Transconductance and tube performance

[atmars]

I have a jackson tube tester that tests transconductance as it's primary measure of tube life. The scale is split down the middle for "good" and "bad". Does anyone have any insight into what this really means for performance? Is a tube with a fifty percent rating half as good as one with a 100 percent rating? Is the cut off arbitrary? Does this measurement mean different things as far as performance is concerned in, say, a power tube versus preamp tube?

Thanks

[Old Tele man]

...not a direct answer to your question (I have no info on that Tube Tester), but RDH3 states that a power tubes' transconductance (gm) value is "useful" all the way down to 70% of its' 100% specified value.

...preamp tubes weren't mentioned, but are generally LESS susceptible to large declines in their gm-values during normal life.

[Prairie Dawg]

I have a jackson tube tester that tests transconductance as it's primary measure of tube life. The scale is split down the middle for "good" and "bad". Does anyone have any insight into what this really means for performance? Is a tube with a fifty percent rating half as good as one with a 100 percent rating? Is the cut off arbitrary? Does this measurement mean different things as far as performance is concerned in, say, a power tube versus preamp tube?

Thanks

What model is it?

[Chuck H]

Somewhat along the lines of what OTM said, As long as you keep the bias in check you shouldn't notice any difference down to about 30% or 40% on that reading. You are more likely to notice other issues like mechanical noise or little sputter and hiss noises from the tube becoming gassy. Or even extra hum from the heater filaments aging and drifting out of balance. IMHE as long as the tube tests within it's "good" range, and isn't microphonic or gassy, there is still good tone left in it. Even if it's days are numbered.

Chuck

[atmars]

What model is it?

It's a version of the 648. Here's a photo of what the meter looks like.

Meter pictures from antiques photos on webshots

As long as you keep the bias in check you shouldn't notice any difference down to about 30% or 40% on that reading.

I come across quite a few used tubes. Although since it's impossible to do a switched a/b test of tubes in the same amp, it's hard to say with absolute certainty, but I haven't noticed any significant difference in tone from a tube that tests, say, 95% as opposed to 65%. I did get one that tested 20 on the the scale that goes from "0" to "130" and when tested in the amp circuit the performance was definitely "poor".

[Steve Conner]

The "good" vs. "bad" thing (also known as the English scale) always struck me as kind of arbitrary. For power tubes at least, the best tester for a tube is the amp that it's used in. A 6550 that couldn't hack it any more in a Fender 400PS would likely still be perfectly adequate for, say, a Leslie amp, and the demarcation between Good and Bad on the tube tester's meter would be somewhere in between those two extreme examples.

[Bruce / Mission Amps]

As far as a vacuum tube that still works, is not shorted and inserted into a guitar amp... the best
tube tester EVER is the one that is connected in between your two ears.
I have used many preamp tubes that test as ' ? ' or less on English scales that still sound amazing in the amp!

[atmars]

I have used many preamp tubes that test as ' ? ' or less on English scales that still sound amazing in the amp!

This, I guess, is at the heart of my question. I just put a a tube into my latest build testing 65/71 and it sounded better that a 112/115 that was in it 10 seconds earlier. I the consensus that the scale has more to do with tube life and that sound is only greatly degraded at the very bottom of the scale (generally - the ears being the final arbiter)?

I was just reading the new weber book and he mentions several times the importance of "fresh" tubes. I had to wonder how "fresh" is "fresh".

[techineer]

Transconductance and tube performance

Transconductance is the relationship between the plate current and the grid-to-cathode voltage of the tube. The transconductance is an electrical property of the tube determined largely by the physical arrangement of the tube elements, and along with the value of resistance in the plate circuit, establishes the voltage gain of the stage.

The value of tube transconductance for signals is found from the change in plate current divided by the change in grid-to-source voltage that produced it. A tube with high transconductance will have a large change in plate current with a small change in grid voltage. Conversely, a tube with lower transconductance will have a smaller change in plate current for the same change in grid voltage.

This means that a tube with high transconductance will reach saturation and cutoff (clipping) much more easily (with smaller grid signals) than one with low transconductance. The graph of grid-voltage vs. plate current (Transconductance curve) may be rather sharply curved, and as a result a signal amplified by a high-gain tube may have more distortion even when it is not driven into clipping.

Tubes with lower transconductance will tend to have a broader curve to their graph, and for small signals may be more linear and have less distortion, depending on where the tube is biased.

Most manufacturers have a rating system for output tubes that is related to transconductance, but is based on the plate current for a given bias condition. Tubes with high transconductance will generally have less DC plate current for a given bias condition than those with low transconductance. So the high-transconductance, low-current tubes will have a low rating number, while the low-transconductance, high-current tubes will have a higher rating number.

With that in mind, musicians who play clean or blues music will probably prefer a tube with a higher rating number, while grunge, heavy metal and other distortion fans will probably prefer a low-rating number on their output tubes.

Tubes do tend to decrease their transconductance during their lifetime, but the value of transconductance is not an absolute indicator of remaining tube life. To use it as such, you would need to know what its transconductance was when it was new.

However, if the transconductance becomes too low, the tube grid can no longer control its plate current, and the plates become overheated and turn a beautiful cherry red. Once this happens, the tube is useful only for mood lighting or heating your apartment.

[Enzo]

I never use a tube tester, it doesn;t tell me anything I need to know. If I suspect a tube, it is far faster and more accurate to simply put a different tube in the socket and verify that indeed the tube was bad or indeed the tube was not the issue. I can do that in seconds, while the tester is still being set up.

Tube testers generally won;t tell me if a tube is noisy, if it has lost its tone, if it is microphonic, or other things. And generally the circuit of a tester does not put remotely real world stresses on the tubes it tests. Mostly what they are good at is verifying that, yep, this tube is bad.

[Old Tele man]

...adding to Enzo's comments: a tube tester doesn't test the tube under actual operating conditions either...especially power tubes.

[Pedro Vecino]

In preamp tubes a high level of transconsductance (in the same model of 12Ax7, for example) produces more gain, a more alive tone and more harmonic content in overdrive.
If the tube has quality, the selection is a way to control these aspects between some limits.
If the tube does not have great quality, a low level can be more desirable. This way Itīs possible to appease a bit the harmonic whirlpool that can produce in overdrive.
Going down a point of transconductance the trend it is to lose gain and (more importantly): lose weight in lows and/or density in the sound.
The tester only serves me for this previous selection. Itīs useful to look for balances between channels and modes.
If the tester does not allow to read relative transconductance in micromhos, it is suitable to have some new tubes (or better with a known measure) for reference.
Regards

[techineer]

I never use a tube tester, it doesn;t tell me anything I need to know. If I suspect a tube, it is far faster and more accurate to simply put a different tube in the socket and verify that indeed the tube was bad or indeed the tube was not the issue. I can do that in seconds, while the tester is still being set up.

Tube testers generally won;t tell me if a tube is noisy, if it has lost its tone, if it is microphonic, or other things. And generally the circuit of a tester does not put remotely real world stresses on the tubes it tests. Mostly what they are good at is verifying that, yep, this tube is bad.

When you deal with many amps a day, identifying the suspect tube by guess is an iffy proposition. And what if more than one tube is bad? With most amps today, the tube tester can be a relatively quick way to identify definitely bad tubes (shorts, low gain, etc.) without having to spend time figuring out which tube to suspect. And most amps only require two setups of the tube tester.... once for the preamp tubes, and once for the output tubes. Once you are sure all the tubes are basically functional, (whether or not they are being tested under "real world" conditions) then you can more confidently focus on looking for the real cause of the symptoms.

In addition, the transconductance tube tester is very useful for evaluating the condition of a set of output tubes. If the output tubes all measure a relatively high value of transconductance and are all close to the same value, that indicates to me that they are most likely good for a while longer. If one tube measures a significantly different value than the other (or the rest), I generally recommend replacement of the set. In 9 years I've never had a customer complain because I told them their output tubes were still usable.

This was what the original poster was asking about - how to interpret and apply the measurement of transconductance. He wasn't asking how to identify a bad tube. I believe he was trying to relate the effects of different values of transconductance to the tonal quality of the amplifier output.

I would be very unhappy without my tube tester.

[Chuck H]

When you deal with many amps a day, identifying the suspect tube by guess is an iffy proposition. And what if more than one tube is bad? With most amps today, the tube tester can be a relatively quick way to identify definitely bad tubes (shorts, low gain, etc.) without having to spend time figuring out which tube to suspect. And most amps only require two setups of the tube tester.... once for the preamp tubes, and once for the output tubes. Once you are sure all the tubes are basically functional, (whether or not they are being tested under "real world" conditions) then you can more confidently focus on looking for the real cause of the symptoms.

You are clearly my superior whith respect to technical knowledge. But I have to say that what works for some doesn't always work for others. IMHE tracing a problem to a bad tube (or even two) is relatively easy based on the symptoms. And since the tester cannot dianose problems like microphonics (and I've even had tubes test "good" that leaked DC onto the grid) taking the testers word for it could actually slow the trouble shooting process. But I digress because I do not do repairs professionally.

In addition, the transconductance tube tester is very useful for evaluating the condition of a set of output tubes. If the output tubes all measure a relatively high value of transconductance and are all close to the same value, that indicates to me that they are most likely good for a while longer. If one tube measures a significantly different value than the other (or the rest), I generally recommend replacement of the set. In 9 years I've never had a customer complain because I told them their output tubes were still usable.

Now I can see the point in this because setting up to test individual power tubes in the amp can be a PITA.

This was what the original poster was asking about - how to interpret and apply the measurement of transconductance. He wasn't asking how to identify a bad tube. I believe he was trying to relate the effects of different values of transconductance to the tonal quality of the amplifier output.

And the points made above by other professionals and myself indicate that unless the tube measures grossely bad the ear test is best.

I would be very unhappy without my tube tester.

It's good you have one then. I have thought about getting one but I always get over it. I do think it's a good tool for the professional to have. But probably not necisarry.

Different strokes and all...

Chuck

[Enzo]

Techineer, I DO service many amps, that is how I make my living. And as Chuck said, if you are happy using yours, the last thing I want to do is tell you not to use it. I offered my own perspective based upon my own experience.

identifying the suspect tube by guess is an iffy proposition. And what if more than one tube is bad?

Whoa there. Guess? No, I would sub a tube for the exact same reason you put it in your tester. I had reason to suspect it. If I think a tube is bad, I swap it out for a good one. That tells me right there. For me that is faster than getting out my tube tester.

Now if you use yours all the time, i am sure you have the settings for 12AX7 and 6L6 and EL34 memorized. I still have to look them up. SO you save some time there.

And what if more than one tube is bad?

Then I will have multiple clues as to that. Without boasting, my skills as a troubleshooter are a little past the rudiments. The very operation of the amp tells me what tubes to suspect.

Do what works best for you.