Testing power tubes
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bluesguitar
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Testing power tubes
I have a question for those of you more knowledgeable on the subject. I don't own a tube tester, but have been wanting to find a way to compare the sound of 6L6's to select the best ones. And even to try and identify poor tubes. Is it safe on the amp to operate it (push/pull) with just one power tube at a time, for the purpose of getting a true representation of that individual tube? And, if so do you think this method will provide the comparative info I'm listening for? If I remember what I read correctly, Gerald Weber says that tube testers can't give you a true representation of tube quality because they don't test the tube at actual operating voltages. I'm not sure how he determines the quality of a tube? Any other suggestions for methods of comparing individual tube quality?
Re: Testing power tubes
Amateur answer here. I'm pretty sure someone else will be along with the pooper scooper for whatever droppings I might leave. This is going to be long.
I would not put one tube in a PP amp that calls for a pair.
A "real" test rig has a regulated power supply so that grid and plate voltages can be held constant while someone measures mA or current draw. (A very brief explanation and not real technical.) Realistic voltages are used, maybe 275V for EL84, and 400+ for 6L6's, etc. The ability to hold voltage constant makes for easier evaluation.
Current draw is one measure used to determine if the tube is strong or weak and it is used in matching.
In general, regardless of the particular test rig and method (see more ranting below) it is possible to measure transconductance if you are able to test at two or more different operating points. Transconductance is a measure of how stable the tube will react as operating conditions change. (Again, not technical, flame suit on.) Apparently, the bigger the number, the better, as a tube sees operating conditions vary with the signal it is being fed. You can look up the calculation for T/C on the internet, so I won't introduce that particular distraction. See here: http://en.wikipedia.org/wiki/Transconductance and scroll down for vacuum tubes.
Between current draw and T/C you can form an idea about the condition of a tube and whether it is matched to another.
If you have an amp, you can probably test tubes. Just be judicious about how you evaluate the findings. In a PP amp, IMO, it is best to have 2 tubes in the circuit. Since there is a certain amount of interaction between the 2 in a pair, it is a good idea to keep one constant and swap the other tube for testing. This should work OK as long as they are not seriously mismatched. Remember, this strategy gives a relative constant and not a real fixed constant (gimme a break on this please.) This is why I say be careful about conclusions.
In an adjustable fixed bias amp, you can take a 2 or three readings at predetermined levels of negative grid voltage. Get enough of a spread in your test points to be meaningful...not 1/2V apart. For example, an EL84 is spec'd at Va=250, Vg1=-12.5. Maybe you can get readings at 11, 12.5, and 14. Just be careful that the grid voltage choices don't cause excessive draw and certainly don't allow red plating. The problem you'll run into is that plate voltage will vary with grid voltage, so your comparisons may not be as perfect as you might like, but you can still get some useful data. When you take the readings, always take them for both tubes -- the one under test and your "control" tube. You'll find the control tube readings shift a bit as the mate changes. If you roll enough tubes through your test procedure, you'll get better at forming a picture of what the test data mean.
The simpler route is to build or convert a SE amp to adjustable fixed bias. You'll have to add a bias circuit, but this eliminates the effect of the control tube on the one under test. I actually built a fixed bias Champ to test a pile of 6AQ5's I have and I thought it worked reasonably well.
You can test on a fixed bias amp or a cathode biased amp. If you aren't too picky about the data, just use a 1 Ohm 1% precision 1W resistor as the sense resistor on the cathode and use that for mA evaluation. For cathode biased, place the 1R between the cathode and the cathode bias resistor. I say to use 1W or bigger because it will have heavier leads. You don't really need 1W. Better yet, drill a pair of holes for test points and use pin jacks to take readings to measure mV, which convert to mA. It helps to have a locking bias pot you can access from the topside of the chassis, too, so you can tinker with the bias voltage easily. This allows you to swap and read tubes with the chassis rightside up.
The other way to capture the mA data is by determining the voltage drop across the OT winding and using Ohms Law to calculate the mA. I call this the modified shunt method. You may need to disconnect your OT so you can get reliable readings of DCR on each half of the OT primary, as you'll need the information to do the calculation. (You can measure mA by the regular shunt method, but I am not fond of this...better to measure volts and calculate.)
As I said, test at 2 or 3 operating points to calculate transconductance (Gm).
To make it easier, run a series of calculations in Excel over what you expect is the relevant range, print the table and then you can read your result from the chart.
Here is my worksheet file from several years ago when I tested those 6AQ5's. BTW if anyone would like some, PM me as I must have 50 of them and we can "talk." It should be noted that while you can see my inventory here, I am not sure it was filed in a way that I can retrieve the tubes. Also, my fixed bias Champ was a failure as an amp and it was subsequently converted to traditional cathode bias with a very good result. Actually, it is a Vibro Champ and the vibro wouldn't oscillate in fixed bias.
I would not put one tube in a PP amp that calls for a pair.
A "real" test rig has a regulated power supply so that grid and plate voltages can be held constant while someone measures mA or current draw. (A very brief explanation and not real technical.) Realistic voltages are used, maybe 275V for EL84, and 400+ for 6L6's, etc. The ability to hold voltage constant makes for easier evaluation.
Current draw is one measure used to determine if the tube is strong or weak and it is used in matching.
In general, regardless of the particular test rig and method (see more ranting below) it is possible to measure transconductance if you are able to test at two or more different operating points. Transconductance is a measure of how stable the tube will react as operating conditions change. (Again, not technical, flame suit on.) Apparently, the bigger the number, the better, as a tube sees operating conditions vary with the signal it is being fed. You can look up the calculation for T/C on the internet, so I won't introduce that particular distraction. See here: http://en.wikipedia.org/wiki/Transconductance and scroll down for vacuum tubes.
Between current draw and T/C you can form an idea about the condition of a tube and whether it is matched to another.
If you have an amp, you can probably test tubes. Just be judicious about how you evaluate the findings. In a PP amp, IMO, it is best to have 2 tubes in the circuit. Since there is a certain amount of interaction between the 2 in a pair, it is a good idea to keep one constant and swap the other tube for testing. This should work OK as long as they are not seriously mismatched. Remember, this strategy gives a relative constant and not a real fixed constant (gimme a break on this please.) This is why I say be careful about conclusions.
In an adjustable fixed bias amp, you can take a 2 or three readings at predetermined levels of negative grid voltage. Get enough of a spread in your test points to be meaningful...not 1/2V apart. For example, an EL84 is spec'd at Va=250, Vg1=-12.5. Maybe you can get readings at 11, 12.5, and 14. Just be careful that the grid voltage choices don't cause excessive draw and certainly don't allow red plating. The problem you'll run into is that plate voltage will vary with grid voltage, so your comparisons may not be as perfect as you might like, but you can still get some useful data. When you take the readings, always take them for both tubes -- the one under test and your "control" tube. You'll find the control tube readings shift a bit as the mate changes. If you roll enough tubes through your test procedure, you'll get better at forming a picture of what the test data mean.
The simpler route is to build or convert a SE amp to adjustable fixed bias. You'll have to add a bias circuit, but this eliminates the effect of the control tube on the one under test. I actually built a fixed bias Champ to test a pile of 6AQ5's I have and I thought it worked reasonably well.
You can test on a fixed bias amp or a cathode biased amp. If you aren't too picky about the data, just use a 1 Ohm 1% precision 1W resistor as the sense resistor on the cathode and use that for mA evaluation. For cathode biased, place the 1R between the cathode and the cathode bias resistor. I say to use 1W or bigger because it will have heavier leads. You don't really need 1W. Better yet, drill a pair of holes for test points and use pin jacks to take readings to measure mV, which convert to mA. It helps to have a locking bias pot you can access from the topside of the chassis, too, so you can tinker with the bias voltage easily. This allows you to swap and read tubes with the chassis rightside up.
The other way to capture the mA data is by determining the voltage drop across the OT winding and using Ohms Law to calculate the mA. I call this the modified shunt method. You may need to disconnect your OT so you can get reliable readings of DCR on each half of the OT primary, as you'll need the information to do the calculation. (You can measure mA by the regular shunt method, but I am not fond of this...better to measure volts and calculate.)
As I said, test at 2 or 3 operating points to calculate transconductance (Gm).
To make it easier, run a series of calculations in Excel over what you expect is the relevant range, print the table and then you can read your result from the chart.
Here is my worksheet file from several years ago when I tested those 6AQ5's. BTW if anyone would like some, PM me as I must have 50 of them and we can "talk." It should be noted that while you can see my inventory here, I am not sure it was filed in a way that I can retrieve the tubes. Also, my fixed bias Champ was a failure as an amp and it was subsequently converted to traditional cathode bias with a very good result. Actually, it is a Vibro Champ and the vibro wouldn't oscillate in fixed bias.
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Re: Testing power tubes
Apologies in advance for a minor self-indulgent hijack.
Really, over 100 reads and no one has something to add? I'm surprised. I'm not believing my post was that good. You think maybe after tinkering with this stuff for a decade I could actually post something coherent on this topic?
Really, over 100 reads and no one has something to add? I'm surprised. I'm not believing my post was that good. You think maybe after tinkering with this stuff for a decade I could actually post something coherent on this topic?
Re: Testing power tubes
You da man!Phil_S wrote:Really, over 100 reads and no one has something to add?
I build and repair tube amps. http://amps.monkeymatic.com
Re: Testing power tubes
I read through it and think it's great.
I will let the smarter dudes correct anything that is not correct.
I will let the smarter dudes correct anything that is not correct.
Tom
Don't let that smoke out!
Don't let that smoke out!
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dorrisant
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Re: Testing power tubes
Awesome info Phil!Phil_S wrote:Apologies in advance for a minor self-indulgent hijack.
Really, over 100 reads and no one has something to add? I'm surprised. I'm not believing my post was that good. You think maybe after tinkering with this stuff for a decade I could actually post something coherent on this topic?
Thanks!
Tony
"Education is what you're left with after you have forgotten what you have learned" - Enzo
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Stevem
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Re: Testing power tubes
I can not see how testing a tube for its sonic qualitys in a SE stage when or if it will get used in a PP stage has much merit, seing as all SE stages are class A and not all PP stages are, somewhat.
All PP stages however output a low amount of pure class A power, you can design for more, in fact a PP stage using 6550s with the needed voltages and OT imp can output 25 watts of class A power and and over 50 watts A/B power.
Sorry, gettin off track here!
The only way to gauge how a tube will sound is in the amp/circuit it will get used in, testing is a different story and usless in regards to tone,clipping point and good or bad sounding harmonics in over drive mode.
All PP stages however output a low amount of pure class A power, you can design for more, in fact a PP stage using 6550s with the needed voltages and OT imp can output 25 watts of class A power and and over 50 watts A/B power.
Sorry, gettin off track here!
The only way to gauge how a tube will sound is in the amp/circuit it will get used in, testing is a different story and usless in regards to tone,clipping point and good or bad sounding harmonics in over drive mode.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Testing power tubes
Stevem: FWIW, I agree with everything you said. That long winded post of mine, at best, addresses the OP's question, "And even to try and identify poor tubes." Finding current draw and gm might separate strong from weak and maybe provide useful data for tube matching (please let's not debate methods and merit of matching here). I think you really did a good job of taking a step back to address the whole of the original post.