Indeed this
It is a measure of how much the current changes with a change in input voltage. It is usually specified in mA/Volt, S (for siemens), Mhos or μMhos, but they all mean the same thing.
Thank you your input. I'm aware the data sheets include a Gm value. I think this value for a new tube in good condition. When I calculate the Gm from the curves it is usualy about the same as the data sheet.
Again, what I'm looking for is the Minimum Acceptable Gm. I would assume this would be something less than the data sheet value.
Again, what I'm looking for is the Minimum Acceptable Gm. I would assume this would be something less than the data sheet value.
I'm with Wavebourn on this. I gave up decades ago on commercial tube testers, and developed my own testers with regulated voltages for all elements that let me test under published operating conditions. That way, there are known standards to test against. But also, I had to develop my own "English" scale along the way to translate the results into a practical rating of condition. On the tubes that I test using Gm as an indicator, I accept 90% or better of published value as "Good", 80% < 90% as "Weak" and below 80% as "Bad".
Dave
Dave
May be used as such, but from a technical standpoint knowing what GM value represents end of life is useful in many applications. Some applications are more or less critical than others so if a tube performs satisfactorily in an application (verified by substitution with a known good/new tube) then it may continue in service until it no longer does..
Tubes designed for special applications like SQ series industrial grade tubes and all telco tubes like the D3A, 5842, 417A usually give end of life transconductance values right in the data sheet.
For some precious Globe Bottle with carbon filament may be even 5% left would be still fine. 
I once bought 4 ofmetal RCA 6L6 tubes made during WW-II, all of them had Gm significantly higher than in specs, Also, all of them were identical, like twin brothers/sisters. I did not dare to use them and sold on ePay to somebody from Japan who needed them more than I did.
I once bought 4 ofmetal RCA 6L6 tubes made during WW-II, all of them had Gm significantly higher than in specs, Also, all of them were identical, like twin brothers/sisters. I did not dare to use them and sold on ePay to somebody from Japan who needed them more than I did.
Thank you.
Until I find a good tube tester I was going to try my own testing. I have a TC-2 to the test the basics. I'm building a simple bench set-up to test Gm. I have a 500V 200mA tube regulated power suppy. I building a simple 0 to -20V DC supply for grids.
I think I can use it to look at other aspects of tube performance as well. Should be fun.
Until I find a good tube tester I was going to try my own testing. I have a TC-2 to the test the basics. I'm building a simple bench set-up to test Gm. I have a 500V 200mA tube regulated power suppy. I building a simple 0 to -20V DC supply for grids.
I think I can use it to look at other aspects of tube performance as well. Should be fun.
Do you have any suggestions, schematics, wisdom regarding the "accurate, regulated filament/heater supply". This is on my list of things to address as the project proceeds, but now that it's on the table....
I agree with you Kevin, the scope of the project is a limited number of tubes. I see using the tester to filter through a batch of the same tube model. I don't intend to build in the features that would make it easy to switch from one tube type to another.
I'm also going to try using it to test tube distortion, noise, harmonics,.. ?. As a newbie I think I can learn a lot.
Tom
I agree with you Kevin, the scope of the project is a limited number of tubes. I see using the tester to filter through a batch of the same tube model. I don't intend to build in the features that would make it easy to switch from one tube type to another.
I'm also going to try using it to test tube distortion, noise, harmonics,.. ?. As a newbie I think I can learn a lot.
Tom
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