gm vs gain

[Mars Amp Repair]

I hesitate to ask this question after being in this field for a long time. None the less here it is:
What is the relationship between gain & gm? I'm assuming Gain = Emission although I understand the meter reading isn't reflective of the actual gain of the tube since it's a Good-?-Bad general meter indication.
I know gain is Vpp out over pp in. Simple.
I also know that gm is the relationship between Vin in vs Plate current out.
Seems that tubes that have higher gm are the tubes with less gain eg 12AU7 or 12AT7 vs 12AX7. Gm for a 12AT7 is 4000 which has a lower gain than the 12AX7 with a gm of 1250.
Tube testers eg my Hickcock 666A of course read both. If you have the 'English' set for reading the gm of the tube, it is just a matter of resetting the 'English' knob to the emission setting to then see the emission reading...SO, they have to be related as it is just a matter of resetting the 'English' parameter to go from a gm reading to the Emissions reading. I guess I'm assuming the Emission reading is a reflection of the gain of the tube.

Anyway I may be way off base in my thinking here....any ideas?
Thanx, glen

[Helmholtz]

See "Emission Tester" here: https://en.wikipedia.org/wiki/Tube_t...mission_tester

From this the emission test essentially evaluates the emissivity of the cathode and the reading is not (directly) related to either gm or intrinsic gain (µ).

Some theory:

Transconductance (gm) is defined as change of plate current divided by change of grid to cathode voltage.
Voltage gain is defined as change of plate voltage divided by change of grid to cathode voltage.
All changes need to be small, i.e correspond to small signal conditions.

In circuit voltage gain G can be calculated from G = gm * Rload, where total load resistance Rload is found from internal plate resistance rp in parallel with plate resistor in parallel with external load (Rload= rp//Rp//Rext).
In circuit voltage gain is always lower than µ.

For a triode gm, rp, and µ are related by the Barkhausen equation: gm*rp = µ.

All these tube parameters vary with plate current.

[Mars Amp Repair]

So if I'm interpreting correctly: Since the Hickok 600A is a mutual conductance tester and it only requires the resetting of the English control to measure 'Good/?/Bad' reading of the tubes performance, seems it would be reasonable to say that the Good/?/Bad test is not a standard emission test.
In the Wikipedia article you so kindly linked, it states that the emission test leaves much of the tubes performance untested given they are basically connecting & testing the tube as if it were a diode. It also states: 'The advantage of an emission tester is that from all types of tube testers, it provides the most reliable warning of tube wear-out. If emission is at 70%, transconductance can be at 90% still, and gain at 100%.'
Seems to me that the Hickok 600A and other iterations of their testers have what they call the 'Life' test, where you push the 'Test' button & set the English setting for a reading on the meter of '2000' on the 0-3000 scale (which I imagine is an arbitrary setting just to judge the decay of the tubes condition) and then decrease the filament setting by one increment with the filament setting and see if the 'emission' reading decreases beyond the '?' portion of the scale. I suppose that is actually a reflection of the 'cathode emission performance.
I suppose that may make up for the deficiency of the Transconductance tube tester vs Emission tester for the issue raised above.
No matter honestly as I find the Hickok provides my needs perfectly in the audio realm of testing tubes for primarily guitar/Bass/etc amps. I find the 'LIfe' test to be particularly helpful as the only indication this tester gives you of 'Life' expectancy is that if the life of the tube IS low, the initial gm reading tends to drift downward when you initially depress the Test button.
I believe I have a better understanding of gm and gain as a result of the information you supplied.
Do you have any comments on the 'Life' test vs advantage of emission test s described above? Do you think the 'Life' test makes up for the Hickok not being strictly being an emsisions tester?
Thanx for your time...glen

[Helmholtz]

I'm not familiar with your tester and have no idea what the "English" setting means.
Don't you have a manual?

All I can say is that to measure gm the tube needs to be appropriately DC biased, an AC signal applied to the grid and the resulting AC plate current be measured.

[Mars Amp Repair]

Yes I have the calibration manual & also the user manual. They don't go into the function of the 'English' control. I look at the name 'English' just a funny name the engineers decided to use almost like the term in Pool 'Putting a bit of English on the ball to change it's trajectory. Only a supposition on my part. I think they could have called it anything,
The tester also has a 'Bias' control which I suppose actually does what that indicates: adjusts DC bias to the control grid.
The 'English' control I suppose compensates for whatever reading they expect for the parameters of the DUT. Most testers have an adjustment like this but call it different names and some just refer to it as control # whatever and provide a setting for the device under test.
All of this really doesn't matter a whole lot. As they say as a universal reply to an answer unknown: Works good, lasts a long time!'
Thanx for the info.
glen

[Helmholtz]

Just wondering, if the tester actually measures gm (as opposed to something loosely related), why don't they proudly state it in the manual?

Do new 12AT7s generally give higher readings than new 12AX7s?

[g1]

Don't you have a manual?

This one should be close.

[DrGonz78]

On my Hickok 6000a there is no English control and it is simply labeled “Shunt”. So I guess English is another term for shunt.

[Helmholtz]

This one should be close.

Thanks.

This manual clearly states that the tester measures mutual conductance (= transconductance gm) in absolute micromho (= µA/V = µS) numbers.
I don't see an "emission" test mentioned but didn't read very carefully. Except that it states that the "English" setting is not an emission test but rather a pass/fail test based on mutual conductance.

[Mars Amp Repair]

Just wondering, if the tester actually measures gm (as opposed to something loosely related), why don't they proudly state it in the manual?

Do new 12AT7s generally give higher readings than new 12AX7s?

Yes as far as gm is concerned. 12AT7 on this tester is rated at 4000umho. 12AX7 is 1250umho. This makes sense as the 12AT7 can drive more current per xnge in grid voltage, or so it seems.
Find attached pic of Hickock panel medallion. Yes, it is a mutual conductance tester. I guess I didn't make that clear. glen

[Helmholtz]

Find attached pic of Hickock panel medallion. Yes, it is a mutual conductance tester. I guess I didn't make that clear. glen

But why were you talking about emission test in your first post?

Emission is not a parameter specified by tube maufacturers.
I can only assume that a new tube is supposed to start out at 100%.

[Mars Amp Repair]

But why were you talking about emission test in your first post?

My bahd as I was surmising it might be an Emission test on the Hickock. Since the Emission test requires connecting the grids & plate together, I guess it should have been obvious just resetting the English control could not be equivalent to an emission test.
I am thinking the 'Life' test might make up for the one advantage listed for the Emission test over the Mutual conductance test.
Either way it appears the Mutual Conductance tester has more flexibility in many ways than the Emission tester.
TI added an adaptor to the octal socket that allows measuring the cathode current across a 1ohm resistor on a standard meter. That allows me to match tubes for gm & idle current measured at this tester settings. Very handy.
Judging by the different ratings of tubes the gm ratings on matched tubes I test do match my tester readings but the cathode current rating does not as I suspect each tester has different parameters as to what part of the load line it tests the tubes. So, the current readings rarely match the labels on the pre-matched tubes I purchase. No matter as I just use the current reading to match tubes for my use.
Thanx, glen