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#1
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exceeding heater to cathode voltage
I'm wondering what nasty effects i can expect, if any, for exceeding a
tube's maximum heater to cathode voltage rating. the tube is a 6072, and it's maximum claimed voltage is +- 90v peak. i'm using one triode as a cathode follower, and so the voltage at the cathode is gonna be around 150v above ground. i'm running AC on the heaters. so, why not cap couple before the follower you say? well, some think even the expensive caps sound better when driven and loaded with lower impedances. i'm saving the cap for the follower's output. i'm actually modifying an existing mic pre, and one thing they did that i don't understand is they return each side of the AC heater supply to ground with a 1k resistor. what is this done for? seems like if it were left floating, i wouldn't have to worry about the voltage limit situation. it probably also causes ground to wiggle a tiny bit at 60hz. thanks! SB |
#2
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well obviously if you exceed the h-c rating you could cause a h-c
breakdown. If you float the filament at 150 V, make sure you don't have any other tubes connected to the same filament winding and make sure the transformer filament winding is rated for 150 V. But I don't understand why the cathode has to be at 150 V above ground for a cathode follower, isn't the cathode a few volts above the grid and the grid is near ground? Of course what I really don't understand is why you want to do this at all , but lets not go there. Mark |
#3
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In article . com,
wrote: I'm wondering what nasty effects i can expect, if any, for exceeding a tube's maximum heater to cathode voltage rating. the tube is a 6072, and it's maximum claimed voltage is +- 90v peak. i'm using one triode as a cathode follower, and so the voltage at the cathode is gonna be around 150v above ground. i'm running AC on the heaters. Depending on the tube, it could be anything from hum to premature heater failures. so, why not cap couple before the follower you say? well, some think even the expensive caps sound better when driven and loaded with lower impedances. i'm saving the cap for the follower's output. So, raise the heater reference voltage. Hang the heater at +150V. i'm actually modifying an existing mic pre, and one thing they did that i don't understand is they return each side of the AC heater supply to ground with a 1k resistor. what is this done for? seems like if it were left floating, i wouldn't have to worry about the voltage limit situation. it probably also causes ground to wiggle a tiny bit at 60hz. The heater will induce more hum if it's left floating. The heater circuit needs to be referenced to ground. It could be referenced to something above ground, if need be. Or you could use a different tube for the output stage. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#4
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#6
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"Scott Dorsey" wrote in message
... In article . com, wrote: I'm wondering what nasty effects i can expect, if any, for exceeding a tube's maximum heater to cathode voltage rating. the tube is a 6072, and it's maximum claimed voltage is +- 90v peak. i'm using one triode as a cathode follower, and so the voltage at the cathode is gonna be around 150v above ground. i'm running AC on the heaters. Depending on the tube, it could be anything from hum to premature heater failures. I've even seen some odd distortion behavior and oscillation. so, why not cap couple before the follower you say? well, some think even the expensive caps sound better when driven and loaded with lower impedances. i'm saving the cap for the follower's output. So, raise the heater reference voltage. Hang the heater at +150V. If you're limited to a single heater circuit for all the tubes, float it at +75V. That'll mean the non-CF tubes will be at about -70V H-C and the CD will be at about +75V. Everyone should be happy. Peace, Paul |
#7
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Or you could use a different tube for the output stage. --scott -- do you think the 6072 makes a poor follower? it'll just be driving the 5 or 10k load of an AD converter, and i don't plan on using long cables between the two. do i need a higher current tube, like a 12AU7 or something? since i'm modding, i'm trying to minimize the pain-in-the-ass factor, and also not bludgeon the PCB too much. the preamp used 12ax7s before. SB |
#8
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In article .com,
wrote: Or you could use a different tube for the output stage. do you think the 6072 makes a poor follower? it'll just be driving the 5 or 10k load of an AD converter, and i don't plan on using long cables between the two. do i need a higher current tube, like a 12AU7 or something? since i'm modding, i'm trying to minimize the pain-in-the-ass factor, and also not bludgeon the PCB too much. the preamp used 12ax7s before. I don't know, I never did the math on the 6072. But if you look up the follower section in the Radiotron Book, there's a method for calculating basic linearity from the load lines, and there's a formula for figuring the effective output impedance with a given cathode resistor. Your whole goal is to balance voltage swing with output impedance and get the best combination for the device you're using. My personal feeling is you probably want the load to be at least four or five times as high as the output impedance of the stage. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#9
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which A/D converter?
some A/D's have a non-linear input Z and need to be driven by a low Z. While this article is not audio related, depending on the A/D you are using, some of the concepts may apply. http://www.sensorsmag.com/articles/0303/20/main.shtml Mark |
#10
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"Scott Dorsey" wrote in message
... In article .com, wrote: Or you could use a different tube for the output stage. do you think the 6072 makes a poor follower? it'll just be driving the 5 or 10k load of an AD converter, and i don't plan on using long cables between the two. do i need a higher current tube, like a 12AU7 or something? since i'm modding, i'm trying to minimize the pain-in-the-ass factor, and also not bludgeon the PCB too much. the preamp used 12ax7s before. I don't know, I never did the math on the 6072. But if you look up the follower section in the Radiotron Book, there's a method for calculating basic linearity from the load lines, and there's a formula for figuring the effective output impedance with a given cathode resistor. Your whole goal is to balance voltage swing with output impedance and get the best combination for the device you're using. My personal feeling is you probably want the load to be at least four or five times as high as the output impedance of the stage. Not really -- you want it to be a good deal higher than that. Typically a cathode follower circuit has an output impedance of some 300-600 ohms, but hang 3k on its output and suddenly it sucks, because you've eaten up most of its open-loop gain. Sean's wanting to drive a soundcard input. If it's 5k input impedance I'd go for something with some real muscle, like a 12FQ7 (same tube as a 6SN7 but a different filament setup, and it's a 9-pin). If it's 10k a 12AU7 will do. The goals are linearity and good current delivery, and those two tubes are good in both respects. The 6072 is basically a mil-spec version of the 12AY7 which has low microphonics, but only so-so linearity and current delivery. Peace, Paul |
#11
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Paul Stamler wrote:
Not really -- you want it to be a good deal higher than that. Typically a cathode follower circuit has an output impedance of some 300-600 ohms, but hang 3k on its output and suddenly it sucks, because you've eaten up most of its open-loop gain. Hi Paul! how much higher? and what 'loop' are you refering to? there's no global feedback happening in this pre. and what performance change are you refering to when you say 'it sucks'? a load that's 10x greater Z than the follower will have a small effect on its output level. but of course, it may turn into a fuzz box. soon i will investigate the effects of loading on distortion behavior with the power of FFTs, i just haven't gotten the unit to my practice space yet where my computer is. also, can you recommend a transparent sounding zener diode series, for the purpose of output clamping? with the swing available from tubes, they really could wipe out any subsequent solid state stage that wasn't input-protected. go over to a friends house, "dude check out my...fftfftttttttssszzzz". oops, toasted your AD. sorry! Sean's wanting to drive a soundcard input. well, it's a Lavry Blue AD. let's call it a fancy, expensive soundcard. it's good to have the blues! SB |
#12
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wrote:
Paul Stamler wrote: Not really -- you want it to be a good deal higher than that. Typically a cathode follower circuit has an output impedance of some 300-600 ohms, but hang 3k on its output and suddenly it sucks, because you've eaten up most of its open-loop gain. Hi Paul! how much higher? and what 'loop' are you refering to? there's no global feedback happening in this pre. There is... that's what a cathode follower is.... it's a stage with 100% feedback. The feedback through the grid circuit is what causes the cathode to precisely follow the grid. And when you load it too much, the cathode suddenly isn't following any more. I think Mr. Stamler is exaggerating a little bit about a load that is a factor of ten higher than the effective output Z being a problem. Note that when the effective output Z is calculated, the gain of the tube is taken into account... the more gain you have, the more the feedback is helping you. and what performance change are you refering to when you say 'it sucks'? a load that's 10x greater Z than the follower will have a small effect on its output level. but of course, it may turn into a fuzz box. soon i will investigate the effects of loading on distortion behavior with the power of FFTs, i just haven't gotten the unit to my practice space yet where my computer is. You don't need an FFT, although it can be nice. Just stick a 1kc square wave through the thing and watch what happens to the edges of the waveform. also, can you recommend a transparent sounding zener diode series, for the purpose of output clamping? with the swing available from tubes, they really could wipe out any subsequent solid state stage that wasn't input-protected. go over to a friends house, "dude check out my...fftfftttttttssszzzz". oops, toasted your AD. sorry! I don't like zener clamps if they can be avoided at all. And quite frankly, you don't get very much output swing out of a cathode follower. At least not if you want low output Z. You really need to read the Radiotron Handbook section on followers. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#13
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wrote in message oups.com... Paul Stamler wrote: Not really -- you want it to be a good deal higher than that. Typically a cathode follower circuit has an output impedance of some 300-600 ohms, but hang 3k on its output and suddenly it sucks, because you've eaten up most of its open-loop gain. Hi Paul! how much higher? and what 'loop' are you refering to? there's no global feedback happening in this pre. No, but there's local feedback. I was guilty of using shorthand; for open-loop read "gain without feedback". A cathode follower basically has 100% feedback, bringing the gain down to unity. How much higher? Basic rule is that you figure out what load the thing can drive based on its current capacity, not its output impedance. Like I said, 10k for a 12AU7, 5k for a 6SN7 if you bias it good and hot, and if you want to drive a 5k load with tubes you're best off setting the jumpers for -10dBV nominal sensitivity rather than +4dBu. and what performance change are you refering to when you say 'it sucks'? a load that's 10x greater Z than the follower will have a small effect on its output level. but of course, it may turn into a fuzz box. Exactly. Look, back to opamps for a second. A 5534 has a specified output impedance of 0.3 ohms, but that doesn't mean you can hang 3 ohms of load on it and expect anything but fuzz. Same goes for tubes. soon i will investigate the effects of loading on distortion behavior with the power of FFTs, i just haven't gotten the unit to my practice space yet where my computer is. You may be horrified at what you see. Tubes really don't like low impedances. also, can you recommend a transparent sounding zener diode series, for the purpose of output clamping? with the swing available from tubes, they really could wipe out any subsequent solid state stage that wasn't input-protected. go over to a friends house, "dude check out my...fftfftttttttssszzzz". oops, toasted your AD. sorry! Check with the manufacturer -- you may find the AD already has clamping diodes installed for exactly such reasons. Peace, Paul |
#14
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"Scott Dorsey" wrote in message
... I think Mr. Stamler is exaggerating a little bit about a load that is a factor of ten higher than the effective output Z being a problem. Note that when the effective output Z is calculated, the gain of the tube is taken into account... the more gain you have, the more the feedback is helping you. Except that into, say, a 6k load, the tube has a lot less gain to begin with, so a lot less feedback when used as a cathode follower. I did some tests a couple of decades ago to illustrate the point. The tubes weren't completely open-loop, since they had unbypassed cathode resistors, but still. For a 12AU7 with a 100k plate resistor and a 1.5k cathode resistor, idling at about 2.2mA: Load / Gain / THD at +15dBu 240k / 10.0 / 0.22 92k / 8.7 / 0.26 48k / 7.2 / 0.32 20k / 5.0 / 0.52 9.6k / 2.9 / 0.90 Note that whereas at a 240k load there's 10x gain, so the chance to reduce distortion by that amount, at a 9.6k load, there's only 2.9x gain, so you can only reduce distortion by that much when you turn the tube into a cathode follower. Hence my suggestion that the 12AU7 be used with an A/D converter of -10dBV nominal sensitivity. For a 6SN7 with a 27k plate resistor and 475 ohm cathode resistor, running at about 5.4mA idle current, the numbers a 240k / 13.0 / 0.072 92k / 12.0 / 0.082 48k / 10.0 / 0.10 20k / 8.0 / 0.18 9.6k / 5.6 / 0.27 Note that at 9.6k you have about twice the gain and less than 1/3 the THD, so a 6SN7 should be about 6x cleaner as a cathode follower under those conditions. It's a nice tube, although a power hog. But it's still happier at higher load impedances; into 240k you have the possibility of around ..006% THD, and in fact that's what I've measured at that level into that impedance. By the way -- yes, I know I've written that I don't think THD is a good measure of audio quality, and I stand by that. But I did these tests back in the 1980s, when I was young and foolish -- and besides, the same pattern of behavior will show up in IM tests. Peace, Paul |
#15
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On 15 Apr 2005 12:44:00 -0700, wrote:
I'm wondering what nasty effects i can expect, if any, for exceeding a tube's maximum heater to cathode voltage rating. the tube is a 6072, and it's maximum claimed voltage is +- 90v peak. i'm using one triode as a cathode follower, and so the voltage at the cathode is gonna be around 150v above ground. i'm running AC on the heaters. so, why not cap couple before the follower you say? well, some think even the expensive caps sound better when driven and loaded with lower impedances. i'm saving the cap for the follower's output. i'm actually modifying an existing mic pre, and one thing they did that i don't understand is they return each side of the AC heater supply to ground with a 1k resistor. what is this done for? It guarantees the capacitive coupling of the AC from the heater to the cathode is balanced: When one side of the heater supply goes positive, the other side is going negative. If it were left to float, it would probably be driven by the capacitive coupling of the transformer winding, where one side likely has more capacitance to ground than the other. seems like if it were left floating, i wouldn't have to worry about the voltage limit situation. Does this tube have a heater winding separate from the other tubes? If not, can you add a separate transformer to run its heater? I presume any transformer will have at least 200 volts isolation. If you have this tube's supply isolated, remove the ground side of the two 1k resistors, and connect them to the cathode connection. Even without isolation and using a single winding for all heaters, you can perhaps 'bias' the heater voltage up to perhaps 80V DC so that all heater-cathode voltages are within spec. it probably also causes ground to wiggle a tiny bit at 60hz. I doubt there's that much current to ground, it's just the capacitively coupled 60 Hz from the secondary winding. thanks! SB ----- http://mindspring.com/~benbradley |
#16
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On 15 Apr 2005 12:44:00 -0700, wrote:
I'm wondering what nasty effects i can expect, if any, for exceeding a tube's maximum heater to cathode voltage rating. the tube is a 6072, and it's maximum claimed voltage is +- 90v peak. i'm using one triode as a cathode follower, and so the voltage at the cathode is gonna be around 150v above ground. i'm running AC on the heaters. so, why not cap couple before the follower you say? well, some think even the expensive caps sound better when driven and loaded with lower impedances. i'm saving the cap for the follower's output. i'm actually modifying an existing mic pre, and one thing they did that i don't understand is they return each side of the AC heater supply to ground with a 1k resistor. what is this done for? It guarantees the capacitive coupling of the AC from the heater to the cathode is balanced: When one side of the heater supply goes positive, the other side is going negative. If it were left to float, it would probably be driven by the capacitive coupling of the transformer winding, where one side likely has more capacitance to ground than the other. seems like if it were left floating, i wouldn't have to worry about the voltage limit situation. Does this tube have a heater winding separate from the other tubes? If not, can you add a separate transformer to run its heater? I presume any transformer will have at least 200 volts isolation. If you have this tube's supply isolated, remove the ground side of the two 1k resistors, and connect them to the cathode connection. Even without isolation and using a single winding for all heaters, you can perhaps 'bias' the heater voltage up to perhaps 80V DC so that all heater-cathode voltages are within spec. it probably also causes ground to wiggle a tiny bit at 60hz. I doubt there's that much current to ground, it's just the capacitively coupled 60 Hz from the secondary winding. thanks! SB ----- http://mindspring.com/~benbradley |
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