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Old 7th January 2013, 12:18 AM   #1
SandyG is offline SandyG  United States
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Default Tube Load Lines and B+

Quick question on what I should use for the B+ on load lines.

Do you use the B+ voltage before all series dropping resistors for all stages for calculating load lines? I 'assume' B+ always to mean the main voltage before all the dropping resistors.

For example if I have several tubes in a circuit - amp, invert, drive, and each is powered by a series dropping resistor off of the main B+ out of the power supply do I use the Main B+ to calculate the load line voltage or do I work from the voltage at each stages dropping resistor before the plate resistor as the voltage for the load line.

Hope this makes sense, I can toss up a simple schematic if that helps.

Thanks

Sandy
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Old 7th January 2013, 12:47 AM   #2
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When doing loadlines, use the DC voltage at the far end of the plate resistor for the point along the Ip= 0 axis for the DC loadline. The AC loadline would include the effects of loading by the driven stage, which would be Rp || Rg of the driven stage. The AC loadline intersects the DC loadline at the Q-Point.
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Old 7th January 2013, 12:56 AM   #3
SandyG is offline SandyG  United States
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Quote:
Originally Posted by Miles Prower View Post
When doing loadlines, use the DC voltage at the far end of the plate resistor for the point along the Ip= 0 axis for the DC loadline. The AC loadline would include the effects of loading by the driven stage, which would be Rp || Rg of the driven stage. The AC loadline intersects the DC loadline at the Q-Point.
Thanks Miles and just to make sure, I use the voltage incoming to the plate resistor of each tube.

Is the process of calculating the dropping resistors (Not plate resistor) work from driver towards input each with their respective current draw to calculate resistors and voltages. (Hope this makes sense).

What I'm really trying to do is understand the process where to start, ie, do I pick my bias points (and thus current draw) on the tubes first then work back from that for B+ voltages...

Thanks

Sandy
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Old 7th January 2013, 09:12 AM   #4
DF96 is offline DF96  England
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You need to work in both directions and iterate a solution, unless you intend to use a custom mains transformer with exactly the secondary voltage you need. Remember that valves come with something like a 20% tolerance on most parameters so don't worry if something is 5% off from 'perfect'.

After getting the basic design right, you may need to remember that at very low frequencies, where the supply rail decoupling caps no longer operate, each stage sees all the resistors from the anode right through to the reservoir cap in the PSU. To avoid subsonic problems, voltage dropper resistors in the supply rails should not be too big when compared with anode load resistors.
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Old 7th January 2013, 04:29 PM   #5
SandyG is offline SandyG  United States
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Thanks for the tips, I feel better knowing I have a bit of leeway! I have a lot of baseline values from amps of similar designs but want to actually do the work to make sure that I have the operating points in a good place at the start given my power supply, which is Heyboer 800VAC CT with Dual 5U4's then CLCL. I have run Duncan's PSUII and come up with some starting values for that and will end up just building it and see if it all matches computed values. It has been very close in the 2 other cases that I built and simulated.

Then I guess start with common plate resistors values for 6SN7's in amp, invert, drivers and work in circles around those values and try to target my values on the load lines.

Sandy
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