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Tail Resistor in long tail pair with CCS.
I'm working on a long tail pair phase inverter using a 12ax7 with a current
sink on the tail. I've noticed in some other designs (ARC) they use a resistor ~5k between the 2 tails and the CC sink. What's the benefit of this resistor? I find very little difference with or without it. It causes the CCS to work a little harder to make up for the series resistance but other than that I see no change in the bias or operating point. There's very little written about using a CCS with long tails. I've searched the web searched the web, but nothing. Any Ideas?? RonL |
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What's the benefit of this resistor? ** Imagine the CCS device fails short cct. .......... Phil |
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Patrick Turner wrote I'm working on a long tail pair phase inverter using a 12ax7 with a current sink on the tail. I've noticed in some other designs (ARC) they use a resistor ~5k between the 2 tails and the CC sink. What's the benefit of this resistor? I find very little difference with or without it. But what are you saying? Are there two CCS, one for each 1/2 of the 12AX7? If so, a 5k between each cathode acts like a current feedback R to linearize the current change in the two tubes; there is none in the CCS. Have you checked the gains of the 12AX7? What *exactly* is the circuit configuation? There are enough clues, as long as you don't get totally hung up on the meaning of "tail". It causes the CCS to work a little harder to make up for the series resistance but other than that I see no change in the bias or operating point. CCS never "work hard" There is no current change; the current is constant, so no signal power is liberated in the CCS, only the DC power of Vdc x Idc across the device. What a jumble of nonsense. It's class A. Instantaneous power varies, although mean power will not, assuming a symmetrical signal and a perfect CCS which yours, by some considerable distance, is not. You may find these equations useful: Ohms Law: V=R.I Power: W=V.I So, to the extent that a CCS is not perfect, it will work harder if the voltage across it varies more. To the extent that it is perfect, it will work harder some of the time, and less hard some of the time, and although that amounts to the same amount of work altogether, peak work rate will be higher. You should take some lessons from Phil, who has seen sense twice in just one week. There's very little written about using a CCS with long tails. I've searched the web searched the web, but nothing. Any Ideas?? A CCS **is** a "long tail", because it acts exactly like a resistor of huge value to some mythical very negative voltage, ( except for stray capacitances ). Good! A typical CCS would have a **real** finite R = 20Mohms, and if there is 5mA of current, then the CCS acts like 20Mohms commom Rk taken to -100,000 volts dc supply, which of course is difficult to arrange in a little box. But you could have an LTP with each grid at 0V bias, cathodes at say +5V bias and a 5mA common cathode current taken to -12V, or taken to some lower negative supply as in the case at http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm What do you think the dynamic resistance of your CCS is? At DC and at 20kHz? If resistance falls to a few kohm at, say 50kHz, does that matter? cheers, Ian |
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PS
I wrote: ...Instantaneous power varies, although mean power will not... This only takes the signal into account, BTW, which will be the same regardless of the resistor, assuming a perfect CCS. But looking at the DC conditions, the resistor drops the voltage seen by the CCS, thus the mean power it dissipates is less. All assuming Phil and I are correct in assuming the resistor is simply in series with the CCS. cheers, Ian |
#6
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Patrick Turner wrote
... http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm ... Could you show the power supply for this? Or perhaps just say what the source resistance is of the -87V bias supply? cheers, Ian |
#7
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Hi Group:
I should have posted this before but here's a link to the actual circuit: http://www.nashaudio.com/Public/12AX7_DiffPair_CCS.pdf It will be the input/phase inverter section of an amp I'm building. A differential driver will follow this stage and then the output section (Kt88's) "Patrick Turner" wrote in message ... wrote: I'm working on a long tail pair phase inverter using a 12ax7 with a current sink on the tail. I've noticed in some other designs (ARC) they use a resistor ~5k between the 2 tails and the CC sink. What's the benefit of this resistor? I find very little difference with or without it. But what are you saying? Are there two CCS, one for each 1/2 of the 12AX7? If so, a 5k between each cathode acts like a current feedback R to linearize the current change in the two tubes; there is none in the CCS. Have you checked the gains of the 12AX7? What *exactly* is the circuit configuation? It causes the CCS to work a little harder to make up for the series resistance but other than that I see no change in the bias or operating point. CCS never "work hard" There is no current change; the current is constant, so no signal power is liberated in the CCS, only the DC power of Vdc x Idc across the device. There's very little written about using a CCS with long tails. I've searched the web searched the web, but nothing. Any Ideas?? A CCS **is** a "long tail", because it acts exactly like a resistor of huge value to some mythical very negative voltage, ( except for stray capacitances ). A typical CCS would have a **real** finite R = 20Mohms, and if there is 5mA of current, then the CCS acts like 20Mohms commom Rk taken to -100,000 volts dc supply, which of course is difficult to arrange in a little box. But you could have an LTP with each grid at 0V bias, cathodes at say +5V bias and a 5mA common cathode current taken to -12V, or taken to some lower negative supply as in the case at http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm Patrick Turner. RonL |
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r.laury wrote
Hi Group: I should have posted this before but here's a link to the actual circuit: http://www.nashaudio.com/Public/12AX7_DiffPair_CCS.pdf Yes Ron, that's what Phil guessed. Have you followed his advice? Current through ECC83 should be about 1mA per side, giving 2mA through the CCS. Take failure of MJE340 to collector/emitter short as your starting point with the present circuit. 2mA through 15k would give -30V at cathodes, so clearly the current will rise. By how much? What will the situation be if the current rises to 4mA? That would give 0V at the cathode but less than 0V at the anodes (!), so it couldn't be quite 4mA. Somewhere between 3.5 and 4mA will give a Vak which allows that current with the consequent largish positive Vgk. What happens at the grids? If there were no extra 5k, there would be just 10k at the cathode. What happens to the grids now? I am surprised you measured no difference, BTW. The added 5k makes for a poorer CCS. What did you measure, just the static current? Try using an AC signal to the input, then measure the AC voltage at the cathodes, and measure the AC current through the CCS. Calculate the AC resistance. Do this at 100Hz and 20kHz if you can, with and without the 5k. Mosfets are better if you can avoid blowing them up. It will be the input/phase inverter section of an amp I'm building. A differential driver will follow this stage and then the output section (Kt88's) It's all the rage... cheers, Ian |
#9
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Ian Iveson wrote: Patrick Turner wrote ... http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm ... Could you show the power supply for this? Or perhaps just say what the source resistance is of the -87V bias supply? There is no need for me to show a power supply. Voltage supplies nominated are voltage supplies as nominated. Voltage supplies are regarded as low impedance supplies. DIY ppl like to make their own PS, with flexible values for the voltages and emitter resistors to suit the power tranny they choose. DIY ppl should be capable of building themsleves a PS before proceeding with building an amp. They should fully understand the schematic, and why the values have been chosen. Then they would know why it is important that where a low impedance voltage supply is advantageous to a circuit's performance, then they should make sure their voltage supplies are low impedance. I never set out to spoon feed everyone with every morsel of information or possible situation. People must have learnt to think for themselves and to have read and educated themselves to a level of competence which allows them to easily follow all of what I have said at my website. Where I get enquiries from time to time from ppl trying to build my designs of amps or transformers, I always offer them sufficient further information so they may proceed. Patrick Turner. cheers, Ian |
#10
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wrote: Hi Group: I should have posted this before but here's a link to the actual circuit: http://www.nashaudio.com/Public/12AX7_DiffPair_CCS.pdf It will be the input/phase inverter section of an amp I'm building. A differential driver will follow this stage and then the output section (Kt88's) The schematic above appears to be OK although an MJE340 used for only 2 mA of constant current seems a bit over the top when a TO92 package smaller bjt or a j-fet would do. The 5k R6 does not have to be in the circuit but it isolates the cathodes from stray C in the bjt, and allows you to check the actual CCS current easily as well as provide some guard in case a tube shorts between anode and cathode, which wouldn't hurt the bjt because there is the high value anode load RL there to limit current. With 50Vrms of signal out of both anodes, and only say 1.4Vrms input to one side of the LTP, there should be about 0.7Vrms at the commom cathodes. if you try to measure the signal voltage across the R6 5k, you should find it to be an extremely small voltage, so signal current in the 5k is low because below the 5k the collector input resistance is very very high, many megohms. Patrick Turner. "Patrick Turner" wrote in message ... wrote: I'm working on a long tail pair phase inverter using a 12ax7 with a current sink on the tail. I've noticed in some other designs (ARC) they use a resistor ~5k between the 2 tails and the CC sink. What's the benefit of this resistor? I find very little difference with or without it. But what are you saying? Are there two CCS, one for each 1/2 of the 12AX7? If so, a 5k between each cathode acts like a current feedback R to linearize the current change in the two tubes; there is none in the CCS. Have you checked the gains of the 12AX7? What *exactly* is the circuit configuation? It causes the CCS to work a little harder to make up for the series resistance but other than that I see no change in the bias or operating point. CCS never "work hard" There is no current change; the current is constant, so no signal power is liberated in the CCS, only the DC power of Vdc x Idc across the device. There's very little written about using a CCS with long tails. I've searched the web searched the web, but nothing. Any Ideas?? A CCS **is** a "long tail", because it acts exactly like a resistor of huge value to some mythical very negative voltage, ( except for stray capacitances ). A typical CCS would have a **real** finite R = 20Mohms, and if there is 5mA of current, then the CCS acts like 20Mohms commom Rk taken to -100,000 volts dc supply, which of course is difficult to arrange in a little box. But you could have an LTP with each grid at 0V bias, cathodes at say +5V bias and a 5mA common cathode current taken to -12V, or taken to some lower negative supply as in the case at http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm Patrick Turner. RonL |
#11
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I was wondering about the one you sold, obviously. If the
unfortunate fool who paid you for it puts some NOS Mullard and GE valves in there, he could be in for a very nasty surprise one day. Don't assume everyone sticks with the cheapies you sell. Please, no need to run and try and hide my questions. I'm not here to embarrass you. I can help:-)) Perhaps you could send a 5k resistor, or perhaps a few resistors and a mosfet, and some solder. Don't be shy: sell it as an "upgrade". Don't you want me babe? cheers, Ian "Patrick Turner" wrote in message ... Ian Iveson wrote: Patrick Turner wrote ... http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm ... Could you show the power supply for this? Or perhaps just say what the source resistance is of the -87V bias supply? There is no need for me to show a power supply. Voltage supplies nominated are voltage supplies as nominated. Voltage supplies are regarded as low impedance supplies. DIY ppl like to make their own PS, with flexible values for the voltages and emitter resistors to suit the power tranny they choose. DIY ppl should be capable of building themsleves a PS before proceeding with building an amp. They should fully understand the schematic, and why the values have been chosen. Then they would know why it is important that where a low impedance voltage supply is advantageous to a circuit's performance, then they should make sure their voltage supplies are low impedance. I never set out to spoon feed everyone with every morsel of information or possible situation. People must have learnt to think for themselves and to have read and educated themselves to a level of competence which allows them to easily follow all of what I have said at my website. Where I get enquiries from time to time from ppl trying to build my designs of amps or transformers, I always offer them sufficient further information so they may proceed. Patrick Turner. cheers, Ian |
#12
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"Patrick Turner" wrote
http://www.nashaudio.com/Public/12AX7_DiffPair_CCS.pdf It will be the input/phase inverter section of an amp I'm building. A differential driver will follow this stage and then the output section (Kt88's) The schematic above appears to be OK although an MJE340 used for only 2 mA of constant current seems a bit over the top when a TO92 package smaller bjt or a j-fet would do. The 5k R6 does not have to be in the circuit but it isolates the cathodes from stray C in the bjt, Er...to what good purpose? There are a heap of factors involved, including the fact that the resistor reduces the DC voltage across the CCS substantially, whilst reducing the AC voltage by a relatively very small amount. The operating conditions of the MJE340 are so changed that it would take more than a casual half-thought to work out whether the effective capacitance seen by the cathode is smaller or greater. My guess is greater, and the resistor slows the response of the CCS. ...and allows you to check the actual CCS current easily as well as provide some guard in case a tube shorts between anode and cathode, Er... which wouldn't hurt the bjt because there is the high value anode load RL there to limit current. Quite, so what "guard" does it provide? With 50Vrms of signal out of both anodes, and only say 1.4Vrms input to one side of the LTP, there should be about 0.7Vrms at the commom cathodes. if you try to measure the signal voltage across the R6 5k, you should find it to be an extremely small voltage, so signal current in the 5k is low because below the 5k the collector input resistance is very very high, many megohms. How many? You claimed 22Mohm for your design, although perhaps it was not the one you have posted now? It is very similar to the one in question, but for the deathwish. And at what frequency? How does it vary? Perhaps you don't think these things are important? cheers, Ian |
#13
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Tail Resistor in long tail pair with CCS.
Ian Iveson wrote: I was wondering about the one you sold, obviously. If the unfortunate fool who paid you for it puts some NOS Mullard and GE valves in there, he could be in for a very nasty surprise one day. Don't assume everyone sticks with the cheapies you sell. Please, no need to run and try and hide my questions. I'm not here to embarrass you. I can help:-)) Perhaps you could send a 5k resistor, or perhaps a few resistors and a mosfet, and some solder. Don't be shy: sell it as an "upgrade". Don't you want me babe? cheers, Ian Ian, you make a boring ****wit out of yourself some days. Patrick Turner. "Patrick Turner" wrote in message ... Ian Iveson wrote: Patrick Turner wrote ... http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm ... Could you show the power supply for this? Or perhaps just say what the source resistance is of the -87V bias supply? There is no need for me to show a power supply. Voltage supplies nominated are voltage supplies as nominated. Voltage supplies are regarded as low impedance supplies. DIY ppl like to make their own PS, with flexible values for the voltages and emitter resistors to suit the power tranny they choose. DIY ppl should be capable of building themsleves a PS before proceeding with building an amp. They should fully understand the schematic, and why the values have been chosen. Then they would know why it is important that where a low impedance voltage supply is advantageous to a circuit's performance, then they should make sure their voltage supplies are low impedance. I never set out to spoon feed everyone with every morsel of information or possible situation. People must have learnt to think for themselves and to have read and educated themselves to a level of competence which allows them to easily follow all of what I have said at my website. Where I get enquiries from time to time from ppl trying to build my designs of amps or transformers, I always offer them sufficient further information so they may proceed. Patrick Turner. cheers, Ian |
#14
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Tail Resistor in long tail pair with CCS.
Ian Iveson wrote: "Patrick Turner" wrote http://www.nashaudio.com/Public/12AX7_DiffPair_CCS.pdf It will be the input/phase inverter section of an amp I'm building. A differential driver will follow this stage and then the output section (Kt88's) The schematic above appears to be OK although an MJE340 used for only 2 mA of constant current seems a bit over the top when a TO92 package smaller bjt or a j-fet would do. The 5k R6 does not have to be in the circuit but it isolates the cathodes from stray C in the bjt, Er...to what good purpose? There are a heap of factors involved, including the fact that the resistor reduces the DC voltage across the CCS substantially, whilst reducing the AC voltage by a relatively very small amount. The operating conditions of the MJE340 are so changed that it would take more than a casual half-thought to work out whether the effective capacitance seen by the cathode is smaller or greater. My guess is greater, and the resistor slows the response of the CCS. In actual fact the operation with the series R in the tail makes no real difference in practical terms even though one could possibly measure what the stray C might do. ...and allows you to check the actual CCS current easily as well as provide some guard in case a tube shorts between anode and cathode, Er... which wouldn't hurt the bjt because there is the high value anode load RL there to limit current. Quite, so what "guard" does it provide? One like those guys at the Palace, big furry hat and red coat, and probably gay. With 50Vrms of signal out of both anodes, and only say 1.4Vrms input to one side of the LTP, there should be about 0.7Vrms at the commom cathodes. if you try to measure the signal voltage across the R6 5k, you should find it to be an extremely small voltage, so signal current in the 5k is low because below the 5k the collector input resistance is very very high, many megohms. How many? You claimed 22Mohm for your design, although perhaps it was not the one you have posted now? It is very similar to the one in question, but for the deathwish. And at what frequency? How does it vary? Perhaps you don't think these things are important? Its obvious you have never measured the aspects of using the circuit under discussion. After 4 years of discussions about such things its time you did something. Patrick Turner. cheers, Ian |
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