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Old 27th August 2011, 04:13 PM   #1
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Default Minimum Acceptable Gm

I see this phras used a lot by tube sellers. Unless I missed something it's not in typical tube data.

Where can I find this value for various tubes.
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Old 27th August 2011, 04:16 PM   #2
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"phrase" Sorry
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Old 27th August 2011, 04:29 PM   #3
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Gm is the Transconductance and will be in the tube's datasheet....
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Old 27th August 2011, 04:38 PM   #4
bigwill is offline bigwill  United Kingdom
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Quote:
Originally Posted by Alastair E View Post
Gm is the Transconductance and will be in the tube's datasheet....
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.
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Old 27th August 2011, 04:47 PM   #5
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I think thats the prob here!

Too many names/acronyms for the same thing!
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Old 27th August 2011, 06:50 PM   #6
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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.
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Old 27th August 2011, 07:05 PM   #7
bigwill is offline bigwill  United Kingdom
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I guess it depends on the circuit. As a broad generalisation lower Gm results in higher noise, higher output impedance and lower gain, so it depends how much of these attributes you can stand to lose in a particular application.
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Old 27th August 2011, 07:19 PM   #8
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Usually 20% loss was accepted as end of life.
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Old 27th August 2011, 08:02 PM   #9
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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".

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Old 27th August 2011, 09:42 PM   #10
DF96 is offline DF96  England
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Some new valves may be only 80%. I would say below 70% is bad.
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