Minimum Acceptable Gm

Troncones · 27th August 2011, 04:13 PM

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.

Troncones · 27th August 2011, 04:16 PM

"phrase" Sorry

Alastair E · 27th August 2011, 04:29 PM

Gm is the Transconductance and will be in the tube's datasheet....

bigwill · 27th August 2011, 04:38 PM

Quote:

Originally Posted by Alastair E

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.

Alastair E · 27th August 2011, 04:47 PM

I think thats the prob here!

Too many names/acronyms for the same thing!

Troncones · 27th August 2011, 06:50 PM

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.

bigwill · 27th August 2011, 07:05 PM

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.

Wavebourn · 27th August 2011, 07:19 PM

Usually 20% loss was accepted as end of life.

dcgillespie · 27th August 2011, 08:02 PM

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

DF96 · 27th August 2011, 09:42 PM

Some new valves may be only 80%. I would say below 70% is bad.

27th August 2011, 04:13 PM	#1
Troncones diyAudio Member Join Date: Dec 2009 Location: Orange County, California.	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. __________________ The Myth: A hand full of Tubes, Caps, Resistors and Iron and you have an Amp. The Reality: You have self induced Dementia. Newbie.

27th August 2011, 04:16 PM	#2
Troncones diyAudio Member Join Date: Dec 2009 Location: Orange County, California.	"phrase" Sorry __________________ The Myth: A hand full of Tubes, Caps, Resistors and Iron and you have an Amp. The Reality: You have self induced Dementia. Newbie.

27th August 2011, 04:29 PM	#3
Alastair E diyAudio Member Join Date: Feb 2004 Location: South Wales	Gm is the Transconductance and will be in the tube's datasheet.... __________________ Das Beste Oder Nichts

27th August 2011, 04:47 PM	#5
Alastair E diyAudio Member Join Date: Feb 2004 Location: South Wales	I think thats the prob here! Too many names/acronyms for the same thing! __________________ Das Beste Oder Nichts

27th August 2011, 06:50 PM	#6
Troncones diyAudio Member Join Date: Dec 2009 Location: Orange County, California.	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. __________________ The Myth: A hand full of Tubes, Caps, Resistors and Iron and you have an Amp. The Reality: You have self induced Dementia. Newbie.

27th August 2011, 07:05 PM	#7
bigwill diyAudio Member Join Date: Dec 2004 Location: UK	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.

27th August 2011, 07:19 PM	#8
Wavebourn diyAudio Member Join Date: Sep 2006 Location: Pleasant Hill, CA	Usually 20% loss was accepted as end of life. __________________ The devil is not so terrible as his mathematical model! Wavebourn: We Create Creativity!

27th August 2011, 08:02 PM	#9
dcgillespie diyAudio Member Join Date: May 2010 Location: Ball Ground, GA	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

27th August 2011, 09:42 PM	#10
DF96 diyAudio Member Join Date: May 2007	Some new valves may be only 80%. I would say below 70% is bad.

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