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Emission and transconductance in 12AX7

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10/26/2004 9:53 AM

Emission and transconductance in 12AX7

I have seen that some companies sell used tubes with exposed transconductance levels in 12AX7愀 (ex: 1150/1250). What relation has those levels with the life utility of the tube?, and has relation these measures with the gain of each triode for selecting purposes?

Others sellers expose the measurement in mA. with respect to a reference (minimun good/full). Is better this indication value with respect to the own gain of each triode or average life?

I take a long time selecting valves by its sound and functions in amps but I d惱nt know the value of these measures in relation to its selection. I know that the quality and sonic shades in 12AX7愀 are not measured in micromhos or milliamperes but I think that it would be interesting that some experts settled down the utilities and the differences between both. Thank you very much. Best regards.

10/26/2004 10:34 AM

Sorry but what are you talking about? Trasconductance? (micromhos or millisiemens units) Trasconductance, internal resistance and mu are strictly related by equation so yes they are very important regarding the gain of a triode stage (but depends more on topology) and regarding the sonic shades too, I don't know how they relate to the remaining lifespan of a tube.

What do you mean by Emission?

teo

10/26/2004 11:56 AM

Thanks, Teo. I have read an article about tube testers (Tone Lizard) that says:
"...testers which checked the 'emission' of the tube. Basically they turn all tubes into diodes, by shorting any and all grids to the plate. Then they measure the current flowing through this virtual diode by sticking an ammeter in series with either the plate or the cathode"
....................
A seller says (about an ECC83):
"triode 1: 3,2mA
triode 2: 3,0mA" "Good begins to 1,9mA"

Other seller says:
"triode 1: 1200 micromho
triode 2: 1080 micromho" "Average gm is 1250"
....................

The first measure is "emission" and the second "mutual conductance". My interest is to know the utility of this values and the relation with gain and if it exists relation with the life remain in a used tube. Thanks again

10/26/2004 1:29 PM

The basic formula is mu = gm * rp, then if you take a look at

http://www.aikenamps.com/CommonCathode.htm

you can find lot of formulas that let you calculate voltage gain of a typical triode stage.

If I understand correctly "emission" is something related to how many electrons are "emitted" from the filament heated cathode: the more the higher the current measured. It could be a good measure for the remaining lifespan of the tube altough I expect it to be higly non-linear with a sudden drop when it's near to end.

Gm should't change while the tube ages, 1250 is a typical measures for current production tubes while NOS are typically in the 1500 range (this partly explains why they sound differently)

I don't know the relation between mu/gm/rp and emission but anyway there's a relation with plate current so I expect a relation with the formers too.

teo

10/26/2004 2:42 PM

On the Hickock tube tester, a new 12AX7 will have readings of 1100 to 1500 for Gm for each triode.

The tube chart shows 875 as the minimum Gm which is considered OK for an acceptable 12AX7.

When you see a figure like 1250 for a used 12AX7, the seller is trying to give you an idea that the tube is as good as new.

Sometimes a very worn-out sounding tube will still have good Gm in the 1200 to 1400 range.

Other times a brand new or NOS tube will have wonderful tone better than most specimens, except for a slight defect like microphonics or excess hum which would not be a problem in the multitude of non-audio circuits that these tubes were also intended to be used in.

You never know how good or unsuitable it is until you hear it in your own amp.

However the Gm readings are supposed to be more accurate ways to compare tubes when different testers have been used to test them, since Gm is a determination of an amplifying parameter rather than an emission tester which mainly just picks out the bad tubes.
But even some of the best tube testers like the Hickocks are supposed to be calibrated using a factory reference 6L6 tube, and the factory has been closed for years so I don't expect the agreement to be as close between different shops as it was when tubes were more popular and the testers had more recent calibration.

Mike

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