drawing a loadline for a power stage EL34 tube. - diyAudio
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Old 28th January 2007, 11:22 PM   #1
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Question drawing a loadline for a power stage EL34 tube.

Hello everyone,

I am a bit confused about using loadlines for power stage tubes. I'm building an EL34 SET with a 6SL7 drive stage. So for the EL34 power stage this is what I have:
B+=450V Ia=50mA Rk=680ohms.(cathode bias), Vgk=-34V. I am using a 5k ohm Hammond 628SEA OPT.


I arrived at Ia=50mA by Vgk/Rk; choosing these two values as mentioned above. This is also within the Pomax rating less i.e less than 25W: 0mA8450V=22.5W.

I want to complete my understanding of tube analysis and so I want to know if my values look right to all you experienced people out there? Also I was trying to draw my load line from 450V to the intersection point between 32V(Vgk) and 50mA(Ia) but the loadline was turning out to be too steep, exceeding Pomax. So this non-conformity between my graphical analysis and algaebric analysis seems to suggest that I'm going wrong somewhere?? Or amybe I;m looking at the wrong plate curves??btw, I am looking at the Phillips EL:34 data sheet downloaded from TDSL, the plate curve states Vg2=360V, Vg3=0V in bold.

I think I have a good understanding of loadlines for drive stages. I usually have my loadline slope equal Rp but things seem different with power stages.

I have a feeling that my values are right and that I'm going wrong with the loadline somewhere. Please help me out people and I would be very thankful to you all.
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Old 29th January 2007, 10:53 AM   #2
Merlinb is offline Merlinb  United Kingdom
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Default Drawing the load line:

Sounds like you're forgetting this is a reactive load, not resistive.

First draw it like a resistive load. When Va = 0, Ia = 450/5000 =90mA.
Draw a line from Va = 450V to Ia = 90mA. This tells you the gradient of the line.
Now slide the load line up the graph while maintaining its gradient (basically draw a line in parallel with the one you just did, but higher up).
You want it to cross through the Vgk point at Va = 450V, Ia = 50mA. This will tell you the Vgk. (Actually it will be a bit less than this because you're cathode biasing, but you should quickly get the hang of it.)

It should look a bit like this:
http://www.freewebs.com/valvewizard1/SELoadline1.jpg
Only with the voltages and load that YOU are going to use.
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Old 29th January 2007, 06:22 PM   #3
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You are building an SET amp. That implies triode operation, so you need to use triode curves. Your earlier statement "the plate curve states Vg2=360V" implies pentode curves. These are available on the original Mullard EL34 datasheet.

The following web site gives a good tutorial on load lines:

http://www.members.aol.com/sbench101/

Scroll down the main page about 2/3 of the way down to load lines parts 1 through 5.

Keep in mind that you will drop 30 to 40 volts across the cathode resistor and 10 to 30 volts across the OPT so your dissipation is less than you think it is. I run EL34's (admitedly a little hot) at 60 - 65 mA with a 450 volts of B+. The measured cathode voltage is 34 to 36 volts with a 560 ohm resistor. With 400 volts actually across the tube this is 25 to 27 watts of dissipation. I use a 5 K (1628SEA) load because I like the sound better than a 3 or 3.5K load. You can start with these values and go from there.
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Old 29th January 2007, 08:30 PM   #4
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Thank you gentlemen!

Regards,
Jason.
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