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#1
Posted to rec.audio.tubes
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ASCII sketch of triode model equation
OK, unlike yesterday when I was not in the mood for ASCII art, today I am happy to do an ASCII sketch of the triode equation used in my model. Please remember that you must use a fixed pitch font in your news reader to properly view this sketch. Here is the equation for the triode model as I first posted it several days ago. Ipk = c * (u * Vgk + Vpk) ^ 1.5 For the purposes of this sketch I am going to replace the constant "c" with a new constant "k" which is related to "c" by the following function. c = k / (u ^ 1.5) Making this substitution we get the following equation which I will sketch. Ipk = k * (Vgk + Vpk / u) ^ 1.5 OK, finally here is the sketch showing how the negative feedback works. __________ | | | divide | +----------------| by |---------------------+ | | u | | | |__________| | | | | |\ | | |\ | \ | | | \ __________ | \ | +--| \ | | | \ | | \ | 3/2 | | v-c \ | Output | sum |---| power |---| transcon- |---+------- Input | / | function | | ductance / (plate) --------| / |__________| | (k) / (grid) | / | / |/ | / |/ I hope this sketch makes the negative feedback in the equation, and the spice model derived from it, clear to all. This does not imply one way or the other whether or not there is actually negative feedback involved in the operation of a triode vacuum tube. To properly understand this sketch it is necessary to understand that the transconductance block is connected such that a positive input to the transconductance block causes positive current to flow from the output of the model into the transconductance block. This may be easier to understand if you consider the value of the transconductance to be -k rather than k. I apologize in advance if this sketch doesn't reproduce properly on your computer even when using a fixed pitch font. Regards, John Byrns |
#2
Posted to rec.audio.tubes
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ASCII sketch of triode model equation
In article . com,
"Bret Ludwig" wrote: John Byrns wrote: OK, unlike yesterday when I was not in the mood for ASCII art, today I am happy to do an ASCII sketch of the triode equation used in my model. I'm not trying to be prickish, but ASCII art is a pain in the ass to read. Consider starting a web site and making drawings with CAD if you wish to communicate your ideas with the best clarity. You are obviously a member of the Powerpoint generation. Regards, John Byrns |
#3
Posted to rec.audio.tubes
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ASCII sketch of triode model equation
Bret Ludwig wrote: John Byrns wrote: OK, unlike yesterday when I was not in the mood for ASCII art, today I am happy to do an ASCII sketch of the triode equation used in my model. I'm not trying to be prickish, but ASCII art is a pain in the ass to read. Consider starting a web site and making drawings with CAD if you wish to communicate your ideas with the best clarity. It looked OK on my screen. It allows me to better guess what is is in the picture such as the signage of the triangular amp representations. One should be able to see at a glance what is completely unambiguous with all wave forms so that the level of bickering about WTF was meant is reduced to zero. If someone presents an irrefutably good drawing, I'll post it at my website where i have a spare 180MB of space. Patrick Turner. |
#4
Posted to rec.audio.tubes
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ASCII sketch of triode model equation
Bret wrote:
I'm not trying to be prickish, but ASCII art is a pain in the ass to read. Consider starting a web site and making drawings with CAD if you wish to communicate your ideas with the best clarity. It wouldn't be worth it in this case. I am sure you can imagine your own diagram by glancing at the equation. It simply compounds John's trivialisation of the notion of feedback. Since when has it made sense to draw a diagram of an equation? What is it supposed to mean? What does it add to understanding? Nothing at all in this case. A similar diagram could be drawn of an equation describing Henry's example of a falling parachute. Or pretty much any real process. OTOH, the SPICE code from which this fragment comes is a circuit listing: what SPICE calls a subcircuit model. Perhaps it would be interesting if John were to post the circuit, so it can be compared to the diagram? If it is the same, then the diagram has a little bit of meaning at least, however divorced from reality. But.... cheers, Ian |
#5
Posted to rec.audio.tubes
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ASCII sketch of triode model equation
Ian Iveson wrote: Bret wrote: I'm not trying to be prickish, but ASCII art is a pain in the ass to read. Consider starting a web site and making drawings with CAD if you wish to communicate your ideas with the best clarity. It wouldn't be worth it in this case. I am sure you can imagine your own diagram by glancing at the equation. It simply compounds John's trivialisation of the notion of feedback. Since when has it made sense to draw a diagram of an equation? What is it supposed to mean? What does it add to understanding? Nothing at all in this case. A good picture speaks one thousand words of wisdom. But a thousand of your words could only be described with scribbles. Patrick Turner. A similar diagram could be drawn of an equation describing Henry's example of a falling parachute. Or pretty much any real process. OTOH, the SPICE code from which this fragment comes is a circuit listing: what SPICE calls a subcircuit model. Perhaps it would be interesting if John were to post the circuit, so it can be compared to the diagram? If it is the same, then the diagram has a little bit of meaning at least, however divorced from reality. But.... cheers, Ian |
#6
Posted to rec.audio.tubes
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ASCII sketch of triode model equation
Patrick Turner wrote
Bret wrote: I'm not trying to be prickish, but ASCII art is a pain in the ass to read. Consider starting a web site and making drawings with CAD if you wish to communicate your ideas with the best clarity. It wouldn't be worth it in this case. I am sure you can imagine your own diagram by glancing at the equation. It simply compounds John's trivialisation of the notion of feedback. Since when has it made sense to draw a diagram of an equation? What is it supposed to mean? What does it add to understanding? Nothing at all in this case. A good picture speaks one thousand words of wisdom. Whereas this diagram, which is not good, complicates a single simple equation. In what way do you find it helpful? Does it help you to calculate the result of the equation? Start with an input, compute an output, compute the resulting input, and repeat until you arrive at a stable solution? Unless you naturally, like computers and hence SPICE, use numerical computation rather than analytical methods, that would be the long way round. It's easier to work from the equation after doing a spot of algebra to include the load. Systems theory, including feedback analysis, is not necessary in this case. Unlike with real feedback. Clearly it has nothing to do with how a real triode works. A real triode only has one path between anode and grid, or from any electrode to any other, for that matter. Hence there cannot be a separate feedback path, and definitely not with a different function, like John has put in his diagram of Duncan Munro's model. It's a pity I can no longer give a link, BTW, to the documentation for the much-improved, later version of the model John took his equation from. Unfortunately it is no longer available from the author, and I don't want to breach copyright in this case. I can tell you that the diagram is rather different. If you want a good diagram to put on your site, perhaps you could ask Duncan Munro if he doesn't mind you making commercial use of his work? But then you don't bother about copyright do you? But a thousand of your words could only be described with scribbles. Yep. Like those Japanese drawings. Every word a work of art. Er...or are they Chinese? Ian A similar diagram could be drawn of an equation describing Henry's example of a falling parachute. Or pretty much any real process. OTOH, the SPICE code from which this fragment comes is a circuit listing: what SPICE calls a subcircuit model. Perhaps it would be interesting if John were to post the circuit, so it can be compared to the diagram? If it is the same, then the diagram has a little bit of meaning at least, however divorced from reality. But.... cheers, Ian |
#7
Posted to rec.audio.tubes
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ASCII sketch of triode model equation
If the ASCII is not clear, do this:
Copy the diagram and paste in notepad. Should be very very clear to understand "John Byrns" wrote in message ... OK, unlike yesterday when I was not in the mood for ASCII art, today I am happy to do an ASCII sketch of the triode equation used in my model. Please remember that you must use a fixed pitch font in your news reader to properly view this sketch. Here is the equation for the triode model as I first posted it several days ago. Ipk = c * (u * Vgk + Vpk) ^ 1.5 For the purposes of this sketch I am going to replace the constant "c" with a new constant "k" which is related to "c" by the following function. c = k / (u ^ 1.5) Making this substitution we get the following equation which I will sketch. Ipk = k * (Vgk + Vpk / u) ^ 1.5 OK, finally here is the sketch showing how the negative feedback works. __________ | | | divide | +----------------| by |---------------------+ | | u | | | |__________| | | | | |\ | | |\ | \ | | | \ __________ | \ | +--| \ | | | \ | | \ | 3/2 | | v-c \ | Output | sum |---| power |---| transcon- |---+------- Input | / | function | | ductance / (plate) --------| / |__________| | (k) / (grid) | / | / |/ | / |/ I hope this sketch makes the negative feedback in the equation, and the spice model derived from it, clear to all. This does not imply one way or the other whether or not there is actually negative feedback involved in the operation of a triode vacuum tube. To properly understand this sketch it is necessary to understand that the transconductance block is connected such that a positive input to the transconductance block causes positive current to flow from the output of the model into the transconductance block. This may be easier to understand if you consider the value of the transconductance to be -k rather than k. I apologize in advance if this sketch doesn't reproduce properly on your computer even when using a fixed pitch font. Regards, John Byrns |
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