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Re: I've taken the plunge.


 
8/31/2004 6:44 PM
Enzo
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Re: I've taken the plunge.
Kirchhoffs law is to electronics what this might be to a river - the amount of water flowing into a point in the river is equal to that flowing out. Well, of course, otherwise the water would pile up.  
 
I realize if I am tracing down a spurious signal, like hum, that it has to come from somewhere, but I have yet to find a practical situation wherein I invoked K's law. it don't explain why my boat leaks.
 
8/31/2004 6:58 PM
Don Symes
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Rivers only flow downhill. Stacking an AC component on top of that -- especially an AC component from some field that's not in physical contact with your ... boat -- makes for a complicated picture. And a badly mixed metaphor.  
 
This can't have helped.
 
8/31/2004 9:36 PM
Enzo
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Well, a power boat leaves a wake on the river, how's that for stacking on AC?  
 
K's law is of course true, and I suppose an important concept. It is just my favorite example of something I do not use in a shop environment, unlike Ohm's law which I make practical use of daily. Maybe it is that it offers no calculations or measurements to assist me in my work.  
 
If I am tracking down an errent hum signal it does me no favor to tell me it has to come from somewhere.  
 
I genuinely want to hear someone else's view if it is of practical value to them.
 
9/1/2004 5:46 AM
drewl
Kirchoffs laws, Nodal analysis etc. are more design tools than troubleshooting aids.  
simple things like voltage dividers or calculating the gain of an amp stage...but finding hum or a leaking cap requires the "solder sniffing" abilities of a good tech to track down the culprit!
 
9/1/2004 6:11 AM
Dave H
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quote:
"I genuinely want to hear someone else's view if it is of practical value to them…..It is just my favorite example of something I do not use in a shop environment"
 
 
Oh yes you do! but you don’t realize you’re doing it. :) As Andy said it’s just common sense. If the plate current is 35mA and the screen grid current is 5mA what is the cathode current? It’s 40mA because “The algebraic sum of the currents at a junction is zero” (K’s law #1). In a 12AX7 input stage the supply is 300V and the plate voltage is 150V. What is the voltage across the plate resistor? It’s 150V because “The sum of the voltage drops around a closed mesh is zero” (K’s law #2)  
 
If I remembered correctly those are the definitions of K’s laws I was taught at school. I think the wording was carefully chosen to make it appear that the laws have no practical use. ;)  
 
Dave
 
9/1/2004 5:19 PM
Enzo
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Thanks, Dave, but I think the plate resistor example demonstrats Ohm's Law.  
 
I think the distinction I make is that while K's Law describes how the world works, at least in part it offers no isight into what I do. I want a formula to plug numbers into. Then again, I suppose the two currents sharing a path might be more like what K describes. But once the concept is understood, K is not a calculating aid. Maybe that's my beef.  
 
And I bever used DeMorgan's Theorem either...
 
9/1/2004 6:38 PM
Don Symes
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Once upon a time a PHB (with an MSEE) forbade me to use DeMorgan's-equivalent symbols for gates because he couldn't find a vendor who SOLD them.  
 
When you're doing discrete gate logic design you use DeMorgan's Theorem to reduce and simplify the number of gates you need to produce a given function. I find (found, really, it's been YEARS) useful when drawing a schematic to use the DeMorganized symbols to make what the gates are doing in the circuit more obvious to the poor schmuck debugging the thing (me).  
 
Karnaugh maps were something I never had any use for.  
 
Oh, and cheers, Jack.
 
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