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27th August 2010, 06:30 PM  #9  
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Were it truly 500K then this could make one heck of a ccs which it really doesn't..
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"Extraordinary claims require extraordinary evidence."  Carl Sagan Last edited by kevinkr; 27th August 2010 at 06:32 PM. 

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