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checking bias on single-ended EL84 amp


 
5/29/2000 1:07 AM
Fred G.
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checking bias on single-ended EL84 amp
How do I check the bias on a single-ended EL84 amp ? I've only worked on PP Fenders, so I have no experience checking bias on the single-ended stuff. Where do I hook up my meter, what settings do I set it to, and what kind of readings do I want to see ?  
 
Thanks and regards,  
 
Fred G.
 
5/29/2000 1:45 AM
Bruce
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If it's cathode biased, just measure the cathode resistor to ground when the amp is off, and then with the amp on and hot, measure the DC voltage from the cathode to ground of the EL84 and divide that voltage by that measured resistance of the cathode bias resistor.  
That's the current through the tube.  
Bruce
 
 
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5/29/2000 4:12 AM
Fred G.
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Thanks, Bruce.  
 
I get a resistance of 154 ohms, a voltage of 7 VDC. So, 7/154=.045, 45 milliamps, then ? I'm reading 276 VCD on the plate. From the chart I'm looking at, an EL84 at this plate voltage wants to see about 41 mA. So, I need to replace the existing cathode resistor with a 180 ohm one, then ( please correct me if I screwed up the calculation here )! What wattage resistor should I use here - 2W or 5W, or ?  
 
Thanks and regards,  
 
Fred G.
 
5/29/2000 2:36 PM
jason
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Fred,  
Since this amp is cathode biased the plate voltage is equal to plate voltage - cathode voltage, or in your case...276-7= 269v.  
As for the cathode resistors wattage, I usually use a 10watter there (Never had el84 cathode biased amp before, only 6v6...both SE and pp), probably overkill, but better safe than sorry. You can probably get away with the 2 watt in your case.  
jason
 
5/29/2000 3:16 PM
Eric H
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Jason, my amp is 2-6V6 SE. I checked this about 2 weeks ago and got 78 ma (39ma each) with a plate voltage of 320 (200ohm resistor). IIRC thats 75-80 percent max dissipation? Is this too high?  
 
-Thanks,  
Eric
 
5/29/2000 3:50 PM
jason
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Is the plate voltage of 320 taking the cathode voltage drop into account. If not, 78maX200 ohms is around 15.6 volts so the plate voltage is really 304.4v. 304x39mA is 11.8 watts disipation per tube. One thing to remember with SE amps...they are class A so the old "bias to 70%" no longer is the deal. Class A amps are biased right at max disipation at idle, because unlike a class AB PP amp, disipation will not increase with an applied signal.  
jason
 
5/30/2000 1:14 AM
Eric H
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Thanks a lot, Jason.  
 
-Eric
 
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