Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Gary_S on December 28, 2012, 10:19:18 am
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Currently working my way slowly through this tech stuff from AX84 where they discuss all the tech details to do with this amp. Anyway to cut a long story short there's a bit at the start (within the first 20 pages of the document) where they ask you to have a go at working out the details of the cathode resistance and those things from a load line. The answer is given in the back of the document but it's kind of hard trying to work it out when you don't have it written out in front of you.
Does anyone know any things you can use for drawing out load lines just for practicing and for working out the parameters of a tubes gain stages and so on?
How do you experienced techs go about using load lines? do you just print a copy off the net to use? or do you print one out on graph paper or.....?
I first heard about the load line concept by reading the sample chapter on Merlin's site. I ordered his book so i'm hoping to learn a lot from that.
This is the link below i've been working on in case anyone didn't see my other equation thread.
http://www.ax84.com/static/p1x/p1-ex-theory.pdf (http://www.ax84.com/static/p1x/p1-ex-theory.pdf)
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Look here:
http://www.ampbooks.com/home/amplifier-calculators/12AX7/ (http://www.ampbooks.com/home/amplifier-calculators/12AX7/)
You can get a quick idea of what's going on.
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Fantastic link frank! Thanks very much, i've never seen that site before.
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Merlin has a loadline spreadsheet here: http://www.freewebs.com/valvewizard2/LoadLinePlotter.xls (http://www.freewebs.com/valvewizard2/LoadLinePlotter.xls)
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Thanks for the link on the software.
To get another view of loadlines, look at this link http://ampbooks.com/home/classic-circuits/fender-champ-5e1-preamp/ (http://ampbooks.com/home/classic-circuits/fender-champ-5e1-preamp/) It goes into a little more detail than the ax84 paper.
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Maybe I do it the hard way...
I've downloaded a bunch of pdf datasheets from Frank's (http://www.mif.pg.gda.pl/homepages/frank/) for various tubes.
I open the pdf, get the desired set of curves to where they fill up the computer screen, do "print screen", paste that in Paint, save the file as a GIF.
Later, I can open that blank curve file, save under a new name, draw my lines on it in the computer, post the result here if it comes up. Saves over lots of printed copies, 0.5mm mechanical pencils and a clear ruler (which is the other way I'd do it).
But yeah, there are other programs that spit the answer for you. I'd use those as well, but if you had a tutor step you through drawing the lines and what they mean/imply, you'd gain a much deeper understanding of what's going on in a tube (as well as reinforce ohm's law, which is helpful in all kinds of places).
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I've been using a method pretty similar to HBP's for anything involving power tubes or unusual preamp tubes. I save the graph out of a datasheet PDF and Photoshop the load lines in. The great thing about using Photoshop or GIMP (which is free) is the ability to use layers when drawing. The lines you draw can float over the original file in their own layers, and you can tweak them whenever you want, turn their layers on and off to hide them and try different values in yet more layers, etc.
Merlin's load line plotter spreadsheet has also been great for projects using the most common preamp tubes.
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For triodes, nothing beats Curve Captor. It does load line, estimates the harmonic distortion, draws the output voltage swing and generates various SPICE models for the tube (that's what it was designed for). Get it here: http://sourceforge.net/projects/curvecaptor/ (http://sourceforge.net/projects/curvecaptor/)
Jaz
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This may be a real stupid question but here goes.
What if one like me just builds an amp straight off a fender schematic and when I look at all the fender amps that used a 12ax7 preamp and second gain stage and then the split load PI and power tubes. All the schems show the exact same bias for the preamp tubes and for the split load PI . same resisters and same setup yet each one has a different voltage on the plates . So if you say set the power tube bias hotter you pull down the power tube plate voltage and in turn since the power tubes are now drawing more current then this in turn pulls the plate voltage lower all the way to the preamp . So now everything has changed right?
In reality we are only talking a few volts so no big deal .
So are these tube charts basically used to design an amp ?
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The characteristic charts are a part of the spec sheet to help the designer to select the tube appropriate for the job and to set the operating conditions for the tube. You can design amps without using the charts as many do, there's nothing wrong with that. :icon_biggrin:
Jaz
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This may be a real stupid question but here goes.
What if one like me just builds an amp straight off a fender schematic and when I look at all the fender amps that used a 12ax7 preamp and second gain stage and then the split load PI and power tubes. All the schems show the exact same bias for the preamp tubes and for the split load PI . same resisters and same setup yet each one has a different voltage on the plates . So if you say set the power tube bias hotter you pull down the power tube plate voltage and in turn since the power tubes are now drawing more current then this in turn pulls the plate voltage lower all the way to the preamp . So now everything has changed right?
In reality we are only talking a few volts so no big deal .
So are these tube charts basically used to design an amp ?
Tube charts, are some draft-person's interpretation of collected data. This data could be averaged data from several tubes. So what you see, may not be what your tube or its replacement will provide. Then look at the tolerance of the parts. Any one for Monte Carlo simulations using SPICE.
Bottom line, valve amp building is an art, and not so much science.