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el84 screen voltage


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3/13/2003 6:05 AM
n8b
el84 screen voltage
I just finished up my first SE El84 build (I've done SE 6v6 before and PP 6v6). I'm using a Hammond tranny that puts out 550CT at 50ma with 3 1n4007's on each side to rectify the voltage. When I measure the unrectified voltage with load and tubes, it measures 630V across the secondary. I am getting a B+ of 377v and screen voltage of 388v. The screen voltage goes through a 1k series resistor in the power rail and a 1k resistor to the screen from the power rail. How is it possible that the screen voltage is higher than the B+? Why is the secondary voltage 630v instead of the 550v or slightly higher voltage due to higher line voltage (126v instead of the 115 that the xformer is speced at)?  
 
The amp performs and sounds great under these conditions. The El84 is running at about 16W at 377v plate voltage. Is that a bad thing? Any help or answers are greatly appreciated.  
 
nate
 
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3/13/2003 9:33 AM
pef

Look here  
 
http://www.jj-electronic.sk/tube_el84.htm  
 
JJ EL84 have 300 volts in max plate voltage and 12watts.  
 
Look if the plate don't red glow in full volume, if yes pull down the plate dissipation.  
You need to bias at (12w/377v)*0.7 = 22ma
 
3/13/2003 8:35 PM
Bob R
Since this is a SE amp, I think you don't need to multiply by 0.7 to get your bias. Class A runs at 100%
 
3/13/2003 12:57 PM
Newt

Remember that your plate voltage is dropped by the effective DC resistance of the OT primary.
 
3/13/2003 8:28 PM
jason
nate,  
I'm assuming this single ended amp is cathode biased, is that right? If so, did you subtract the cathode voltage from the plate voltage? This will yeild your true B+.  
As a side note, I've been running my JJ el84's at 16 watts for about a year now. Sound great.  
 
jason
 
3/13/2003 9:41 PM
n8b

>If so, did you subtract the cathode voltage from the plate voltage? This will yeild your true B+.  
 
No, the cathode voltage is 12.9v, so the b+ is only 364 then. That still puts me in the 16w range with a 300ohm cathode resistor. I had a 150 in there with the cathode voltage at 8.8v (around 22w?).  
 
>As a side note, I've been running my JJ el84's at 16 watts for about a year now. Sound great.  
 
That's reassuring. Like I said, it sounds great and performs flawlessly. I was only worried that the voltage was way to high. I've heard of some Mesa Boogie amps that run el84's at around 400v, but I wasn't sure if it was such a good idea.  
 
Thanks,  
 
Nate
 
3/13/2003 11:43 PM
jason
nate,  
12.9/300 = 43mA of current. Remember that not all of that is plate current. A few milliamps of that is screen current so you're still a little lower on the plate dissipation than it seems. If you can get that screen voltage down I think you'll find that 43mA decrease even more.  
Are you using a choke in you power supply or are you using a dropping resistor. If it's a dropping resistor you may want to increase its value a little to drop down that screen voltage a tad.  
jason
 

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