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#1
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Grid current peak??
I think you have that backwards, grid current should be unquestionably
positive for any voltage above the virtual cathode potential (~ -0.8V here), rising to miliamperes for positive voltages. Grid contamination would vary widely with age and use. A hot, contaminated grid will emit a few electrons, some being picked up by the cathode, more by the plate. What's the maximum value of negative grid current you've seen across samples? BTW, what is the resistance of your meter in the modes used? Tim -- Deep Fryer: A very philosophical monk. Website @ http://webpages.charter.net/dawill/tmoranwms "Robert Baer" wrote in message ... Some 12Au7 tubes have "unbalanced" triodes when viewed this way, and some triodes are rather "peaky" like this one. Shouldn't there be no peak if the tube was perfectly OK? What would be the cause? (BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one used for each grid, and one for the plate via a switch) |
#2
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Grid current peak??
Some 12Au7 tubes have "unbalanced" triodes when viewed this way, and
some triodes are rather "peaky" like this one. Shouldn't there be no peak if the tube was perfectly OK? What would be the cause? (BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one used for each grid, and one for the plate via a switch) |
#3
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Grid current peak??
On Sun, 4 Nov 2007 04:24:07 -0600, "Tim Williams"
wrote: I think you have that backwards, grid current should be unquestionably positive for any voltage above the virtual cathode potential (~ -0.8V here), rising to miliamperes for positive voltages. Grid contamination would vary widely with age and use. A hot, contaminated grid will emit a few electrons, some being picked up by the cathode, more by the plate. What's the maximum value of negative grid current you've seen across samples? BTW, what is the resistance of your meter in the modes used? Tim What's the physics of the left part of the curve? Thermonic emission from the warm grid to the plate or the cathode? Are there secondary emission effects at the grid? I haven't thought much about tubes in a long time. People keep threatening to make a device with a semiconductor electron-emitting cathode, a vacuum gap, and a metallic anode. That would be very slick, but none seem to be practical so far. John |
#4
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Grid current peak??
"John Larkin" wrote in message
... What's the physics of the left part of the curve? Thermonic emission from the warm grid to the plate or the cathode? Should be something like that. Although I wonder, since negative current [magnitude] should rise somewhat with decreasing grid voltage as it becomes a better cathode. Of course, it may also be that, as a cathode, it is already saturated (no substantial space charge), where current wouldn't vary much with voltage. I wonder what conditions these measurements were taken in -- probably static, in which case I wonder if the plate temperature, and therefore everything inside the plate, was at thermal equilibrium when these measurements were taken. If so, then towards cutoff, the triode will be running a goodly bit cooler and the grid won't emit much. It follows that high plate dissipation and grid voltage just under the virtual cathode potential (about -1V) should produce the maximum leakage. Are there secondary emission effects at the grid? I don't think so. I suspect the electric field doesn't increase much until the electrons are past the grid (i.e. not enough leaks through the grid spaces), so in the grid region, there aren't any electrons with more energy than the grid's work function. A hot, contaminated grid might have some secondary emission though. I haven't thought much about tubes in a long time. People keep threatening to make a device with a semiconductor electron-emitting cathode, a vacuum gap, and a metallic anode. That would be very slick, but none seem to be practical so far. I would love to see a 300B in a TO-247 package. Well, no, I wouldn't love to *see* one, but it would be quite fun to use. Tim -- Deep Fryer: A very philosophical monk. Website @ http://webpages.charter.net/dawill/tmoranwms |
#5
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Grid current peak??
Robert Baer wrote:
John Larkin wrote: On Sun, 4 Nov 2007 04:24:07 -0600, "Tim Williams" wrote: I think you have that backwards, grid current should be unquestionably positive for any voltage above the virtual cathode potential (~ -0.8V here), rising to miliamperes for positive voltages. Grid contamination would vary widely with age and use. A hot, contaminated grid will emit a few electrons, some being picked up by the cathode, more by the plate. What's the maximum value of negative grid current you've seen across samples? BTW, what is the resistance of your meter in the modes used? Tim What's the physics of the left part of the curve? Thermonic emission from the warm grid to the plate or the cathode? Are there secondary emission effects at the grid? I haven't thought much about tubes in a long time. People keep threatening to make a device with a semiconductor electron-emitting cathode, a vacuum gap, and a metallic anode. That would be very slick, but none seem to be practical so far. John Yesssss! Ages ago, when i saw an article about a semiconductor designed as an electron emitter, i wondered when that would be used as a low power cathode - cut energy requirements in half (roughly)! ** Thanks for the comments - may be helpful when i sit down to think about them. Part of the problem in miniaturizing tubes is that it's really hard to sustain a good enough vacuum, due to having all that surface area per unit volume. That's the limiting factor in micro field emitters, for instance. Organic LEDs use a layer of (I joke not) metallic calcium as an electron source. Cheers, Phil Hobbs |
#6
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Grid current peak??
Tim Williams wrote:
I think you have that backwards, grid current should be unquestionably positive for any voltage above the virtual cathode potential (~ -0.8V here), rising to miliamperes for positive voltages. Grid contamination would vary widely with age and use. A hot, contaminated grid will emit a few electrons, some being picked up by the cathode, more by the plate. What's the maximum value of negative grid current you've seen across samples? BTW, what is the resistance of your meter in the modes used? Tim -- Deep Fryer: A very philosophical monk. Website @ http://webpages.charter.net/dawill/tmoranwms "Robert Baer" wrote in message ... Some 12Au7 tubes have "unbalanced" triodes when viewed this way, and some triodes are rather "peaky" like this one. Shouldn't there be no peak if the tube was perfectly OK? What would be the cause? (BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one used for each grid, and one for the plate via a switch) 1) Positive lead of the DVOM is at the grid and negative lead is at adjustable Vc; values shown aer what was read on the meter. Note the neck of the giraffe tilts negative to the left. 2) The schematic clearly shows that the meter resistance is 1Meg. |
#7
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Grid current peak??
flipper wrote:
On Sun, 04 Nov 2007 03:10:04 -0800, Robert Baer wrote: Some 12Au7 tubes have "unbalanced" triodes when viewed this way, If you mean the two triodes in one bottle are not 'identical' that's typical. They're not solid state devices made on a common substrate with the same process parameters, they're individual mechanical structures that happen to be placed inside the same envelope but 'triode a' in tube 1 could just have easily ended up in tube 2, or tube 202. and some triodes are rather "peaky" like this one. Shouldn't there be no peak if the tube was perfectly OK? What would be the cause? (BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one used for each grid, and one for the plate via a switch) I'd be skeptical of that data because direct grid measurements are notorious for altering grid bias. A normal tube will have a small grid leak until Vgk approaches 0 and then it turns positive, like a diode (in theory right at 0 but it depends on the actual field around the grid and that varies depending on the structure) ) What 'problem' are you trying to solve? See the top curve where i have calculated the grid voltage WRT cathode. |
#8
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Grid current peak??
John Larkin wrote:
On Sun, 4 Nov 2007 04:24:07 -0600, "Tim Williams" wrote: I think you have that backwards, grid current should be unquestionably positive for any voltage above the virtual cathode potential (~ -0.8V here), rising to miliamperes for positive voltages. Grid contamination would vary widely with age and use. A hot, contaminated grid will emit a few electrons, some being picked up by the cathode, more by the plate. What's the maximum value of negative grid current you've seen across samples? BTW, what is the resistance of your meter in the modes used? Tim What's the physics of the left part of the curve? Thermonic emission from the warm grid to the plate or the cathode? Are there secondary emission effects at the grid? I haven't thought much about tubes in a long time. People keep threatening to make a device with a semiconductor electron-emitting cathode, a vacuum gap, and a metallic anode. That would be very slick, but none seem to be practical so far. John Yesssss! Ages ago, when i saw an article about a semiconductor designed as an electron emitter, i wondered when that would be used as a low power cathode - cut energy requirements in half (roughly)! ** Thanks for the comments - may be helpful when i sit down to think about them. |
#9
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Grid current peak??
flipper wrote:
On Sun, 04 Nov 2007 03:10:04 -0800, Robert Baer wrote: Some 12Au7 tubes have "unbalanced" triodes when viewed this way, If you mean the two triodes in one bottle are not 'identical' that's typical. They're not solid state devices made on a common substrate with the same process parameters, they're individual mechanical structures that happen to be placed inside the same envelope but 'triode a' in tube 1 could just have easily ended up in tube 2, or tube 202. and some triodes are rather "peaky" like this one. Shouldn't there be no peak if the tube was perfectly OK? What would be the cause? (BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one used for each grid, and one for the plate via a switch) I'd be skeptical of that data because direct grid measurements are notorious for altering grid bias. A normal tube will have a small grid leak until Vgk approaches 0 and then it turns positive, like a diode (in theory right at 0 but it depends on the actual field around the grid and that varies depending on the structure) ) What 'problem' are you trying to solve? OK, you found me out. What i have is a GR1862B Megohmmeter which uses a 12AU7 as a differential detector in a semi-bridge configuration (don't ask why - go to GR on that Q). I am getting tired of having tubes slowly degrading so that i cannot adjust the infinity at max setting (pot setting slowly crawls from near center to always one end). I was hoping that i could characterize the 2 triodes and use the data as a guide - maybe even selecting a tube that would start on one end so as tholast longer in this application. |
#10
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Grid current peak??
flipper wrote:
On Sun, 04 Nov 2007 15:42:09 -0800, Robert Baer wrote: flipper wrote: On Sun, 04 Nov 2007 03:10:04 -0800, Robert Baer wrote: Some 12Au7 tubes have "unbalanced" triodes when viewed this way, If you mean the two triodes in one bottle are not 'identical' that's typical. They're not solid state devices made on a common substrate with the same process parameters, they're individual mechanical structures that happen to be placed inside the same envelope but 'triode a' in tube 1 could just have easily ended up in tube 2, or tube 202. and some triodes are rather "peaky" like this one. Shouldn't there be no peak if the tube was perfectly OK? What would be the cause? (BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one used for each grid, and one for the plate via a switch) I'd be skeptical of that data because direct grid measurements are notorious for altering grid bias. A normal tube will have a small grid leak until Vgk approaches 0 and then it turns positive, like a diode (in theory right at 0 but it depends on the actual field around the grid and that varies depending on the structure) ) What 'problem' are you trying to solve? OK, you found me out. I apologize if that was supposed to be a secret. What i have is a GR1862B Megohmmeter which uses a 12AU7 as a differential detector in a semi-bridge configuration (don't ask why - go to GR on that Q). I am getting tired of having tubes slowly degrading so that i cannot adjust the infinity at max setting (pot setting slowly crawls from near center to always one end). I was hoping that i could characterize the 2 triodes and use the data as a guide - maybe even selecting a tube that would start on one end so as tholast longer in this application. Well, tubes do age and I'm not familiar with a GR1862B so I can't help much on that score but I'd suggest you test your measuring setup by checking anode current with and without the grid probe attached to see if it remains constant. I have been looking at the new graph via Hexcel and am not happy with it; it appears that it lies. The meters are hard-wired in series with the grids; if the current is small then the grid voltage is not much different than the setting (100mV per 100nA). Using those el-cheapo DVOMs in this app is ideal in that "recommended" grid resistance is 1meg and the DCV input is 1meg, allowing a 200nA FS reading. |
#11
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Grid current peak??
On Sun, 04 Nov 2007 15:35:36 -0800, Robert Baer
wrote: flipper wrote: On Sun, 04 Nov 2007 03:10:04 -0800, Robert Baer wrote: Some 12Au7 tubes have "unbalanced" triodes when viewed this way, If you mean the two triodes in one bottle are not 'identical' that's typical. They're not solid state devices made on a common substrate with the same process parameters, they're individual mechanical structures that happen to be placed inside the same envelope but 'triode a' in tube 1 could just have easily ended up in tube 2, or tube 202. and some triodes are rather "peaky" like this one. Shouldn't there be no peak if the tube was perfectly OK? What would be the cause? (BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one used for each grid, and one for the plate via a switch) I'd be skeptical of that data because direct grid measurements are notorious for altering grid bias. A normal tube will have a small grid leak until Vgk approaches 0 and then it turns positive, like a diode (in theory right at 0 but it depends on the actual field around the grid and that varies depending on the structure) ) What 'problem' are you trying to solve? See the top curve where i have calculated the grid voltage WRT cathode. Your calculations appear to be inconsistent with the info given. For example 141 nA through 1 megohm generates 0.141 volts, not 14.1 volts. The whole table looks like it was calculated based on a 100 megohm grid resistor, not 1 megohm as stated elsewhere. |
#12
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Grid current peak??
Robert Baer wrote:
Some 12Au7 tubes have "unbalanced" triodes when viewed this way, and some triodes are rather "peaky" like this one. Shouldn't there be no peak if the tube was perfectly OK? What would be the cause? (BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one used for each grid, and one for the plate via a switch) I found my error; wrong neck (multiplier) for the giraffe. Basically, in the range of interest; grid current does not affect the set grid voltage. |
#13
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Grid current peak??
On Sun, 04 Nov 2007 18:24:16 -0500, Phil Hobbs
wrote: Robert Baer wrote: John Larkin wrote: On Sun, 4 Nov 2007 04:24:07 -0600, "Tim Williams" wrote: I think you have that backwards, grid current should be unquestionably positive for any voltage above the virtual cathode potential (~ -0.8V here), rising to miliamperes for positive voltages. Grid contamination would vary widely with age and use. A hot, contaminated grid will emit a few electrons, some being picked up by the cathode, more by the plate. What's the maximum value of negative grid current you've seen across samples? BTW, what is the resistance of your meter in the modes used? Tim What's the physics of the left part of the curve? Thermonic emission from the warm grid to the plate or the cathode? Are there secondary emission effects at the grid? I haven't thought much about tubes in a long time. People keep threatening to make a device with a semiconductor electron-emitting cathode, a vacuum gap, and a metallic anode. That would be very slick, but none seem to be practical so far. John Yesssss! Ages ago, when i saw an article about a semiconductor designed as an electron emitter, i wondered when that would be used as a low power cathode - cut energy requirements in half (roughly)! ** Thanks for the comments - may be helpful when i sit down to think about them. Part of the problem in miniaturizing tubes is that it's really hard to sustain a good enough vacuum, due to having all that surface area per unit volume. That's the limiting factor in micro field emitters, for instance. They usually have to work in UHV to prevent ion damage and stuff. Nanotube bundles work OK, for a while, until the ends get flayed off; they *are* noisy. Nanotips wear out fast, too. There was a diode that just flung electrons out the junction into space, but I don't know what's become of that. Organic LEDs use a layer of (I joke not) metallic calcium as an electron source. I hope the humidity seals are good. John |
#14
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Grid current peak??
BFoelsch wrote:
On Sun, 04 Nov 2007 15:35:36 -0800, Robert Baer wrote: flipper wrote: On Sun, 04 Nov 2007 03:10:04 -0800, Robert Baer wrote: Some 12Au7 tubes have "unbalanced" triodes when viewed this way, If you mean the two triodes in one bottle are not 'identical' that's typical. They're not solid state devices made on a common substrate with the same process parameters, they're individual mechanical structures that happen to be placed inside the same envelope but 'triode a' in tube 1 could just have easily ended up in tube 2, or tube 202. and some triodes are rather "peaky" like this one. Shouldn't there be no peak if the tube was perfectly OK? What would be the cause? (BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one used for each grid, and one for the plate via a switch) I'd be skeptical of that data because direct grid measurements are notorious for altering grid bias. A normal tube will have a small grid leak until Vgk approaches 0 and then it turns positive, like a diode (in theory right at 0 but it depends on the actual field around the grid and that varies depending on the structure) ) What 'problem' are you trying to solve? See the top curve where i have calculated the grid voltage WRT cathode. Your calculations appear to be inconsistent with the info given. For example 141 nA through 1 megohm generates 0.141 volts, not 14.1 volts. The whole table looks like it was calculated based on a 100 megohm grid resistor, not 1 megohm as stated elsewhere. Check; has been corrected - see previous posting and most recent. |
#15
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Grid current peak??
Robert Baer wrote:
Some 12Au7 tubes have "unbalanced" triodes when viewed this way, and some triodes are rather "peaky" like this one. Shouldn't there be no peak if the tube was perfectly OK? What would be the cause? (BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one used for each grid, and one for the plate via a switch) OK; measured cathode-to-plate voltage in the GR Megohmmeter; is 38.7V. Best i could do was to change my supply to 42.9V as choice of zeners was rather limited. Also measured grid-to-cathode voltage where the source was stiff WRT a 10Meg DVM (Check: -2.138, Disch: -1.859, 100Kmeg: -1.860V). Then i pulled the (almost unbalanced) tube from the Megohmmeter and made measurements and saw a few surprises. First, and most delightful, was that "peak" was almost totally gone. Second, the grid current was a lot larger at -0.5V relative to 100V plate. Not too surprising was that the grid current was substantially lower at "reasonable" biases; a lot less that i would have guessed. The Red curve is for the grid in the hi-z arm, where the unknown resistor goes. Seems that there is very little difference in the grid currents, which tends to imply that the "meter balance" pot setting may be rather sensitive. No adjustment pots are shown in the simplified schematic. Suffice to say the FP controls "Check" and "set infinity" are between the cathodes, and the thru-panel "meter balance" (unlabelled) adjustment seems to vary the voltage on the reference and unknown resistors. ** Now i do not know what the heck to look for in the tube data... |
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