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#1
Posted to rec.audio.tubes
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long tail pair grounded grid question?
I am a hobby guitar tube amp builder and student of circuit analysis
for tube guitar amps.I need some help with the reason for the capacitor to ground from the grid of the grounded grid part of the long tail pair inverter.I have studied quite a bit, but since I don't work with it regularly I will need a clear explanation,more than one sentence and not on the level of an electronic engineer.I couldn't design a long tail pair, but I probably understand about 90% of its basic functioning. Thanks |
#2
Posted to rec.audio.tubes
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long tail pair grounded grid question?
MarkX wrote:
I am a hobby guitar tube amp builder and student of circuit analysis for tube guitar amps.I need some help with the reason for the capacitor to ground from the grid of the grounded grid part of the long tail pair inverter.I have studied quite a bit, but since I don't work with it regularly I will need a clear explanation,more than one sentence and not on the level of an electronic engineer.I couldn't design a long tail pair, but I probably understand about 90% of its basic functioning. Thanks To ground the grid for AC, but not for DC, where the DC voltage of the grid must be other than ground for the purpose of bias. That's one possible answer. It's not clear what you think you don't know. There are umpteen variants of the LTP and, in guitar amps especially, the "grounded" grid is used to accept feedback from the transformer secondary, so it's not grounded. Best thing would be to post a circuit diagram of the particular LTP you mean. There are many guitar amp schemos he http://tdsl.duncanamps.com/schematics.php Find one like yours (a Marshall JMP would be a pretty archetypal example) and post a link to it. Then we'll know what we're discussing. The related question of why a pure grounded grid needs grounding isn't as simple as it may seem, maybe. If you drive a triode from its cathode, and leave the grid floating (with no connection), what happens? Why and in what way does the grid follow the signal? Anyway, a LTP amplifies the difference between the signals at the two grids, so in order to act just as an inverter, there should be your signal on one grid, and zero signal on the other, which means it must be connected to signal ground. Ian |
#3
Posted to rec.audio.tubes
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long tail pair grounded grid question?
There are many guitar amp schemos he
http://tdsl.duncanamps.com/schematics.php er...choose "schematics" from he http://www.duncanamps.com/ Duncan would probably argue that American is the language of the internet, but the use of American idiom by British folk really gets up my nose. So there. Maybe he's Scottish and doesn't care. Ian |
#4
Posted to rec.audio.tubes
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long tail pair grounded grid question?
MarkX wrote:
I am a hobby guitar tube amp builder and student of circuit analysis for tube guitar amps.I need some help with the reason for the capacitor to ground from the grid of the grounded grid part of the long tail pair inverter.I have studied quite a bit, but since I don't work with it regularly I will need a clear explanation,more than one sentence and not on the level of an electronic engineer.I couldn't design a long tail pair, but I probably understand about 90% of its basic functioning. Thanks The output signals of a long tailed pair (the anodes) are in anti-phase which is just what you want from a phase splitter. The value of the output signal is the difference between the input signals (the grids). Since you often only have one input signal you want to make sure the other signal input is zero which you do by connecting its grid to ground via a capacitor. HTH Cheers Ian |
#5
Posted to rec.audio.tubes
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long tail pair grounded grid question?
On Aug 24, 8:07*am, MarkX wrote:
I am a hobby guitar tube amp builder and student of circuit analysis for tube guitar amps.I need some help with the reason for the capacitor to ground from the grid of the grounded grid part of the long tail pair inverter.I have studied quite a bit, but since I don't work with it regularly I will need a clear explanation,more than one sentence and not on the level of an electronic engineer.I couldn't design a long tail pair, but I probably understand about 90% of its basic functioning. Thanks It's to ground the grid to the signal while allowing its DC potential to be at the higher, shared cathode potential from the large coupling resistor (or a constant current source if used) and DC coupling from the prior stage. Thus, this triode section is cathode driven for no phase inversion, giving P-P drive to the o/p stage. Cheers, Roger |
#6
Posted to rec.audio.tubes
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long tail pair grounded grid question?
"MarkX" I am a hobby guitar tube amp builder and student of circuit analysis for tube guitar amps.I need some help with the reason for the capacitor to ground from the grid of the grounded grid part of the long tail pair inverter.I have studied quite a bit, but since I don't work with it regularly I will need a clear explanation,more than one sentence and not on the level of an electronic engineer. ** That capacitor is what " grounds " the undriven grid. Without it, that grid will have the same audio signal on it as found at the common cathode bias point - ie where the other end of the grid bias resistor connects. This would result in the second half of the LTP being NOT driven with signal. Consider a LTP where the common cathode resistor is supplied from a negative DC rail of say -100 volts. Then there need only be one cathode resistor and the undriven grid may be connected directly to ground. ....... Phil |
#7
Posted to rec.audio.tubes
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long tail pair grounded grid question?
On Aug 24, 7:07�am, MarkX wrote:
I am a hobby guitar tube amp builder and student of circuit analysis for tube guitar amps.I need some help with the reason for the capacitor to ground from the grid of the grounded grid part of the long tail pair inverter.I have studied quite a bit, but since I don't work with it regularly I will need a clear explanation,more than one sentence and not on the level of an electronic engineer.I couldn't design a long tail pair, but I probably understand about 90% of its basic functioning. Thanks Thanks for all the excellent answers.That is exactly what I was hoping for.All were clear and not something out of Radio Designers Handbook, 4th.I was asking about a LTP without neg.feedback.I'll be back with another question about the neg feedback and will reference a circuit that can be seen at a website. MarkX |
#8
Posted to rec.audio.tubes
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long tail pair grounded grid question?
On Aug 24, 8:14�pm, MarkX wrote:
On Aug 24, 7:07 am, MarkX wrote: I am a hobby guitar tube amp builder and student of circuit analysis for tube guitar amps.I need some help with the reason for the capacitor to ground from the grid of the grounded grid part of the long tail pair inverter.I have studied quite a bit, but since I don't work with it regularly I will need a clear explanation,more than one sentence and not on the level of an electronic engineer.I couldn't design a long tail pair, but I probably understand about 90% of its basic functioning. Thanks Thanks for all the excellent answers.That is exactly what I was hoping for.All were clear and not something out of Radio Designers Handbook, 4th.I was asking about a LTP without neg.feedback.I'll be back with another question about the neg feedback and will reference a circuit that can be seen at a website. �MarkX If you put a sine wave where it belongs on the 1st section, could you see a sine wave across the capacitor that gives the grounded grid 2nd section it's AC ground? |
#9
Posted to rec.audio.tubes
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long tail pair grounded grid question?
"MarkX" If you put a sine wave where it belongs on the 1st section, could you see a sine wave across the capacitor that gives the grounded grid 2nd section it's AC ground? ** Oh dear ... Anyone see that last Scud missile ??? ....... Phil |
#10
Posted to rec.audio.tubes
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long tail pair grounded grid question?
MarkX wrote:
On Aug 24, 8:14�pm, MarkX wrote: On Aug 24, 7:07 am, MarkX wrote: I am a hobby guitar tube amp builder and student of circuit analysis for tube guitar amps.I need some help with the reason for the capacitor to ground from the grid of the grounded grid part of the long tail pair inverter.I have studied quite a bit, but since I don't work with it regularly I will need a clear explanation,more than one sentence and not on the level of an electronic engineer.I couldn't design a long tail pair, but I probably understand about 90% of its basic functioning. Thanks Thanks for all the excellent answers.That is exactly what I was hoping for.All were clear and not something out of Radio Designers Handbook, 4th.I was asking about a LTP without neg.feedback.I'll be back with another question about the neg feedback and will reference a circuit that can be seen at a website. �MarkX If you put a sine wave where it belongs on the 1st section, could you see a sine wave across the capacitor that gives the grounded grid 2nd section it's AC ground? No, because it is grounded for ac so there will be (almost) no signal across it. Cheers Ian |
#11
Posted to rec.audio.tubes
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long tail pair grounded grid question?
On Aug 25, 6:48*am, MarkX wrote:
On Aug 24, 8:14 pm, MarkX wrote: On Aug 24, 7:07 am, MarkX wrote: I am a hobby guitar tube amp builder and student of circuit analysis for tube guitar amps.I need some help with the reason for the capacitor to ground from the grid of the grounded grid part of the long tail pair inverter.I have studied quite a bit, but since I don't work with it regularly I will need a clear explanation,more than one sentence and not on the level of an electronic engineer.I couldn't design a long tail pair, but I probably understand about 90% of its basic functioning. Thanks Thanks for all the excellent answers.That is exactly what I was hoping for.All were clear and not something out of Radio Designers Handbook, 4th.I was asking about a LTP without neg.feedback.I'll be back with another question about the neg feedback and will reference a circuit that can be seen at a website. MarkX If you put a sine wave where it belongs on the 1st section, could you see a sine wave across the capacitor that gives the grounded grid 2nd section it's AC ground? Good question. Why don't you do the experiment and see? |
#12
Posted to rec.audio.tubes
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long tail pair grounded grid question?
If you put a sine wave where it belongs on the 1st
section, could you see a sine wave across the capacitor that gives the grounded grid 2nd section it's AC ground? Still trying to work out exactly what you don't understand. You've made Phil grumpy, probably because you've already been given the answer before you asked, assuming you know what "ground" means. We say that the capacitor is there to ensure that there is no signal on the grid, and then you ask if there is a signal on the grid. We wonder quite what part of "no signal" you don't understand. Basically, signal ground is the 0V reference from which signals are measured. A point is considered to be grounded if it is connected via a very low impedance path to ground so that in normal circuit operation *no significant voltage can be present at that point*. The very low impedance connection in the case of your imaginary grounded grid stage is the capacitor from grid to ground. In order to be very low, its impedance at audio frequencies must much less that the source impedance of the grid. In most real circuits there will be other stuff connected to the same grid, reducing the source impedance seen by the capacitor. Now, it could be that you don't understand: * How to calculate the impedance of a capacitor. * What "ground" means. * Ohm's law * What "source impedance" means. So here's an example question: ** Let's say you drive the cathode of your "grounded grid" stage with a 1kHz signal such that the grid, when not connected to anything, develops a voltage of 1V. Assuming the grid itself has an impedance of 1 Megohm, what voltage will be on the grid when it is connected to ground with a 10nF capacitor? What will it be if the experiment is repeated at 50Hz? What value of capacitor would you choose for this application? Somebody's going to tell me that's a stupid question but I don't care as long as it's answerable. Ian |
#13
Posted to rec.audio.tubes
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long tail pair grounded grid question?
On Aug 24, 7:07�am, MarkX wrote:
I am a hobby guitar tube amp builder and student of circuit analysis for tube guitar amps.I need some help with the reason for the capacitor to ground from the grid of the grounded grid part of the long tail pair inverter.I have studied quite a bit, but since I don't work with it regularly I will need a clear explanation,more than one sentence and not on the level of an electronic engineer.I couldn't design a long tail pair, but I probably understand about 90% of its basic functioning. Thanks I had to seek medical attention from Phil's right cross but he is very knowledgable as are all of you, and my follow up question was such that it looked like I learned nothing from the first round of answers.I was trying to see if even a trace of signal could be on the grounded grid even though the A/C ground was there .I do understand Xc .The most recent amp I built was modeled on a Fender 1955 Pro 5E5, allegedly used by Buddy Holly.That was built three years ago, so I haven't worked with or thought about electronics for a long time and it shows.As I recall,that amp has a concertina inverter/cathodyne maybe it's called so I couldn't experiment with it.I very much appreciate all the information,including the follow up information.I really did learn a lot from each person contributing. |
#14
Posted to rec.audio.tubes
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long tail pair grounded grid question?
On Aug 26, 9:16*pm, MarkX wrote:
On Aug 24, 7:07 am, MarkX wrote: I am a hobby guitar tube amp builder and student of circuit analysis for tube guitar amps.I need some help with the reason for the capacitor to ground from the grid of the grounded grid part of the long tail pair inverter.I have studied quite a bit, but since I don't work with it regularly I will need a clear explanation,more than one sentence and not on the level of an electronic engineer.I couldn't design a long tail pair, but I probably understand about 90% of its basic functioning. Thanks I had to seek medical attention from Phil's right cross but he is very knowledgable as are all of you, and my follow up question was such that it looked like I learned nothing from the first round of answers.I was trying to see if even a trace of signal could be on the grounded grid even though the A/C ground was there .I do understand Xc .The most recent amp I built was modeled on a Fender 1955 Pro 5E5, allegedly used by Buddy Holly.That was built three years ago, so I haven't worked with or thought about electronics for a long time and it shows.As I recall,that amp has a concertina inverter/cathodyne maybe it's called so I couldn't experiment with it.I very much appreciate all the information,including the follow up information.I really did learn a lot from each person contributing. There are many schematics of power amps using LTP stages which have one live grid input, and two anode outputs with opposite phase. See my website at http://www.turneraudio.com.au Now there are a variety of ways of setting up up the LTP and most involve two triodes and a single B+ supply of say +400V. The anodes of the two triodes will be at about +250V, and the commoned cathodes at about 100V. Each grid will have the same Vdc bias of slightly lower Vdc than the common Ek Vdc. One grid has a cap to 0V which effectively grounds this grid at audio frequencies, but not at dc voltages. But you could have both grids biased at 0V and have a negative B- voltage supply for connection to the common cathode resistance. Or the common cathode R could be replaced by an active circuit arrangement called a constant current sink, CCS, which offers the common cathodes a constant flow of current regardless to cathode voltage changes. I prefer this method of LTP set up. Consider the LTP with two triodes drawn side by side with the left one being V1 and with an active grid input and the right one being V2 and with its grid grounded by a cap to 0V. The action of the LTP is as follows:- Consider a positive going voltage input signal applied to the live V1 grid input. This turns on more current in V1, and the anode voltage becomes lower, or more negative, because more current flows in the RL. The cathode voltage of both triodes V1 and V2 will rise slightly, but always less than the applied grid voltage to V1. The rise in cathode voltages causes the relative voltage between cathodes and V2 grid to become more negative, ie, the action is as if a negative going grid voltage was applied to V2. So less current flows in V2. Therefore the anode voltage of V2 rises, ie, becomes more positive. Now thus you have two phases of signals with similar amplitude being produced by the ONE input signal. And there is considerable voltage gain and good cancelation of 2H distortion so the LTP offers a way to make a stage offering good voltage gain and low THD. If a CCS is used as the common source of current to both V1 and V2 cathodes, the two anode output voltages will be equal in amplitude. However, there are some finer points to remember when using the normal LTP which usually uses a simple common cathode resistor. In this case the anode load of V1 must be adjusted to a lower value than the anode load for V2 or else you will get more anode output voltage at V1 anode than at V2 anode. This is because the current change in V1 equals the current change in V2 *plus* the current change across the common cathode Rk. The higher the gain of the triodes used and the larger the value of the common Rk, the better the natural balance becomes of output voltages and with 12AX7 to get accurate balance the anode R load values need not be very different. Patrick Turner. |
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