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Old 4th September 2009, 02:49 AM   #1
TheGimp is offline TheGimp  United States
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Default Measuring Tube Gm

I tried searching and couldn't find any ref to measuring tube GM so I'm hoping this will be helpful.

I've got several 6BA6s that look like they are new. THis is the 'Cal tube' for the Mercury 1000/2000 Tube Test sets.

The tubes measure close to spec for Gm on my Mercury 1000 (which I recently calibrated). I'm trying to measure the tube Gm to verify the Mercury 1000 calibration.

I built a simple amp circuit using cathode bias (560 Ohm with 2u2 bypass), 4900 Ohm Plate resistor, 165V Plate supply, 1M grid resistor to gnd, 2nd and 3rd screens tied to plate. 2u2 Coupling cap on input from a signal generator.

The amp comes up with the plate voltage at 125V and cathode resistor drop of 4.5V for a bias current of roughly 8mA instead of the target 10mA. Close enough to start out with.

Problem is if I put a 3Vrms (1Khz) input, I only get an output of 15.2Vrms. With a load resistance of 4900 ohms this yields a Gm of 1034u-Mho. The tube is spec'd at 4300u-Mho.

I've calculated gain as Ip / Vi = (Vo / Rp) / Vi ....

With Cathode Bypass I was expecting to see the full gain of the tube. even though I'm 35V below the design spec on the tube.

Where am I going wrong?

Thanks.

Steven
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Old 4th September 2009, 02:53 AM   #2
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Hi,

You need a reference tube for calibration, because Gm will change with grid/plate voltages and current.

That's why a tube may measure one Gm on a TV7 and another on a B+K 747 (both acclaimed top-of-the-line).

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Old 4th September 2009, 03:11 AM   #3
TheGimp is offline TheGimp  United States
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Precisely.

I'm Trying to understand how to test a tube to the manufacturer's data sheet to verify Gm, In which case it becomes a reference tube.

The 6BA6 is spec'd at 4300-4400u-Mho. But, with what voltages? The data sheet (on nj7p) shows two sets of "Characteristics and Typical Operation" one at 250V Plate voltage and 11mA bias current (4400u-Mho), the second at 100V Plate voltage and 10.8mA bias (4300u-Mho). I assumed (yea, you know where that gets you) that I could set the tube up between the two and get near the same Gm since there was not much of a spread.

So why am I measuring only 1/4th what the spec is? Crank it up from 8ma to 11ma? Seems like that shouldn't make much difference.

I also tried a lm317 10 mA current source for cathode drive and it made little difference.

Steven
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Old 4th September 2009, 05:11 AM   #4
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Hi Steven,

The formula for transconductance or Gm of a vacuum tube is as follows. Gm=delta Ia delta Vg. This is the difference in plate current divided by the difference in grid voltage that produced it. So in simpler terms it would be plate current divided by grid voltage. These terms are for RMS plate current & RMS grid voltage. Not DC values. Basically, a 1V signal that causes a 1 milliamp change in plate current is said to be a Gm of 1000.

DC plate current affects Gm much more then voltage, which only affects it to the extent that it can help determine current. It is also much easier to use even numbers in multiples of 10. In my large tube tester I use 1V RMS on the grid and measure RMS plate current across a 10 ohm resistor. Because I use much higher voltage, I have the 10 ohm resistor at the bottom of the plate supply just before ground. You will need a good RMS millivolt meter to make this reading. I use an HP 3400A but there are many other suitable types.

So as an example based on an AC Vg of 1, an AC plate current of 10 millivolts across 10 ohms would be a Gm of 1000, (referenced to a predetermined DC plate current). This could be scaled up accordingly to 100 millivolts across 100 ohms etc. Please note that on my system the plate PS is the plate load. No other load resister is used. For small signal tubes like yours a larger load resistor would be more useful, perhaps 1K or even 10K.

You will need a well filtered DC supply to avoid including hum in your measurment. For what you are doing, this should not be a problem. I would suggest that you provide some way of setting your tube under test to the required plate current. A bench supply with meters would be perfect. Perhaps keep us posted on your results.
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Old 4th September 2009, 05:23 AM   #5
TheGimp is offline TheGimp  United States
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Thanks.

I've been setting the input signal (1KHz) to 3vrms, and measuring the rms voltage at the plate with a 4K97 resistor for the plate resistor. I chose 1KHz as my input signal to avoid interference from 60/120hz ripple.

I used the rms plate voltage divided by the 4K97 resistance to determine the plate rms current, then divided it by the input rms voltage to get Gm.

I found the RCA tube spec to be more informative. I've changed to a separate bias supply for the grid(1), and tied grid 2 to a resistor divider to supply 100V to it. This made a big difference.

It looks like I need separate supplies for plate, grid(1), and Grid (2) to be able to adjust them to the spec in the book for each tube I test.

I'm up to around 3000u-Mho now from my initial 1000 u-Mho readings so I'm moving in the right direction.

Steven
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Old 4th September 2009, 06:28 AM   #6
Yvesm is offline Yvesm  France
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Quote:
Originally Posted by HollowState View Post
Hi Steven,

The formula for transconductance or Gm of a vacuum tube is as follows. Gm=delta Ia delta Vg. This is the difference in plate current divided by the difference in grid voltage that produced it. So in simpler terms it would be plate current divided by grid voltage. These terms are for RMS plate current & RMS grid voltage. Not DC values. Basically, a 1V signal that causes a 1 milliamp change in plate current is said to be a Gm of 1000.
Absolutly true !
Except you sould explicitly state "at constant plate voltage".

Steven, the deviation you noted is because you inserted a resistor in the plate circuit to measure the AC voltage accross it and this obviously means that plate voltage is no longer constant.
Use a very lo value for this resistor, as lo as your can regarding your meter sensitivity.

Yves.
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Old 4th September 2009, 01:53 PM   #7
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A couple of other suggestions to get around the problem load resistor changing the plate voltage:

1)Use a milliameter and measure Ip directly while keeping Vp constant and varying Vg. Of course I realize that if you had a milliameter you probably would already be doing this, but I thought I would mention it.

2) Use a smallish load resistor by all means, but, you may not have any in the range of 1 ohm etc. So instead take measurements using 2 different load resistors of some reasonable size you do have, a few K or whatever. Use the same ac value of Vg both times. Call the first load resistor R1, and the second R2. Call the corresponding output ac voltages at those resistors V1 and V2. Then based on the equation V = Mu * Vg * Rl /(Rl + Rp) and the necessary algebra you get

Rp = R1*R2*(V2-V1)/(V1*R2 - V2*R1)

then Mu = V1 * (R1+ Rp)/(R1*Vg)

and finally Gm = Mu/Rp.

Now it can still be objected that Vp is not held constant. But this is the case in real circuits anyway, Vp is not constant, Ip varies, Gm and Rp vary. Thats reality. The best you can do is use a value estimated at the bias point. Gm, Mu and Rp are small signal concepts. Derivatives, tangents to curves at a point and all that. Dont worry about it when you build a circuit with 4900 ohms load and swing 15.2 volts across it that Gm doesnt match the spec sheet. You are now in the large signal world, Gm, Mu and Rp dont mean so much. Mu stays more level than the other two, use Mu and loadlines for these voltage levels.
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Old 4th September 2009, 02:11 PM   #8
Yvesm is offline Yvesm  France
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I second that of course!

May be good to point that neither Gm nor Rp are constant, but variable according to plate current and voltage.
Fortunatly they move in opposite direction that's why their product (that is Mu) remains more stable.

Mu is also defined as the plate voltage variation for a grid voltage change at constant plate current.

On published plate curves Mu is the lenght of an horizontal (constant current) line between two grid curves while the slope/transconductance is the lenght of a vertical (constant voltage) line between grid curves.

Yves.
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Old 4th September 2009, 04:51 PM   #9
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Quote:
Originally Posted by TheGimp View Post
I built a simple amp circuit using cathode bias (560 Ohm with 2u2 bypass), 4900 Ohm Plate resistor, 165V Plate supply, 1M grid resistor to gnd, 2nd and 3rd screens tied to plate. 2u2 Coupling cap on input from a signal generator.
Steven
I did read this originally but forgot to comment on it. Yes, testing a 6BA6 as a triode instead of a pentode will lower the Gm value.

Quote:
Problem is if I put a 3Vrms (1Khz) input, I only get an output of 15.2Vrms.
I'm thinking 3V RMS may be too high for this small tube. Three volts may be causing it to limit by going into partial cutoff. Perhaps looking at the waveform with a scope would verify this. I'd suggest no more then a IV signal.
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Old 4th September 2009, 05:33 PM   #10
kevinkr is offline kevinkr  United States
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I have actually measured the transconductance of 211 at varying grid and plate voltages (900V - 1100V) with good success using static dc measurements at several points along the characteristic curve of the device in question.

I found that you generally need to stay in the most linear portion of the curve and you need at least two points in that region as far apart as you can reasonably manage. The transconductance you measure is strongly affected as well by how big the difference in plate currents measured actually is, and by the current range over which you measure. (As you know transconductance is not a constant.)

I use precisely measured fixed grid bias (not cathode bias) and carefully measure the plate current at a known (and carefully measured) plate voltage for each known grid bias voltage. This provides pretty consistent results. This works for any tube, including your reference tube.

I use precision digital meters and avoid current sampling resistors when possible. With triodes you can usually measure current in the cathode circuit which at very high voltages is somewhat safer than measuring plate current directly in the plate circuit. With pentodes and tetrodes you have to measure the plate current and the comment about using a small current sampling resistor is relevant, although with static dc measurements as long as you measure the plate voltage as well you are usually all set. You should of course also measure the screen current and voltage. Bear in mind that the curves shown in a data book are generally the average of the measurements from a relatively large sample set. No tube you measure is going to have exactly the same transconductance as that based on the curves and data book specification - however it should generally be very close - with large sample sizes it may be possible to select a tube that closely matches the average, why you would want to is less clear..

Note that if you measure enough discrete dc op points (Vg vs Ip) at a given plate voltage you can generate very reasonable looking curves that match what you see in the data sheet somewhat closely with the caveat mentioned above. Because this is a slow process you generally can't replicate the curves into the area where dissipation ratings are exceeded except very briefly.

Calculate Delta Ip/Delta Vg per Hollowstate's remark in post #4

Don't put too many hours on your calibration reference tube as that will result in parametric changes (aging) that will ruin it as a calibration reference.
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Last edited by kevinkr; 4th September 2009 at 05:46 PM.
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