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Old 6th May 2009, 10:30 PM   #1
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Default Testing Gm without tube tester

What would be the simplest way to check Gm of various tubes without a tube tester? I am thinking of just setting up a quick and dirty jig. Don't really want to get fancy with opamps, A/D, and meter displays.

I have three multimeters, three oscilloscopes, audio xfmrs for isolation, and function generator. Have no problem setting up my conditions to match actual operating conditions in my amps.

Maybe measure mu, too, since I'm at it --

Thanks !
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Old 6th May 2009, 10:50 PM   #2
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You can do this in-circuit.  Put one meter, set to AC volts, on the grid (referenced to ground).  Insert another meter, set to AC milliamps, into the plate circuit right 'above' the plate.  Supply the grid with a sine signal that is well within its capabilities.  For a front end tube, say 0.1Vac; for an output tube, 1Vac.  Measure the AC milliamps on the plate meter and divide out.  gm is specified in mA / V.

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Old 6th May 2009, 11:02 PM   #3
cerrem is offline cerrem  United States
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To properly measure valves, it is paramount to regulate all voltages.... Without regulation the data is eroneous.....
95% of tube testers old and new are useless as far as I am concerned...
WIth tetrodes and and pentodes the screen voltage needs to be locked down tight with regulator... The plate should be regulated but can get away with no reg on the plate under the conditions of small signal testing and that you are in the satuaration region of curves.... Then you will use a AC mA meter in series with the plate to read gm...
Another problem I see is many testers don't regulate the heaters... This can be disaster when pluging in various number of tubes in the tester, thus a load change and input line variation....
A tube's heater is also not consistent from valve to valve of the same type... You can choose to voltage reg the heaters and you will see variation of the heater current....or you can choose to use a current regulator and then you will see variation of the heater voltage developed...
Keep in mind that gm will move about based on the operating conditions the valve sees...
So if you are operating the output valves in Class AB1, then you will have a dillema in testing... If you use small signal testing as is typicall, then the test will be at same conditions at amplifier idle voltages and bias, thus a Class A test is conducted with sine wave injection at input grid.... But the valve in the amplifier will swing into cut-off durring large signal excursion... This is where averaging the gm comes in.... Your best off testing the valve in a Class A condition which is approx 1/2 the operating conditions of the amp...this small signal gm WILL be the average gm you would get at full operting conditions...

cerrem
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Old 7th May 2009, 03:06 AM   #4
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How about using a 1 or 10 ohm resistor, and measuring voltage drop? Measurement of AC voltage is easier than AC current, and typically more accurate.

Am I correct in assuming Gm is (within reason) independent of load ? That is, plate voltage and current determines Gm, not the particular load ?
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Old 7th May 2009, 04:22 AM   #5
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Quote:
How about using a 1 or 10 ohm resistor, and measuring voltage drop? Measurement of AC voltage is easier than AC current, and typically more accurate.
Isn't that how all digital meters work?
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Old 7th May 2009, 05:17 AM   #6
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Quote:
Originally posted by zigzagflux
How about using a 1 or 10 ohm resistor, and measuring voltage drop? Measurement of AC voltage is easier than AC current, and typically more accurate.

Am I correct in assuming Gm is (within reason) independent of load ? That is, plate voltage and current determines Gm, not the particular load ?
Yes Zigzag, you can measure AC voltage across a 10 resistor. For a Gm of 1000, 1 ma of AC current must flow. (referenced to 1v AC on grid) A 10 ohm resistor would provide 10 millivolts across it. You will need a good sensitive AC voltmeter to accurately measure that. That's what I do on my rack setup for transmitting tubes. The resistor is on the bottom (negative end) of the B+ supply just before ground. I use an HP 3400A TRMS meter in case the sine is distorted. (seldom is) An average responding meter would also serve ok.

And yes, Gm is a function of current through the tube. Not plate voltage or load. My load is the PS itself, nothing else. Of course, grid, screen and plate voltages will affect tube current, and so the Gm.
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Old 7th May 2009, 08:21 AM   #7
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Measuring AC voltage (at 15kHz) across a 10 Ohm resistor is exactly how the AVO VCM163 valve tester determines gm.
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Old 7th May 2009, 12:44 PM   #8
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Excellent. Thanks !!
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Old 8th May 2009, 06:17 PM   #9
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How's this look?

Operating point chosen to match what I use in my amp.
Attached Files
File Type: pdf testing.pdf (14.1 KB, 293 views)
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Old 9th May 2009, 10:10 AM   #10
EC8010 is offline EC8010  United Kingdom
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The first will work. The second needs grid bias or a capacitor across that cathode resistor. Also, you're going to need a transformer coupled via a capacitor to extract your (very small) audio signal without blowing up your measurement electronics.
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