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 Post subject: Power output load line question
PostPosted: Aug Mon 13, 2018 3:03 pm 
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I've always been much better at fixing tube items than designing them, but the last couple weeks I've been changing that. I need to make sure I'm not missing something here. There's actually a couple things I'm hazy on.

As I understand it, when i do a load line calculation for a push-pull amp, I use the supply voltage and 1/2 of the input impedance of the output transformer to make my line, (same as singled ended) then move the line up, keeping the gradient, to just below the power dissipation curve. Wherever the end of the line is at the supply voltage will be the bias point, using the bias lines. That's the first part to see if I'm understanding this right.

Then, from what I've read, I draw another load line, using 1/4 of the input impedance, since it's push-pull, and look for where the two load lines intersect, and basically erase the parts of both lines that are farthest from the power dissipation curve, leaving a bent line. So #1 am I on the right track on this part as well and #2 if so, why bother with the second line? I've already established my bias point.

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Last edited by TPAairman on Aug Mon 13, 2018 8:20 pm, edited 1 time in total.

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 Post subject: Re: Power output load line question
PostPosted: Aug Mon 13, 2018 3:55 pm 
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This is hard to do without a chalkboard.....

First, there are typically TWO load lines: One defines the quiescent (DC) operating point, and the other defines the dynamic (AC) behavior.

When you talk about moving a line to fit into the maximum power curve, it sounds like you are fitting the dynamic load line into the curves, and using that to decide the bias point. Once you do that, you would then use the DC load line to actually set the parameters. The DC load line is set by the DC resistance of the OPT ( a few hundred ohms), and the AC load line is set by the speaker impedance and reflected thru the OPT. This is in the thousands of ohms.

To facilitate discussion, how about posting a sketch that illustrates the method you are using.

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Power output load line question
PostPosted: Aug Mon 13, 2018 5:12 pm 
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Here's an approximate picture for a 6L6. this is not exact because the specified AB1 operating point is with the screen at 270, but the curves are for a screen voltage of 250
Attachment:
6L6_loadlines.jpg
6L6_loadlines.jpg [ 57.7 KiB | Viewed 1120 times ]

this uses the operating point of Ep = 360 and Eg = -22.5
The DC and AC load lines are shown:
AC is 4kohms (the tube data sheet says use 3.8K)
DC is my arbitrary guess using a DCR of 350 and a 370-volt supply.

I understand moving the AC load line around to maximize some parameter. To me, that determines the desired operating point, which is then set by manipulating the DC load line.

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Power output load line question
PostPosted: Aug Mon 13, 2018 8:20 pm 
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This is probably the easiest to post. http://www.valvewizard.co.uk/pp.html Pretty much scroll down to the first load line chart, and read the second paragraph to the left of it, and the one after that.

He's doing the two lines in the opposite order as I stated above, though in the end, we get to the same point. He's describing the the lines as the Class A line and Class B line. The Class A line is the first one I described, which I need to make a correction (and am correcting my original post) - you use half the impedance value for your resistance value, not the full value. The class B line is the one with 1/4 the impedance value.

There is that website, one YouTube video on the subject, and a book that I have, which all describe the Class A line, and moving it up to the power dissipation curve.

The problem I've run into with quite a few subjects on tube theory is you will have several sources and each seem to leave out some detail or are vague on something, but it's a different part for each one, so you have to look at all of them and piece things together.

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 Post subject: Re: Power output load line question
PostPosted: Sep Wed 12, 2018 5:19 pm 
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This is sold as an audio amp?.....specing power at 120 ohms doesn't make a lot of sense for typical speakers that are in the range of 4 to 16 ohms.

Can you post a link to a manual or service info?

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Power output load line question
PostPosted: Sep Thu 27, 2018 6:49 pm 
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Mark, this guy isn't a legitimate account. He has posted here before under the names Anastasia Gray, Audionoobpt, and Ninelsox. He basically leaves off-topic posts on threads. He even tried to impersonate Alan Voorhees in the thread that discussed the website being moved to a different server. I've just reported all of his posts, but he'll probably come back again. I guess there's no way to stop him. Maybe he will eventually get bored and give up.

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 Post subject: Re: Power output load line question
PostPosted: Sep Fri 28, 2018 12:21 am 
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And he got my hopes up, seeing a new post on my question.......

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