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Old 2nd April 2011, 03:47 AM   #1
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Default Did I do this loadline correctly

I have an amplifier that has jumpers that are intended allow one to select between 6SN7 and 6SL7 for the first stage tube. Since I built a lightspeed clone I thought I'd give the 6SL7 a go for the extra gain. I first wanted to have a look at the loadline and I didn't like what I saw. I'm not really sure I drew them correctly, especially given how horrid the factory specs look.

The first attachment is for the 6SL7 with a PV of 268, a 68K plate resistor and a 3 volt bias. I measured these values.

The second attachment is what I think might work better. It's still 268 PV but I changed the plate resistor to 200K and changed the bias to 1.5 volts.

Question is, did I do it correctly and is my version the best choice?

Thanks
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File Type: jpg 6SL7_268PV_68K_3v.jpg (294.3 KB, 293 views)
File Type: jpg 6SL7_268PV_200K_1.5vbias.jpg (292.9 KB, 280 views)
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Old 2nd April 2011, 03:55 AM   #2
tomchr is offline tomchr  United States
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The 68k load line with bias at 3.5 V will result in quite a bit of distortion. Note how the load lines bunch up near the end of the load line.

It looks like the second load line (1.5 V) is actually 100 kOhm not 200 kOhm. But that looks like a reasonable operating point.

I've run 6SL7's at 1.7 V cathode bias (red LED) and a 2~3 mA CCS. Works like a charm.

~Tom
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Old 2nd April 2011, 06:06 AM   #3
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Quote:
Originally Posted by tomchr View Post
It looks like the second load line (1.5 V) is actually 100 kOhm not 200 kOhm. But that looks like a reasonable operating point.

~Tom
Yeah Tom, that's a typo but I had intended it to be 120K. 268/120=2.23ma
I used that resistance to get the load line more perpendicular to the lines of constant Ec.

I like your operating point better, see first attachment, as it is nearly equal each side of the bias voltage and with a higher bias value.

The second attachment is 2 volt bias which might work even better for me since I'm not using LEDs. Hummm...well maybe...

So, somewhere I got the idea that the load line should be perpendicular to the lines of constant Ec for lowest distortion, but I can't remember now where I got that idea. Is there any reason why that would be so?
Attached Images
File Type: jpg 6SL7_268PV_100K_1.7bias.jpg (292.7 KB, 258 views)
File Type: jpg 6SL7_268PV_100K,2vBias.jpg (292.9 KB, 234 views)

Last edited by Captn Dave; 2nd April 2011 at 06:21 AM.
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Old 2nd April 2011, 06:29 AM   #4
Svein_B is offline Svein_B  Norway
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Do not forget to also consider your AC load-line.

The AC load will be the impedance of the load (next stage grid-leak) in parallel to the plate resistor.

Illustrated with the red line added to your chart.

If the load is 500K or higher, the effect will however be small.

--
SveinB.
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File Type: gif AC-loadline.gif (156.2 KB, 237 views)

Last edited by Svein_B; 2nd April 2011 at 06:32 AM.
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Old 2nd April 2011, 07:32 AM   #5
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Originally Posted by Svein_B View Post
Do not forget to also consider your AC load-line.

The AC load will be the impedance of the load (next stage grid-leak) in parallel to the plate resistor.

Illustrated with the red line added to your chart.

If the load is 500K or higher, the effect will however be small.

--
SveinB.
Well, I had a different understanding of that. I was just reading in Morgan Jones that one uses anode resistance for the dynamic or AC loadline. That anode resistance was charted by drawing a line as shown in the attachment below. The anode resistance is the slope. I'm sure I'm confusing the two. Can you shed some more light on the subject?

It looks like you used about 350K for the grid-leak resistor for the red line - is that correct?
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File Type: jpg 6SL7_268PV_120K_1.5vbias.jpg (294.7 KB, 105 views)
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Old 2nd April 2011, 04:40 PM   #6
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by Captn Dave View Post
So, somewhere I got the idea that the load line should be perpendicular to the lines of constant Ec for lowest distortion, but I can't remember now where I got that idea. Is there any reason why that would be so?
No reason. Generally, the lowest distortion will be with a horizontal load line (constant current plate load, infinite impedance into next stage). But, the question is: are you designing for lowest stage distortion, or lowest overall amplifier distortion? They are not necessarily the same things.

You can calculate the distortion from the plate curves and loadline. Look here in the section on loadlines, about halfway down the page: Steve's Tube Pages

Quote:
Originally Posted by Captn Dave View Post
Well, I had a different understanding of that. I was just reading in Morgan Jones that one uses anode resistance for the dynamic or AC loadline. That anode resistance was charted by drawing a line as shown in the attachment below. The anode resistance is the slope. I'm sure I'm confusing the two. Can you shed some more light on the subject?
If you have Jone's third edition, read page 118. It's in the section on the u-follower. He probably should have addressed it earlier in the discussion on loadlines.

Sheldon
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Old 2nd April 2011, 05:46 PM   #7
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There is also one very practical method to obtain best possible working condition.
Use potentiometers as a part of anode and cathode resistors. Then simply adjust both resistors to minimum distortion by using correct input voltage for the tube.
Then take the readings of pots and replace with fixed resistors.
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Old 2nd April 2011, 06:35 PM   #8
Sheldon is offline Sheldon  United States
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Originally Posted by artosalo View Post
There is also one very practical method to obtain best possible working condition.
Use potentiometers as a part of anode and cathode resistors. Then simply adjust both resistors to minimum distortion by using correct input voltage for the tube.
Then take the readings of pots and replace with fixed resistors.
I'll repeat the point I hinted at earlier, but a little more clear. We don't listen to stages - unless it's a single stage amp - we listen to the system. Sometimes distortion in one stage can cancel distortion in the following stage. This applies particularly for 2nd order distortion in a commonly used grounded cathode, driving a typical SET output. Checking distortion on the fly is a good idea, but you have to do the entire amp.

Interesting graphics on distortion: Harmonic Distortion Pictures

Sheldon
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Old 2nd April 2011, 09:49 PM   #9
kevinkr is offline kevinkr  United States
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I find in general that 6SL7 sound much better and generate less distortion if the quiescent operating point is centered around 1.5 - 2mA - this is particularly the case where large voltage swings are required. (At 1mA you are not operating in the most linear region of the characteristic curves over significant voltage swings.)

Take heed of Sheldon's comments despite the above.
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Last edited by kevinkr; 2nd April 2011 at 09:57 PM.
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Old 3rd April 2011, 04:20 AM   #10
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Hello,
I agree with the thoughts of how the voltage amplifier stage integrates into the complete amplifier; more current and gm are better. The *SL7 is on the weak end of transductance scale. The Miller capacitance of the next stage needs careful consideration.
Load Lines are fine but incomplete theory. The next stage will be an active load. The load line is less a line than an ellipse.
DT
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