Tube datasheets specify a certain gm. gm varies according to the setup, operating points, etc. When the specs were published, was there a standard setup that was used to measure those values?
I am trying to establish a good reference point for measuring the gm of various tubes.
I am trying to establish a good reference point for measuring the gm of various tubes.
Each tube is specified under what conditions Gm is measured. +/-20% is a good guideline for measurements of new tubes.
For example, the 2A3 is specified with 250Vak, 45Vgk, and 60mA Ia.
Try to get a tube to run that.
Best you can do is pick two parameters and let the third go where it will (dependant on the tube).
Gm is measured under fixed anode voltage conditions, so you have to use a very small sense resistor to measure the change incurrent for a small change in grid voltage.
You can chose to run the tube at Vgk=45V and Vak=250V and just measure Ia to see where it is, or you can set Vak=250 and adjust Vgk to get Ia=60mA and measure Vgk, or you can set Vgk=45V amd Ia=60mA and see where Vgk goes.You can only pick two. The tube determines the third.
For example, the 2A3 is specified with 250Vak, 45Vgk, and 60mA Ia.
Try to get a tube to run that.
Best you can do is pick two parameters and let the third go where it will (dependant on the tube).
Gm is measured under fixed anode voltage conditions, so you have to use a very small sense resistor to measure the change incurrent for a small change in grid voltage.
You can chose to run the tube at Vgk=45V and Vak=250V and just measure Ia to see where it is, or you can set Vak=250 and adjust Vgk to get Ia=60mA and measure Vgk, or you can set Vgk=45V amd Ia=60mA and see where Vgk goes.You can only pick two. The tube determines the third.
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As gm increases with quiescent current, manufacturers tend to specify gm at a current somewhat higher than you would normally use.
Cheers
Ian
Cheers
Ian
You can tell how linear the tube is by measuring the gm1 at two different (like 2X) plate currents in the operating range. This is a factor that should be on the data sheets but isn't.
Ideal 3/2 power law tubes will vary gm at the 1/3 power of the current (screen grids and plates usually measure this way), but typical tubes vary g1 gm around the 2/3 power due to grid wire proximity effects. (hence the residual variation of Mu with current)
(a truly linear tube would not vary gm at all on any element versus current, or voltage for that matter)
Ideal 3/2 power law tubes will vary gm at the 1/3 power of the current (screen grids and plates usually measure this way), but typical tubes vary g1 gm around the 2/3 power due to grid wire proximity effects. (hence the residual variation of Mu with current)
(a truly linear tube would not vary gm at all on any element versus current, or voltage for that matter)
Mu = gm1/gm2 or gm1/gm(p)
ie, from the two tracking well
One would like to know the power law for the mistracking. (the difference in the gm power laws of the two relevent tube elements.)
ie, from the two tracking well
One would like to know the power law for the mistracking. (the difference in the gm power laws of the two relevent tube elements.)
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I have two examples of a simulated measuring circuit here. One for a 12AX7, and the other a 12BH7.
Each have a 3k tone injected into the grid. The ratio of the 1k shunt RMS voltage divided by the shunt value to the RMS grid signal is taken. That is the gm.
So would it be safe to make whatever measurement as found at that operating point to be a reference gm, assuming the spice models are ideal?
Each have a 3k tone injected into the grid. The ratio of the 1k shunt RMS voltage divided by the shunt value to the RMS grid signal is taken. That is the gm.
So would it be safe to make whatever measurement as found at that operating point to be a reference gm, assuming the spice models are ideal?
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I'm not sure what you mean by 'reference gm'. The circuit you give will allow measurement of gm (approximately) at your operating point. Using a 100R resistor instead of 1k will be even more accurate.
Reference gm means that the spice measurement is used, at the selected operating point, as a reference (a statistical mean) when testing tubes.
I expect much statistical variation from this point and expect the tube to be weak if its is considerably less than the mean (around 3100 mmhos).
I expect much statistical variation from this point and expect the tube to be weak if its is considerably less than the mean (around 3100 mmhos).
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