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Voltage Divider Question


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6/1/2005 1:24 PM
Bob DeRosa Voltage Divider Question
I understand the “standard” voltage divider where the signal goes thru a series resistor and is then shunted to ground, while taking the signal off of the junction of the two resistors.  
 
What happens in the case where the signal enters the junction of the two resistors, is shunted to ground and then goes thru the series resistor?  
 
Is the voltage dividing effect the same?
 
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6/1/2005 2:32 PM
Shea

No, the voltage dividing effect isn't the same.  
 
What you're describing is just like the #1 input jack setup of most amps -- first the signal passes over a 1 meg grid bias resistor, then through a series resistance of 34k (two 68k grid stoppers in parallel).  
 
Ignoring the tube's internal capacitances, any attenuation would be the result of the impedance of the source (i.e., the pickup's impedance) and the shunt resistance (the 1 meg grid resistor in parallel with the volume pot in the guitar). But the grid stoppers wouldn't have any effect at that point. Usually, the output impedance of the pickup is low enough that the 1-meg resistor on the amp's input jack doesn't attenuate the signal to any discernable degree. However, we usually use volume pots in our guitars that are just low enough in value that they bleed off some high end by loading down the pickup at those frequencies. If you double the size of the volume pot, you'll probably get a brighter, edgier sound, but it'll sound a tad harsh to most people.  
 
The grid stoppers do cause some attenuation at high-frequencies, however, because the internal capacitances of the tube act like the shunt capacitor in a low-pass filter, which is kind of a frequency-dependent voltage divider.  
 
Shea
 
6/2/2005 7:19 AM
anonymous
quote:
"No, the voltage dividing effect isn't the same."
 
 
It’s more like a pad attenuator actually, having almost no effect.  
 
quote:
"Ignoring the tube's internal capacitances, any attenuation would be the result of the impedance of the source (i.e., the pickup's impedance) and the shunt resistance (the 1 meg grid resistor in parallel with the volume pot in the guitar). But the grid stoppers wouldn't have any effect at that point. Usually, the output impedance of the pickup is low enough that the 1-meg resistor on the amp's input jack doesn't attenuate the signal to any discernable degree. However, we usually use volume pots in our guitars that are just low enough in value that they bleed off some high end by loading down the pickup at those frequencies. If you double the size of the volume pot, you'll probably get a brighter, edgier sound, but it'll sound a tad harsh to most people."
 
 
This is wrong. The guitar/pickup/control is a passive system. If you plug a guitar that has no controls whatsoever into an amp, you will get all the highs the pickup has to give. Increasing the control resistance value can not improve this.  
The attenuation comments are a nit. The attenuation caused by all of this is in the order of 1-5%  
 
quote:
"The grid stoppers do cause some attenuation at high-frequencies, however, because the internal capacitances of the tube act like the shunt capacitor in a low-pass filter, which is kind of a frequency-dependent voltage divider."
 
 
Another nit. The grid stoppers add almost no attenuation and are added precisely to limit frequency effects.  
The 3dB point is well out of the frequency range of guitars, speakers and human hearing.
 
6/2/2005 10:20 AM
Chuck
"This is wrong. The guitar/pickup/control is a passive system. If you plug a guitar that has no controls whatsoever into an amp, you will get all the highs the pickup has to give. Increasing the control resistance value can not improve this.  
The attenuation comments are a nit. The attenuation caused by all of this is in the order of 1-5%"  
 
In case you haven't tried it, the difference between a 250k and a 1M guitar volume pot is quite easy to hear. If, as you say, all this attenuation only amounts to 1 to 5% then it must be a sensitive margin here. I don't do electronics per se but I do build guitars and amps. And IIRC there are more considerable impedance changes with the guitar volume control and even the amps 1M input load resistor than you propose making the guitar output/amp input arrangement more frequency sensitive to the componant values than you recognize in your response.  
 
"Another nit. The grid stoppers add almost no attenuation and are added precisely to limit frequency effects.  
The 3dB point is well out of the frequency range of guitars, speakers and human hearing."  
 
Same thing here. If you can't hear the effects of different grid stoppers then you aren't building amps. You seem to be interested in using the semantics of theory to negate what is clearly audible. But it just doesn't work like that.  
 
I know the numbers say that I probably won't die in a firey crash today. But those statistics gave very little solace to the consistant number of people that it happens to. Using the semantics of statistics to avoid regognizing whats right in front of your face is far more ignorant than using practical experience to achieve an actual goal.  
 
I have a statistic for you. How about the number of anonymous posters that get any respect around here.  
 
Hiding behind an anonymous post to spout drivel that is useless in practical amp building is unsolicited antagonism and I think your wife/girlfriend should be treating you better so that you won't come here and bother us...stupid.  
 
Chuck
 
6/2/2005 10:30 AM
Chuck
Hey..Just realized that you are actually trying to be helpful below...But everyone likes Shea here and you pretty much rolled over him needlessly. Maybe practice a little more tolerance and get a name to use.  
 
Oh, and I'm sorry for the "stupid" remark in my post... The rest can stay though.  
 
Chuck
 
6/2/2005 10:50 AM
anonymous
Chuck,  
 
We're talking about two different things.  
The fact that you don't understand that, and what I'm writing about is for you, a problematic, third thing.  
The fact that you can't be objective and find personal slight in simple reasoning is an additional problem you will need to work on. I don't give a rip about your problems - so stuff your apology.  
 
Oh - and your hearing problem is interesting and must give you communication commonality with the canine world...;)  
 
The input capacitance of a typical preamp stage is a few puffs. Form a filter between that and a typical grid stopper resistance of say 36K and where are you, Fido?
 
6/2/2005 6:06 PM
novosibir

quote:
"The input capacitance of a typical preamp stage is a few puffs."
 
 
Did you ever hear about the "Miller Capacitance", which is calculated with the grid to plate capacitance and the amplifying factor of the tube.  
 
Then things become huge different and ALSO YOU wouldt hear every step from a 22K to a 68K to a 100K to a 220K or to a 470K grid stopper.  
 
Did you ever buildt or only tweak a tube amp? Or did you only read books about amps with half of one eye?
 

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