Home |
Search |
Today's Posts |
#1
|
|||
|
|||
curve tracer for big tubes?
I've always been frustrated when trying to test large tubes (813, 805,
826, etc) because none of my tube testers can handle them. I suppose its possible to make an adaptor to use a regular tube tester, but using an external filament supply, but that would be a crude test at best. After thinking about it for a while, I've decided to build a curve tracer that will handle large tubes (and smaller ones as well). Actually, its not that complex. I'll use a Variac driving a 20 volt @10 amp filament transformer for the filament supply, and another Variac to drive a 1600 volt transformer for the plate. By stepping the grid between +100 v and -100 v (DC, and variable ranges), a set of I-V plate curves should be possible. At this point, I'm just thinkng about it. Has anyone else built something like this? Steve |
#2
|
|||
|
|||
Steve,
Yep, I have. An example is in eBay auction # 5725492214 (no one's bought it, alas). I used something similar to what you say, although I will add that I think it would be best if the filament tranny is center tapped (which I did not do). I used a 1 ohm resistor between the cathode and ground to get the current measurement -- which means the filament supply cannot be grounded -- and a voltage divider to get the plate voltage, a 100 : 1 ratio seems to work pretty well. It doesn't work very well for the small tubes, however. I think the 1 ohm current resistor is the main culprit (I need 10 ohm and 100 ohm option). It might eventually be necessary to use a different voltage ratio as well, but I don't think so. Note that a resistor in the cathode lead introduces an error by changing the grid-cathode voltage as the plate current rises (unless you can tie the grid supply to the cathode), but it's not much of an error in most cases (0.1 V at 100 mA for a 211 tube, for example). I have a Fluke voltage supply with switches that make it pretty easy to get a set of curves in the 15 seconds that my digital camera will allow. Lessons learned: Most scopes have a "z" input which will blank the traces while switching from one grid voltage to another. You could press a button while switching and eliminate the "extra" traces like you see on my photos. You really need some kind of power limit. Withough some type of power limit, it is almost impossible to get a decent set of curves, because not only is it difficult to hand adjust the plate voltage for each curve, from memory of the proceeding curve, to the correct point, but in addition, my scope (a Tek 2215A), and probably others, shift the curves as a function of the peak x-y position! A good approximation for power is a resistor in series with the primary of the transformer, although this makes a series of curves that descend in a straight line, actually like you see in many of the old sets of plate curves. ;-) I am in the process of updating my tracer, and I am going to try something much fancier, but I have an old 1000 W 50 ohm rheostat that I am repairing (bought off eBay). It works as is (now that I cleaned up the contacts), but the ends were cracked, and they don't fit properly into the metal pieces at each end, so I'm going to try to fix that before putting it back up on eBay (or selling it to you, if you wish). That and a few big power resistors, which I also have, would make it possible to set the plate voltage once, and then simply switch the grid voltage, and wind up with a nice looking set of curves like you can see in auction # 5719015311 (this happened to be the unusually high resistance of my variac and output transformers). I used the natural diode ability of a tube to take care of the negative voltage swing, but I suspect it would be much better to use a real diode on the input of the HV tranny. That's all I can think of for now, but we can obviously discuss this much more! Phil Steve wrote: I've always been frustrated when trying to test large tubes (813, 805, 826, etc) because none of my tube testers can handle them. I suppose its possible to make an adaptor to use a regular tube tester, but using an external filament supply, but that would be a crude test at best. After thinking about it for a while, I've decided to build a curve tracer that will handle large tubes (and smaller ones as well). Actually, its not that complex. I'll use a Variac driving a 20 volt @10 amp filament transformer for the filament supply, and another Variac to drive a 1600 volt transformer for the plate. By stepping the grid between +100 v and -100 v (DC, and variable ranges), a set of I-V plate curves should be possible. At this point, I'm just thinkng about it. Has anyone else built something like this? Steve |
#3
|
|||
|
|||
As I thought about this, the thing to consider is using a differential or
instrumentation amplifier on the current sense resistor -- you can get an off-the-shelf device with gain of 1000 set by a single resistor -- plenty of devices from Analog Devices or Burr-Brown (TI). Some of the diff amps already have clamp diodes tied to the V+/V- rails on the input pins. You can get away with a resistor small enough not to affect the voltage in tube applications. The problem with any of these devices is that tubes tend to be noisy (compared to other things you might want to measure) so the measurements have to be integrated... You might want to consider using an oscillator and a power amp to drive the variac at 1kHz -- the 60Hz signal is pretty darn slow for viewing meaningful information with a scope. If you have a signal generator with RAMP function, you don't have to worry about the retrace. Of course, if you are doing this you can get rid of the variac too, just control the oscillator output. The tube tracer article in one of the first issues of GLASS AUDIO used the 60 Hz to sweep the plate voltage, and ganged C- supplies which were controlled by a couple CMOS chips. I think that the author pointed out that you needed to mentally integrate the traces ! Jack "Phil" wrote in message ... Steve, Yep, I have. An example is in eBay auction # 5725492214 (no one's bought it, alas). I used something similar to what you say, although I will add that I think it would be best if the filament tranny is center tapped (which I did not do). I used a 1 ohm resistor between the cathode and ground to get the current measurement -- which means the filament supply cannot be grounded -- and a voltage divider to get the plate voltage, a 100 : 1 ratio seems to work pretty well. It doesn't work very well for the small tubes, however. I think the 1 ohm current resistor is the main culprit (I need 10 ohm and 100 ohm option). It might eventually be necessary to use a different voltage ratio as well, but I don't think so. Note that a resistor in the cathode lead introduces an error by changing the grid-cathode voltage as the plate current rises (unless you can tie the grid supply to the cathode), but it's not much of an error in most cases (0.1 V at 100 mA for a 211 tube, for example). I have a Fluke voltage supply with switches that make it pretty easy to get a set of curves in the 15 seconds that my digital camera will allow. Lessons learned: Most scopes have a "z" input which will blank the traces while switching from one grid voltage to another. You could press a button while switching and eliminate the "extra" traces like you see on my photos. You really need some kind of power limit. Withough some type of power limit, it is almost impossible to get a decent set of curves, because not only is it difficult to hand adjust the plate voltage for each curve, from memory of the proceeding curve, to the correct point, but in addition, my scope (a Tek 2215A), and probably others, shift the curves as a function of the peak x-y position! A good approximation for power is a resistor in series with the primary of the transformer, although this makes a series of curves that descend in a straight line, actually like you see in many of the old sets of plate curves. ;-) I am in the process of updating my tracer, and I am going to try something much fancier, but I have an old 1000 W 50 ohm rheostat that I am repairing (bought off eBay). It works as is (now that I cleaned up the contacts), but the ends were cracked, and they don't fit properly into the metal pieces at each end, so I'm going to try to fix that before putting it back up on eBay (or selling it to you, if you wish). That and a few big power resistors, which I also have, would make it possible to set the plate voltage once, and then simply switch the grid voltage, and wind up with a nice looking set of curves like you can see in auction # 5719015311 (this happened to be the unusually high resistance of my variac and output transformers). I used the natural diode ability of a tube to take care of the negative voltage swing, but I suspect it would be much better to use a real diode on the input of the HV tranny. That's all I can think of for now, but we can obviously discuss this much more! Phil Steve wrote: I've always been frustrated when trying to test large tubes (813, 805, 826, etc) because none of my tube testers can handle them. I suppose its possible to make an adaptor to use a regular tube tester, but using an external filament supply, but that would be a crude test at best. After thinking about it for a while, I've decided to build a curve tracer that will handle large tubes (and smaller ones as well). Actually, its not that complex. I'll use a Variac driving a 20 volt @10 amp filament transformer for the filament supply, and another Variac to drive a 1600 volt transformer for the plate. By stepping the grid between +100 v and -100 v (DC, and variable ranges), a set of I-V plate curves should be possible. At this point, I'm just thinkng about it. Has anyone else built something like this? Steve |
#4
|
|||
|
|||
John Walton wrote: As I thought about this, the thing to consider is using a differential or instrumentation amplifier on the current sense resistor -- you can get an off-the-shelf device with gain of 1000 set by a single resistor -- plenty of devices from Analog Devices or Burr-Brown (TI). Some of the diff amps already have clamp diodes tied to the V+/V- rails on the input pins. You can get away with a resistor small enough not to affect the voltage in tube applications. The problem with any of these devices is that tubes tend to be noisy (compared to other things you might want to measure) so the measurements have to be integrated... You might want to consider using an oscillator and a power amp to drive the variac at 1kHz -- the 60Hz signal is pretty darn slow for viewing meaningful information with a scope. The early TEK curve tracers used full wave rectified AC for the main sweep. After getting that info from the TEK salesman of the time (1960) I built a curve tracer which used a Polaroid camera on a TEK 541 to do some curve tracing. The G1 voltage was switched manually. In the end I got a complete set of curves on the photo for a number of tubes. Some of those photos are still here, somewhere. Cheers, John Stewart If you have a signal generator with RAMP function, you don't have to worry about the retrace. Of course, if you are doing this you can get rid of the variac too, just control the oscillator output. The tube tracer article in one of the first issues of GLASS AUDIO used the 60 Hz to sweep the plate voltage, and ganged C- supplies which were controlled by a couple CMOS chips. I think that the author pointed out that you needed to mentally integrate the traces ! Jack "Phil" wrote in message ... Steve, Yep, I have. An example is in eBay auction # 5725492214 (no one's bought it, alas). I used something similar to what you say, although I will add that I think it would be best if the filament tranny is center tapped (which I did not do). I used a 1 ohm resistor between the cathode and ground to get the current measurement -- which means the filament supply cannot be grounded -- and a voltage divider to get the plate voltage, a 100 : 1 ratio seems to work pretty well. It doesn't work very well for the small tubes, however. I think the 1 ohm current resistor is the main culprit (I need 10 ohm and 100 ohm option). It might eventually be necessary to use a different voltage ratio as well, but I don't think so. Note that a resistor in the cathode lead introduces an error by changing the grid-cathode voltage as the plate current rises (unless you can tie the grid supply to the cathode), but it's not much of an error in most cases (0.1 V at 100 mA for a 211 tube, for example). I have a Fluke voltage supply with switches that make it pretty easy to get a set of curves in the 15 seconds that my digital camera will allow. Lessons learned: Most scopes have a "z" input which will blank the traces while switching from one grid voltage to another. You could press a button while switching and eliminate the "extra" traces like you see on my photos. You really need some kind of power limit. Withough some type of power limit, it is almost impossible to get a decent set of curves, because not only is it difficult to hand adjust the plate voltage for each curve, from memory of the proceeding curve, to the correct point, but in addition, my scope (a Tek 2215A), and probably others, shift the curves as a function of the peak x-y position! A good approximation for power is a resistor in series with the primary of the transformer, although this makes a series of curves that descend in a straight line, actually like you see in many of the old sets of plate curves. ;-) I am in the process of updating my tracer, and I am going to try something much fancier, but I have an old 1000 W 50 ohm rheostat that I am repairing (bought off eBay). It works as is (now that I cleaned up the contacts), but the ends were cracked, and they don't fit properly into the metal pieces at each end, so I'm going to try to fix that before putting it back up on eBay (or selling it to you, if you wish). That and a few big power resistors, which I also have, would make it possible to set the plate voltage once, and then simply switch the grid voltage, and wind up with a nice looking set of curves like you can see in auction # 5719015311 (this happened to be the unusually high resistance of my variac and output transformers). I used the natural diode ability of a tube to take care of the negative voltage swing, but I suspect it would be much better to use a real diode on the input of the HV tranny. That's all I can think of for now, but we can obviously discuss this much more! Phil Steve wrote: I've always been frustrated when trying to test large tubes (813, 805, 826, etc) because none of my tube testers can handle them. I suppose its possible to make an adaptor to use a regular tube tester, but using an external filament supply, but that would be a crude test at best. After thinking about it for a while, I've decided to build a curve tracer that will handle large tubes (and smaller ones as well). Actually, its not that complex. I'll use a Variac driving a 20 volt @10 amp filament transformer for the filament supply, and another Variac to drive a 1600 volt transformer for the plate. By stepping the grid between +100 v and -100 v (DC, and variable ranges), a set of I-V plate curves should be possible. At this point, I'm just thinkng about it. Has anyone else built something like this? Steve |
#5
|
|||
|
|||
!doctype html public "-//w3c//dtd html 4.0 transitional//en"
html Yeah, 60 Hz will leave a lot of flicker, no doubt -- especially half-wave 60 Hz -- but I also found out, probably like you, that although you can get a rough feel for a tube on the scope, you need a picture to really do anything, like match tubes, or answer those questions like, "For this design, I wonder how tube X would work?" You will always want to go straight to a hardcopy of the plate curves. At present, therefore, I'm willing to make a tracer with flicker, the tradeoff being a relatively inexpensive 3000 V, 1000 W, 1 A tracer, since I'm going to want a digital pic anyway. Looking at some Tek plate curve photos, I noticed that they switched from one curve to the next at the top of the curves, so they really got 240 curves per second with their full wave power supply! pPhil pJohn Stewart wrote: blockquote TYPE=CITEJohn Walton wrote: p As I thought about this, the thing to consider is using a differential or br instrumentation amplifier on the current sense resistor -- you can get an br off-the-shelf device with gain of 1000 set by a single resistor -- plenty of br devices from Analog Devices or Burr-Brown (TI). Some of the diff amps br already have clamp diodes tied to the V+/V- rails on the input pins. You br can get away with a resistor small enough not to affect the voltage in tube br applications. br br The problem with any of these devices is that tubes tend to be noisy br (compared to other things you might want to measure) so the measurements br have to be integrated... br br You might want to consider using an oscillator and a power amp to drive the br variac at 1kHz -- the 60Hz signal is pretty darn slow for viewing meaningful br information with a scope. pThe early TEK curve tracers used full wave rectified AC for the main sweep. brAfter getting that info from the TEK salesman of the time (1960) I built a brcurve tracer which used a Polaroid camera on a TEK 541 to do some brcurve tracing. The G1 voltage was switched manually. In the end I got bra complete set of curves on the photo for a number of tubes. pSome of those photos are still here, somewhere. pCheers, John Stewart p If you have a signal generator with RAMP br function, you don't have to worry about the retrace. Of course, if you are br doing this you can get rid of the variac too, just control the oscillator br output. The tube tracer article in one of the first issues of GLASS AUDIO br used the 60 Hz to sweep the plate voltage, and ganged C- supplies which were br controlled by a couple CMOS chips. I think that the author pointed out that br you needed to mentally integrate the traces ! br br Jack br br "Phil" wrote in message a "news:416F36 /a... br Steve, br br Yep, I have. An example is in eBay auction # 5725492214 (no one's bought br it, br alas). I used something similar to what you say, although I will add that br I br think it would be best if the filament tranny is center tapped (which I br did not br do). I used a 1 ohm resistor between the cathode and ground to get the br current br measurement -- which means the filament supply cannot be grounded -- and a br voltage divider to get the plate voltage, a 100 : 1 ratio seems to work br pretty br well. It doesn't work very well for the small tubes, however. I think the br 1 ohm br current resistor is the main culprit (I need 10 ohm and 100 ohm option). br It br might eventually be necessary to use a different voltage ratio as well, br but I br don't think so. Note that a resistor in the cathode lead introduces an br error by br changing the grid-cathode voltage as the plate current rises (unless you br can br tie the grid supply to the cathode), but it's not much of an error in most br cases (0.1 V at 100 mA for a 211 tube, for example). I have a Fluke br voltage br supply with switches that make it pretty easy to get a set of curves in br the 15 br seconds that my digital camera will allow. br br Lessons learned: Most scopes have a "z" input which will blank the traces br while br switching from one grid voltage to another. You could press a button while br switching and eliminate the "extra" traces like you see on my photos. br br You really need some kind of power limit. Withough some type of power br limit, it br is almost impossible to get a decent set of curves, because not only is it br difficult to hand adjust the plate voltage for each curve, from memory of br the br proceeding curve, to the correct point, but in addition, my scope (a Tek br 2215A), and probably others, shift the curves as a function of the peak br x-y br position! A good approximation for power is a resistor in series with the br primary of the transformer, although this makes a series of curves that br descend br in a straight line, actually like you see in many of the old sets of plate br curves. ;-) I am in the process of updating my tracer, and I am going to br try br something much fancier, but I have an old 1000 W 50 ohm rheostat that I am br repairing (bought off eBay). It works as is (now that I cleaned up the br contacts), but the ends were cracked, and they don't fit properly into the br metal pieces at each end, so I'm going to try to fix that before putting br it br back up on eBay (or selling it to you, if you wish). That and a few big br power br resistors, which I also have, would make it possible to set the plate br voltage br once, and then simply switch the grid voltage, and wind up with a nice br looking br set of curves like you can see in auction # 5719015311 (this happened to br be the br unusually high resistance of my variac and output transformers). br br I used the natural diode ability of a tube to take care of the negative br voltage br swing, but I suspect it would be much better to use a real diode on the br input br of the HV tranny. br br That's all I can think of for now, but we can obviously discuss this much br more! br br Phil br br Steve wrote: br br I've always been frustrated when trying to test large tubes (813, 805, br 826, etc) because none of my tube testers can handle them. I suppose br its possible to make an adaptor to use a regular tube tester, but using br an br external filament supply, but that would be a crude test at best. After br thinking br about it for a while, I've decided to build a curve tracer that will br handle br large tubes (and smaller ones as well). Actually, its not that complex. br I'll use a Variac driving a 20 volt @10 amp filament transformer for the br filament supply, and another Variac to drive a 1600 volt transformer for br the plate. By stepping the grid between +100 v and -100 v (DC, and br variable br ranges), a set of I-V plate curves should be possible. br br At this point, I'm just thinkng about it. Has anyone else built br something br like this? br br & nbsp; Steve br /blockquote /html |
#6
|
|||
|
|||
Maybe you will want to borrow the power supply for my HT45 "Loudenboomer" -- will run full legal limit.
One of the guys on the DIYAUDIO site worked at Eimac in the 1960's. Jack "Phil" wrote in message ... Yeah, 60 Hz will leave a lot of flicker, no doubt -- especially half-wave 60 Hz -- but I also found out, probably like you, that although you can get a rough feel for a tube on the scope, you need a picture to really do anything, like match tubes, or answer those questions like, "For this design, I wonder how tube X would work?" You will always want to go straight to a hardcopy of the plate curves. At present, therefore, I'm willing to make a tracer with flicker, the tradeoff being a relatively inexpensive 3000 V, 1000 W, 1 A tracer, since I'm going to want a digital pic anyway. Looking at some Tek plate curve photos, I noticed that they switched from one curve to the next at the top of the curves, so they really got 240 curves per second with their full wave power supply! Phil John Stewart wrote: John Walton wrote: As I thought about this, the thing to consider is using a differential or instrumentation amplifier on the current sense resistor -- you can get an off-the-shelf device with gain of 1000 set by a single resistor -- plenty of devices from Analog Devices or Burr-Brown (TI). Some of the diff amps already have clamp diodes tied to the V+/V- rails on the input pins. You can get away with a resistor small enough not to affect the voltage in tube applications. The problem with any of these devices is that tubes tend to be noisy (compared to other things you might want to measure) so the measurements have to be integrated... You might want to consider using an oscillator and a power amp to drive the variac at 1kHz -- the 60Hz signal is pretty darn slow for viewing meaningful information with a scope. The early TEK curve tracers used full wave rectified AC for the main sweep. After getting that info from the TEK salesman of the time (1960) I built a curve tracer which used a Polaroid camera on a TEK 541 to do some curve tracing. The G1 voltage was switched manually. In the end I got a complete set of curves on the photo for a number of tubes. Some of those photos are still here, somewhere. Cheers, John Stewart If you have a signal generator with RAMP function, you don't have to worry about the retrace. Of course, if you are doing this you can get rid of the variac too, just control the oscillator output. The tube tracer article in one of the first issues of GLASS AUDIO used the 60 Hz to sweep the plate voltage, and ganged C- supplies which were controlled by a couple CMOS chips. I think that the author pointed out that you needed to mentally integrate the traces ! Jack "Phil" wrote in message ... Steve, Yep, I have. An example is in eBay auction # 5725492214 (no one's bought it, alas). I used something similar to what you say, although I will add that I think it would be best if the filament tranny is center tapped (which I did not do). I used a 1 ohm resistor between the cathode and ground to get the current measurement -- which means the filament supply cannot be grounded -- and a voltage divider to get the plate voltage, a 100 : 1 ratio seems to work pretty well. It doesn't work very well for the small tubes, however. I think the 1 ohm current resistor is the main culprit (I need 10 ohm and 100 ohm option). It might eventually be necessary to use a different voltage ratio as well, but I don't think so. Note that a resistor in the cathode lead introduces an error by changing the grid-cathode voltage as the plate current rises (unless you can tie the grid supply to the cathode), but it's not much of an error in most cases (0.1 V at 100 mA for a 211 tube, for example). I have a Fluke voltage supply with switches that make it pretty easy to get a set of curves in the 15 seconds that my digital camera will allow. Lessons learned: Most scopes have a "z" input which will blank the traces while switching from one grid voltage to another. You could press a button while switching and eliminate the "extra" traces like you see on my photos. You really need some kind of power limit. Withough some type of power limit, it is almost impossible to get a decent set of curves, because not only is it difficult to hand adjust the plate voltage for each curve, from memory of the proceeding curve, to the correct point, but in addition, my scope (a Tek 2215A), and probably others, shift the curves as a function of the peak x-y position! A good approximation for power is a resistor in series with the primary of the transformer, although this makes a series of curves that descend in a straight line, actually like you see in many of the old sets of plate curves. ;-) I am in the process of updating my tracer, and I am going to try something much fancier, but I have an old 1000 W 50 ohm rheostat that I am repairing (bought off eBay). It works as is (now that I cleaned up the contacts), but the ends were cracked, and they don't fit properly into the metal pieces at each end, so I'm going to try to fix that before putting it back up on eBay (or selling it to you, if you wish). That and a few big power resistors, which I also have, would make it possible to set the plate voltage once, and then simply switch the grid voltage, and wind up with a nice looking set of curves like you can see in auction # 5719015311 (this happened to be the unusually high resistance of my variac and output transformers). I used the natural diode ability of a tube to take care of the negative voltage swing, but I suspect it would be much better to use a real diode on the input of the HV tranny. That's all I can think of for now, but we can obviously discuss this much more! Phil Steve wrote: I've always been frustrated when trying to test large tubes (813, 805, 826, etc) because none of my tube testers can handle them. I suppose its possible to make an adaptor to use a regular tube tester, but using an external filament supply, but that would be a crude test at best. After thinking about it for a while, I've decided to build a curve tracer that will handle large tubes (and smaller ones as well). Actually, its not that complex. I'll use a Variac driving a 20 volt @10 amp filament transformer for the filament supply, and another Variac to drive a 1600 volt transformer for the plate. By stepping the grid between +100 v and -100 v (DC, and variable ranges), a set of I-V plate curves should be possible. At this point, I'm just thinkng about it. Has anyone else built something like this? Steve |
Reply |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Some tube history about 6L6. | Vacuum Tubes | |||
Lots Of Great Tubes For Sale | Marketplace | |||
Lots Of Great Tubes For Sale | Marketplace | |||
Curve Tracer: UPDATE | Tech | |||
Lots Of Great Audio Tubes For Sale! | Marketplace |