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#1
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Tube Question
What happens to Gm, Rp, and mu when two identical triode sections are
connected together in parallel (for a given bias current & plate voltage)? Thanks, Sean B |
#2
Posted to rec.audio.pro
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Tube Question
wrote:
What happens to Gm, Rp, and mu when two identical triode sections are connected together in parallel (for a given bias current & plate voltage)? This sounds like a homework question. Think about the tube as a variable resistor whose value is controlled by the grid voltage, and which has zero grid current. If you parallel two such devices, what happens? Twice the current should flow, but do you think anything else is going to happen? For the mu to change, the slope of the curve would have to change. What could cause that? --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#3
Posted to rec.audio.pro
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Tube Question
wrote in message
... What happens to Gm, Rp, and mu when two identical triode sections are connected together in parallel (for a given bias current & plate voltage)? Do you mean that the bias current through each section is the same? In other words, if one section was biased to 2mA, would the two in parallel be conducting 4mA? Also, what are you doing with the plate resistor(s)? Paralleling them too? Peace, Paul |
#4
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Tube Question
Sorry about the multiple posts, my first one wasn't popping up after a
few minutes, so i tried again. To answer Paul's question, think of having the ability to switch in and out a second triode in parallel, all other conditions being the same. Consider one or no plate resistor, not two. So if I read Scott's post correctly, Gm doubles, Rp halves, and Mu stays the same? SB |
#5
Posted to rec.audio.pro
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Tube Question
wrote in message
... Sorry about the multiple posts, my first one wasn't popping up after a few minutes, so i tried again. To answer Paul's question, think of having the ability to switch in and out a second triode in parallel, all other conditions being the same. Consider one or no plate resistor, not two. So if I read Scott's post correctly, Gm doubles, Rp halves, and Mu stays the same? Yup. Which gives you a 50% lower output impedance and 3dB lower noise contribution from the tube circuit. The tradeoff is that you have twice the input capacitance, which may or may not be important in a given application. And of course twice the cost and current consumption, both for filament and plate circuits. And twice the space and heat. Peace, Paul |
#6
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Tube Question
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#7
Posted to rec.audio.pro
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Tube Question
Paul Stamler wrote:
wrote in message ... Sorry about the multiple posts, my first one wasn't popping up after a few minutes, so i tried again. To answer Paul's question, think of having the ability to switch in and out a second triode in parallel, all other conditions being the same. Consider one or no plate resistor, not two. So if I read Scott's post correctly, Gm doubles, Rp halves, and Mu stays the same? Yup. Which gives you a 50% lower output impedance and 3dB lower noise contribution from the tube circuit. The tradeoff is that you have twice the input capacitance, which may or may not be important in a given application. It gives you 3 dB lower thermal noise! It does not always give you lower flicker noise and in the case of radiation-induced noise and hum from filament leakage it may be no lower at all. Incidentally this is a hum-reduction trick that has been used with the 12AX7... if you run 12V across the filaments they are effectively out of phase with one another. So if the filament leakage is the same on both halves, when you parallel the two halves the hum is cancelled out. And of course twice the cost and current consumption, both for filament and plate circuits. And twice the space and heat. Sheesh, if we were worried about space and heat we'd use a 2N2222 instead. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#8
Posted to rec.audio.pro
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Tube Question
"Scott Dorsey" wrote in message
... Paul Stamler wrote: wrote in message ... So if I read Scott's post correctly, Gm doubles, Rp halves, and Mu stays the same? Yup. Which gives you a 50% lower output impedance and 3dB lower noise contribution from the tube circuit. The tradeoff is that you have twice the input capacitance, which may or may not be important in a given application. It gives you 3 dB lower thermal noise! It does not always give you lower flicker noise and in the case of radiation-induced noise and hum from filament leakage it may be no lower at all. True. And in a well-designed circuit, the active device's noise is significantly lower than the source noise, and so contributes little to the overall noise. But if you're trying to squeeze the last little fraction of a dB from the noise figure in a particular circuit, parallel devices may help. Peace, Paul |
#9
Posted to rec.audio.pro
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Tube Question
Paul Stamler wrote:
"Scott Dorsey" wrote in message ... Paul Stamler wrote: wrote in message ... So if I read Scott's post correctly, Gm doubles, Rp halves, and Mu stays the same? Yup. Which gives you a 50% lower output impedance and 3dB lower noise contribution from the tube circuit. The tradeoff is that you have twice the input capacitance, which may or may not be important in a given application. It gives you 3 dB lower thermal noise! It does not always give you lower flicker noise and in the case of radiation-induced noise and hum from filament leakage it may be no lower at all. True. And in a well-designed circuit, the active device's noise is significantly lower than the source noise, and so contributes little to the overall noise. But if you're trying to squeeze the last little fraction of a dB from the noise figure in a particular circuit, parallel devices may help. Peace, Paul Paralleling devices doesn't actually reduce noise; what it does is alter the balance between current and voltage noise. What this does is change the optimum source impedance downwards, so if you are driving from a low impedance, noise will improve, while if the source impedance is higher, it may well get worse. d |
#10
Posted to rec.audio.pro
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Tube Question
Don Pearce wrote:
Paralleling devices doesn't actually reduce noise; what it does is alter the balance between current and voltage noise. What this does is change the optimum source impedance downwards, so if you are driving from a low impedance, noise will improve, while if the source impedance is higher, it may well get worse. Right. The thing is, in the tube audio world, the source impedance is _always_ lower than the input impedance of the stage.... and since the input impedance of the stage is set by the leak resistor rather than the tube input capacitance, the resistor noise is sometimes a significant issue. In the RF world where the input capacitance of the tube means that it sometimes takes appreciable current to drive it, or in the bipolar transistor world, that's not a good assumption to make. But for audio tubes, it's a normal one. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#11
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Tube Question
On Oct 28, 3:27*pm, Don Pearce wrote:
Paul Stamler wrote: "Scott Dorsey" wrote in message ... Paul Stamler wrote: wrote in message .... So if I read Scott's post correctly, Gm doubles, Rp halves, and Mu stays the same? Yup. Which gives you a 50% lower output impedance and 3dB lower noise contribution from the tube circuit. The tradeoff is that you have twice the input capacitance, which may or may not be important in a given application. It gives you 3 dB lower thermal noise! *It does not always give you lower flicker noise and in the case of radiation-induced noise and hum from filament leakage it may be no lower at all. True. And in a well-designed circuit, the active device's noise is significantly lower than the source noise, and so contributes little to the overall noise. But if you're trying to squeeze the last little fraction of a dB from the noise figure in a particular circuit, parallel devices may help. Peace, Paul Paralleling devices doesn't actually reduce noise; what it does is alter the balance between current and voltage noise. What this does is change the optimum source impedance downwards, so if you are driving from a low impedance, noise will improve, while if the source impedance is higher, it may well get worse. d- Hide quoted text - - Show quoted text - in some cases conbining multiple devices DOES ACTUALLY improve the S/N ratio..... this is becasue the desired signal from the multiple devices is correlated and adds constructively as +6 dB, but the noise in each device is random and uncorrelated so adds only randomly for +3 dB. So you get a 3 dB advantage (in theory) for each doubling of the number of devices that are combined. Mark |
#12
Posted to rec.audio.pro
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Tube Question
"Don Pearce" wrote in message
... True. And in a well-designed circuit, the active device's noise is significantly lower than the source noise, and so contributes little to the overall noise. But if you're trying to squeeze the last little fraction of a dB from the noise figure in a particular circuit, parallel devices may help. Paralleling devices doesn't actually reduce noise; what it does is alter the balance between current and voltage noise. What this does is change the optimum source impedance downwards, so if you are driving from a low impedance, noise will improve, while if the source impedance is higher, it may well get worse. You're thinking of bipolar devices. FETs and tubes (see the title of this thread) have virtually no current noise. Parallel them, and the noise goes down, but the capacitance goes up. Peace, Paul |
#13
Posted to rec.audio.pro
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Tube Question
I have one more tube question. What is the actual input impedance of
a triode's grid, say a 12AU7, at low frequency, say 20 Hz? Sean B |
#14
Posted to rec.audio.pro
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Tube Question
On Wed, 29 Oct 2008 08:29:45 -0700, wrote:
I have one more tube question. What is the actual input impedance of a triode's grid, say a 12AU7, at low frequency, say 20 Hz? Close to infinity. As long as you don't make it positive w.r.t. cathode, no current flows. -- Anahata ==//== 01638 720444 http://www.treewind.co.uk ==//== http://www.myspace.com/maryanahata |
#16
Posted to rec.audio.pro
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Tube Question
wrote in message
... I have one more tube question. What is the actual input impedance of a triode's grid, say a 12AU7, at low frequency, say 20 Hz? Whatever the grid leak resistor is -- that's the resistor going from the grid to ground. Unless you're in the (very nonlinear) region where you're actually drawing grid current, but under normal circumstances you don't want to be there. Peace, Paul |
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