[bcowen]

If u r into tube rolling, it is advisable to start collecting the datasheet of each makes, go here http://www.r-type.org/
and download the pdf files. Then, search for the supplement or data table for each tester like TV-7D/U, etc, they have their own recommended reading...you can find these online, just google.

A good place to start for tester data:

http://bama.edebris.com/manuals

 

[JTbbb]

Here is a link to an article that goes over the basics of the two most common types of testers, emission and mutual conductance (aka transconductance).

http://www.tungsol.com/html/faqs7.html

Emission testers basically operate the tube as a diode and give a measurement of the emissive capabilities of the cathode, the result is given as a percentage, with 100% being considered NOS. This is the most basic type of tester and gives some idea of remaining tube lifespan. However, it doesn't tell you much about the actual operating conditions of the tube.

A more advanced type of tester is a mutual conductance tester, the likes of a Hickok. These types of testers measure mutual conductance of the tube at a specific operating point. Mutual conductance (gm) is one of the three tube constants, the others being amplification factor (mu), and plate resistance (rp). They are related by the equation mu = gm x rp. The amplification factor (mu) is what is going to affect the practical voltage gain of the tube. If mu for two sections of a tube or two different channels of an amplifier differs significantly, you may hear a channel imbalance. But these testers do not measure mu, they measure gm. By the equation, they are directly proportional assuming rp is constant (it isn't, but close enough to be assumed). So basically you can infer that mu will be at its proper specification if gm is too, that's the idea at least, reality is different but good enough to be practical.

Gm testers will either give a direct value for gm (2600 micromhos for the 6SN7, for instance) OR they will give a value based on a normalized scale created by the tester manufacturer. For example, the Hickok TV-7D/U has its own gm scale and gives a value based on that scale that must be compared to the reference value given in the tester manual.

Okay, so for emission testers, you will see the test result as a percentage. For gm testers, you will see either A) a direct value of gm in micromhos that must be compared to the reference value from the tube datasheet or B) a value based on the normalized scale for that specific tester. For B, the seller will often say what the minimum reference value is for that tester, which is often considered to be 60% of a NOS value. So they might say something like "tested 64 on a calibrated Hickok TV-7D/U with 45 being minimum", or they might just say "tested 64/45 on a calibrated Hickok TV-7D/U". If 45 is the 60% of NOS gm based on the Hickok scale, some simple math would tell you that 64 would be roughly 85% of a NOS gm value.

That's the basic idea. However some sellers will use backslashes when tubes have two different sections, like a 6SN7. They might write it as "2700/2600 with 2600 being NOS" or "66/68 with 45 being minimum" in the case of gm testers with normalized scales, so watch out for that.

One more thing, the units of transconductance. By Ohm's low, Ohm = V/A. The units of the mutual conductance, A/V, are the inverse, the reason it is written as mho (yeah, it's literally just ohm backwards). So a millimho is mA/V, a micromho is μA/V. Transconductance of solid state components is much higher these days, and everything is expressed in Siemens (S) which is equivalent to mhos (A/V).

So, 1S = 1mho = 1 A/V = 1,000 mA/V = 1,000,000 μA/V = 1,000,000 micromhos.

That is the crash course.

Very interesting reading, thank you. Now I have to admit that I am one of the many who struggle to fully understand this subject, thus have bow to those with more knowledge/professionally trained. I have come to know an electronics engineer who specialises in tube amp and tube tester repair and rebuilds, so have thus entrusted some of my tubes to be tested. He has told me that the tests that really show a tubes worth is measuring
Plate Current/Trans Conductance PC/TC. The tester he uses is an AVO MK IV Valve Characteristic Tester.

Below is a pair of 5998’s I have had tested with the results. They sound wonderful as a pair in my Euforia amp. It can be seen though that there is a marked difference in the Plate Current on one of the tubes, which is utterly non detectable with it’s current use. However, several posts prior to this one, there is some discussion about just when would a difference be detectable? I am at some point taking delivery of a much modified Bottlehead Crack, and it will be interesting to see if this tube has any detectable channel imbalance.

 

[David222]

A good place to start for tester data:

http://bama.edebris.com/manuals

.

 

[David222]

If u r into tube rolling, it is advisable to start collecting the datasheet of each makes, go here http://www.r-type.org/
and download the pdf files. Then, search for the supplement or data table for each tester like TV-7D/U, etc, they have their own recommended reading...you can find these online, just google.

+1 for The Valve Museum ...super helpful tool. Another useful database for looking up specs is http://tdsl.duncanamps.com/tubesearch.php

 

[bcowen]

+1 for The Valve Museum ...super helpful tool. Another useful database for looking up specs is http://tdsl.duncanamps.com/tubesearch.php

Yup, and Duncan also has a program (TDSL) you can download:

http://www.duncanamps.com/tdslpe/download.html

Free and very small in size. It hasn't been updated in quite a while so a lot of the external links in the 'Data sheet links" tab no longer work, but the basic data is right and gives you a very quick reference to a lot of parameters for a lot of different tubes. I've used it for many, many years.

 

[L0rdGwyn]

Very interesting reading, thank you. Now I have to admit that I am one of the many who struggle to fully understand this subject, thus have bow to those with more knowledge/professionally trained. I have come to know an electronics engineer who specialises in tube amp and tube tester repair and rebuilds, so have thus entrusted some of my tubes to be tested. He has told me that the tests that really show a tubes worth is measuring
Plate Current/Trans Conductance PC/TC. The tester he uses is an AVO MK IV Valve Characteristic Tester.

Below is a pair of 5998’s I have had tested with the results. They sound wonderful as a pair in my Euforia amp. It can be seen though that there is a marked difference in the Plate Current on one of the tubes, which is utterly non detectable with it’s current use. However, several posts prior to this one, there is some discussion about just when would a difference be detectable? I am at some point taking delivery of a much modified Bottlehead Crack, and it will be interesting to see if this tube has any detectable channel imbalance.

Well the thing is the Euforia is a cathode follower OTL. The output stage does not result in any voltage gain in this type of circuit, in fact it actually will lose a little voltage gain. The output tubes in this case do not increase volume so to speak, what they do is generate current gain, meaning they lower the output impedance of the amplifier. A cathode follower operates at what is called "unity gain", meaning a voltage gain of 1 (in reality, it is a little less than 1). That is why you can use a 6AS7G and a 5998 in the same circuit and get roughly the same volume despite the fact that they have different amplification factors (mu). So something like a channel imbalance will likely be even harder to detect in this case. Large differences in transconductance between the two cathode follower tubes might create a perceptible channel imbalance however.

In a common cathode amplifier stage, a more typical tube amplifier stage like the Euphoria 6SN7 input, differences in mu will result in a channel imbalance. But as I said, mu is not measured, we measure things like gm and plate current to get an understanding of how close the tube is to its stock operating specs. In terms of when it would make a difference, I guess it depends at what point an imbalance between two channel becomes perceptible. A 1dB difference between two channel translates to around a 12% difference in voltage gain. If we make the assumption that transconductance is directly proportional to mu, then perhaps 12% a good rule of thumb. Someone else said they go by a 10%, which seems reasonable.

I actually use an electronic curve tracer to measure my tubes, so I am able to measure all of the tube constants, including mu, as well as trace operating curves for tubes whose datasheets are not readily available, which is very valuable for designing circuits.

 

[David222]

NU Grey Glass arrived today. Have not tried NU before -- looking forward to it based on what I've read on this thread.

 

[bcowen]

NU Grey Glass arrived today. Have not tried NU before -- looking forward to it based on what I've read on this thread.

Nice!!

 

[paradoxper]

Took me a few months to find these stupid tubes, more troubling than any previous tube.Ha.

 

[LoryWiv]

Took me a few months to find these stupid tubes, more troubling than any previous tube.Ha.

Worth the effort?

 

[paradoxper]

Worth the effort?

Monday I'll put them through their paces and we'll see.

 

[Odin412]

Took me a few months to find these stupid tubes, more troubling than any previous tube.Ha.

Very nice! Are the tubes actually blue or is it a refection?

 

[paradoxper]

Very nice! Are the tubes actually blue or is it a refection?

They are blue. They're referred to as Blue Glass just as well.

 

[Pondoro]

So I got a Sylvania 6SN7 free, I’m told it tests good, but the plastic locator pin is broken off. I’m afraid I’ll put it in wrong. I don’t want to damage my Saga. Should I just throw it away?

 

[DenverW]

So I got a Sylvania 6SN7 free, I’m told it tests good, but the plastic locator pin is broken off. I’m afraid I’ll put it in wrong. I don’t want to damage my Saga. Should I just throw it away?

You can usually tell where the locator is by looking at the base. Then you just make sure it is aligned properly, and be sure. I had a Ken rad with no guide rod and it worked fine, I just lined it up.