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<td width="59" align="center" height="18"><b><a style="text-decoration: none" href="http://www.audiotubes.com" target="_self"><font size="1" color="#FF0000" face="Verdana">Home</font></a></b></td>
<td width="85" align="center" height="18"><b><a style="text-decoration: none" href="http://www.audiotubes.com/bjtubes.htm" target="_self"><font size="1" color="#FF0000" face="Verdana">Tubes</font></a></b></td>
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<td width="85" align="center" height="18"><b><a style="text-decoration: none" href="http://www.audiotubes.com/12ax7.htm" target="_self"><font size="1" color="#FF0000" face="Verdana">12AX7</font></a></b></td>
<td width="59" align="center" height="18"><b><a style="text-decoration: none" href="http://www.audiotubes.com/12au7.htm" target="_self"><font size="1" color="#FF0000" face="Verdana">12AU7</font></a></b></td>
<td width="85" align="center" height="18"><b><a style="text-decoration: none" href="http://www.audiotubes.com/12at7.htm" target="_self"><font size="1" color="#FF0000" face="Verdana">12AT7</font></a></b></td>
<td width="59" align="center" height="18"><b><a style="text-decoration: none" href="http://www.audiotubes.com/6bq5.htm" target="_self"><font size="1" color="#FF0000" face="Verdana">6BQ5/EL84</font></a></b></td>
<td width="85" align="center" height="18"><b><a style="text-decoration: none" href="http://www.audiotubes.com/6sn7.htm" target="_self"><font size="1" color="#FF0000" face="Verdana">6SN7</font></a></b></td>
<td width="59" align="center" height="18"><b><a style="text-decoration: none" href="http://www.audiotubes.com/6sl7.htm" target="_self"><font size="1" color="#FF0000" face="Verdana">6SL7</font></a></b></td>
<td width="85" align="center" height="18"><b><a style="text-decoration: none" href="http://www.audiotubes.com/6dj8.htm" target="_self"><font size="1" color="#FF0000" face="Verdana">6DJ8</font></a></b></td>
<td width="59" align="center" height="18"><b><a style="text-decoration: none" href="http://www.audiotubes.com/6v6.htm" target="_self"><font size="1" color="#FF0000" face="Verdana">6V6</font></a></b></td>
<td width="85" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/el34.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>EL34/6CA7</b></font></a></td>
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<td width="59" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/caps.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Capacitors</b></font></a></td>
<td width="85" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/fender.htm" target="_self"><b><font size="1" color="#FF0000" face="Verdana">Fender Amp Tubes</font></b></a></td>
<td width="59" align="center" height="16"><a style="text-decoration: none" href="http://www.audiotubes.com/bidlist.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Tubes We Buy</b></font></a></td>
<td width="85" align="center" height="16"><a style="text-decoration: none" href="http://www.audiotubes.com/bjlinks.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Links</b></font></a></td>
<td width="59" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/faq.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>FAQ</b></font></a></td>
<td width="85" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/mullcode.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Euro Tube Codes</b></font></a></td>
<td width="59" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/teledate.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Telefunken Tube Codes</b></font></a></td>
<td width="85" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/order.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>How to Order</b></font></a></td>
<td width="59" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/ordernow.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Order Online</b></font></a></td>
<td width="85" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/ordertip.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Ordering Tips</b></font></a></td>
<td width="59" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/phifi.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Hi-Fi Accessories</b></font></a></td>
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<td width="59" align="center" height="18"><a style="text-decoration: none" href="http://audiotubes.com/books.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Tube Accessories & Books</b></font></a></td>
<td width="85" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/shirts.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Tube Shirts</b></font></a></td>
<td width="59" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/speaking.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Speaking of Tubes</b></font></a></td>
<td width="85" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/tubesave.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Free Screensaver</b></font></a></td>
<td width="59" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/tubetest.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Tube Testers</b></font></a></td>
<td width="85" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/warranty.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Our Guarantee</b></font></a></td>
<td width="59" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/radios.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Old Radios</b></font></a></td>
<td width="85" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/wbdj.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Radio Station</b></font></a></td>
<td width="59" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/radios.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Old Radios</b></font></a></td>
<td width="85" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/amps.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Tube Amps</b></font></a></td>
<td width="59" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/remote.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Remote Controls</b></font></a></td>
<td width="85" align="center" height="18"><a style="text-decoration: none" href="http://www.audiotubes.com/bjrlegal.htm" target="_self"><font size="1" color="#FF0000" face="Verdana"><b>Legal Disclaimer</b></font></a></td>
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<H1>SPEAKING OF TUBES.....</H1><BR>
<P><CENTER><EM><STRONG>A series of articles appearing on this page from time to time, tossing my two cents worth in about those glowing bottles.</STRONG></EM></CENTER><BR>
<P><IMG SRC="hr_glow1.gif" WIDTH=588 HEIGHT=25 ALIGN=top><BR>
<P><CENTER><H2>TUBE TESTS, TRANSCONDUCTANCE.....FABLES AND FACTS</H2></CENTER>
<P><CENTER><H3><STRONG>Two thought provoking e-mails I received from an electrical engineer on a subject filled with misconceptions and mystery. The writer is Edwin Pettis of Pettis Engineering, and I thank him for his insight and valuable information he has given me.</STRONG></H3></CENTER><BR>
<P><H2>Dear Brent:
<P>You've got quite an inquiry here.
<P>First off, the tube testers. Emission testers are the lower end of the tube tester hierarchy. Basically they tie all the grids and plate together and test the tube as a diode, checking for cathode emission or current capability. I have one of these which I use every now and then. They are not considered to be a very good indication of the tube's overall health. This reading is often referred to as an English reading tube tester (good or bad). The "life test" is accomplished by reducing the filament voltage about 10% and checking the reading, it is only a very rough approximation of life left.
<P>The next step up is the transconductance or mutual tube tester. These were made in a wide range of models, accuracy and tests applied. Basically an AC voltage is applied to the control grid and the tube's amplification factor (or mu) is measured under dynamic operating conditions. This is generally the best method of testing tubes because an emission test only tells part of the tube conditions. The sophistication of the mutual testers varied a lot. Some of them were little better than emission testers. Mutual testers allowed the user to set the bias for the tube and test for several important items such as gas, transconductance, shorts, noise and life. I have a Hickock Model 752A which was one of the better mutuals made. Calibration of these testers were generally done by using a "standardized" tube. This tube's characteristics were measured by other instruments, such as the handy dandy VOM and "selected" for the proper calibration values. This tube was then placed in the tester and its circuits calibrated to matched the tubes values. Manufacturers were, for some reason, loath to release the calibration tube's values to anyone outside of the factory. However, over the years, a backdoor approach was found to calibrate some models and in other instances, the actual calibration procedure was found and leaked out to the public.
<P>There is also one other class of tube testers which were designed to actually simulate the actual operating conditions for a particular tube, be it small signal or power tubes. These were considered laboratory grade instruments and were quite expensive even compared to the expensive top-of-the-line mutuals. These had a full set of variable power supplies to set all the various grid and plate voltages to actual operating conditions. These could perform tests that none of the others could. The Weston Electrical Instrument Corp. Model 686 was one of these. There were lots of dials and meters for monitoring just about every thing. About the only measurement they didn't do was distortion.
<P>Another type of tube tester that was a kind of hybrid between an emission and an mutual tester. These performed an AC dynamic test on the tube like the mutual but readout the test on an English scale like the emission tester. I have one of these beasts from Precision Instrument Corp. who made a few versions of them. Mine is about in the middle of the group as far as capabilities go. They claim to produce a "true" reading of the tube's ability to work in a circuit.
<P>I use this one every now and then when I run into a tube which gives me border line readings on the 752A (transconductance). Interestingly enough, a tube which reads near border line or below minimum on the 752A will read rather good on the Precision. Sometimes, in the case of a dual triode (like a 6SL7 or 6SN7), there will be a fairly large difference in reading between section on the 752A. On the Precision, there might be much smaller or no difference at all between sections.
<P>When it comes to testing tubes, there is a GOLDEN RULE which should always be observed! First, go ahead and check the tube in a tester for shorts and quality. Any tube with a short should be thrown out. Second, even if the tester says a tube is border line or weak, DOES IT WORK IN THE CIRCUIT PROPERLY? This is the final test for a tube, if it works in the circuit then there is NO reason to change it. Many circuits (well designed) are designed to work perfectly well with a tube until it is near the end of its life and, depending on the application, is anywhere from 80% (military again) to 50% or even lower of "nominal" transconductance or other pertinent parameters.
<P>Someone who returns a tube because its transconductance reading is below some arbitrary value is wasting their money, big time! If you look at a tube's data sheet, characteristics such as gain and transconductance given are NOT guaranteed minimum values. These values are for a large number of tubes which were tested and the results averaged and compared to calculated values which the tube designer was aiming for. These data sheet values can vary 20% or more from the given value and still be perfectly good tubes which will operate in just about any circuit they are put in. This doesn't mean they will operate in EVERY circuit well. This fully depends on what the circuit designer was trying to accomplish and his parameters for the circuit to operate correctly.
<P>In the case of an audio output tube whose Gm isn't above a given value, no sweat, no effect on operation if the circuit was well designed. If the circuit has deficiencies then there could be trouble. If the driver circuit can't provide sufficient "drive" (voltage or current) then there will be a drop in output power and a small increase in distortion. This condition is not the fault of the tube but poor circuit design (homemade perhaps? Commercial too). Low emission will cause a drop in power output and, because they turn up the volume to compensate, more distortion. A well designed circuit should be able to operate correctly and meet specs even at 60-65% of transconductance. The minimum Gm given on a tube tester should be viewed as a warning only that the tube is nearing the end of its useful life but its not time to chuck it yet. I've seen circuits work just fine up until the tube was all but dead on a tester.
<P>The bottom line is: if it works in the circuit don't fret about the tester's reading, it's alright. When it gets old enough to influence the sound (in the case of an audio amp), then its time to change it. Anybody who goes by the numbers only is a fool who is soon parted from his money! Especially those who have no idea what the numbers should be in the first place!
I<P>'ve got an amplifier that is nearly 30 years old and still has the original tubes in it save for the 6286/EF86 which was getting quite low in its readings. The 6L6GB (SYL) are still reading well above the minimum Gm value after all this time on them. You think I'm going to change them any time soon?
<P>As to why a set of tubes won't "bias up" in a particular amplifier is open to speculation without more information, such as the schematic and operating voltages. He may have a problem with his bias circuit, resistors drifted out of tolerance perhaps. In general, if a set of tubes won't bias in, I'd suspect the bias circuits first before the tubes. It is unlikely that there was a problem with the tubes since they operated just fine in another amplifier. I have come across an amp or two with a tube that wouldn't work right, the voltages were off but the tube seemed fine otherwise. Changing the tube made no difference and I could not find any parts that were out of tolerance but that was many years ago when I was younger and didn't have quite the data base I do now. At that time I couldn't test coupling capacitor like I can now and I'd bet a donut that the coupling capacitor was "leaking" a little plate voltage into the control grid of the next stage where the problem was. Those wax covered capacitors had a weakness for leaking after a few years in a tube circuit.
<P>A gassy tube can become difficult to bias in some circuits. Don't confuse the particular BLUE ionic glow present in a lot of power tubes with gas. THE TWO ARE NOT THE SAME!!! Ionic glow is quite normal even if it isn't in every tube. Power tubes aren't the only types that have ionic glow. I've seen it in a lot of tubes such as the 6SN7 and similar tubes which can handel several watts. Perfectly normal and doesn't affect operation in the slightest. I'll bet those bone-heads will toss the tube if they see it. Both of my SYL 6L6GBs glow in the amp, so what!
<P>Any way, there's my opinion on your inquiry. I hope it gives you some useful information.
<P>Best regards,
<P>Edwin
<P>OK? ARE YOU STILL WITH ME? EDWIN HAS MORE, AND IF YOU ARE CONFUSED AND CONCERNED BY TRANSCONDUCTANCE READINGS, READ THE FOLLOWING E-MAIL AND RELAX!
<P>Dear Brent:
<P>Caught a quick look at your e-mail from yesterday this morning but didn't the time to do much with it. Like just about everything about tubes, this subject has no short answers. The general overview I sent you does well to begin with, but that's about as far as that could go. So I'll try to get more specific without getting too "technical".
<P>The term "mutual conductance" originated by Hazeltine in 1919 to express the conductivity of a vacuum tube. This term has been replaced with "transconductance", the two are interchangeable.
<P>Transconductance is the change in plate current expressed in microamps divided by the signal voltage (DC or AC) at the control grid of the tube. Conductance is the opposite of resistance and the original name of Mho was adopted for this unit of measurement. Mho has now been replaced by the term Siemens (no, it has nothing to do with sex), also interchangeable with each other. The basic unit is quite large for practical use so the term uS is used for 1,000,000th of a Siemen. 1,000 uS is equal to .001 Siemens. Of course, you won't see Siemens on any of the tube data sheets most likely (except for currently manufactured tubes maybe) since the change came after most tube manufacturers stopped making them. You'll see Gm, umho or both on the data sheets, all the same thing.
<P>Now we'll depart from the simple and move on the a little more involved detail beginning with the equation which defines conductance.
<P><u>Gm = the change in plate current </u> where the plate voltage is held constant!
the change of signal voltage ........................................ at the control grid, DC or AC
<P>Now this is VERY IMPORTANT, the transconductance measured at this point is only valid for this one point and no others! You'll notice an attached file for a 12AX7, if you look at the graphs, you'll see that transconductance varies (like everything else in a tube) in a significant way but I'll get to that a little later.
<P>Theoretically, a transconductance tube tester will consider all the dynamic constants of the tube under test to give its one single reading of transconductance on the meter. An AC signal (usually 60Hz from the power supply) of "calibrated" amount is applied to the tube's control grid and the tube's assimulated operating conditions are measured as actual transconductance. Note, emission type testers only measure the emission of the emitting surfaces of the filament or cathode elements. One big difference between the tube tester and an actual circuit is that the tester measures a tube at a fairly low voltage, usually somewhere in the neighborhood of 200-250 volts. Some testers also have an even lower plate voltage setting for tubes that can't handle that much voltage.
<P>One very good thing going for the transconductance tester is that it is a dynamic test, not static like emission testers. The drawback is that it doesn't simulate the actual circuit the tube is going to be working in. As I mentioned before, there were testers designed with this purpose in mind but finding one today is extremely unlikely. They were mostly found in the laboratory and few ever made it to the surplus market.
<P>Transconductance was usually measured on the flatter part of the curve to minimize variations in tube to tube readings. Portions of the transconductance curve are fairly "flat" as such, as you can see on the 12AX7 graph. I choose the 12AX7 as an example because it is one of the relatively few tubes that have complete curves available. Very few tubes will have a set of curves like this, although there may have been such data available from the tube manufacturers for their biggest customers. Such data was very time consuming and expensive to come up with and, as always with tubes, were only "average" characteristics from selected tubes from several production runs (back to statistics again). This type of data was very valuable to the design engineer because if a "tight" design was required these curves were very helpful otherwise he might have had to make them himself.
<P>The so-called standard method for measuring transconductance was to apply 10 millivolts at the control grid through a transformer at 60Hz. An AC milliamp meter calibrated to read out in transconductance is in the plate circuit, Gm=Ipac X 1000. Normal bias and plate voltages are applied, both being regulated.
<P>Now, as to when is a tube to be considered unsatisfactory and rejected for low transconductance? The answer depends on several factors, including the application and circuit design. On many circuits and applications, when the transconductance fell to 75 to 80% of "normal" (there's one of those gray area terms again). For some types of tubes and applications, they may be permitted to fall to 60% or even 50% of "normal". Unfortunately, this term of "normal" usually isn't known to most people except the designer or, if you were very lucky, it might be specified in the service manual for the instrument. You might also be lucky if your application is running at a specified point that is given in a tube manual and the transconductance is given for that point. More often than not, that information will not be given. In which case, you either go by a reliable transconductance tube tester or your ears.
<P>I have one data sheet from GEC (Britain) for the KT66 (6L6GC) which actually did spellout when to replace this tube, again, this was for a specific operating condition. I have attached this file also, interestingly, there were no curves for this tube. The specified setting for measuring transconductance, you'll notice, are not at any of the operating voltages normally encountered for this tube but are similar to what was used in a transconductance tube tester. However, it is unlikely that the tester would run the tube at 85mA as specified. Another interesting item is the power level at which the tube is considered to be replaced.....50%!
<P>For use in a high fidelity amplifier, I would use the 75% figure of transconductance (assuming you knew what "normal" was in the first place) and probably use a little higher figure for power output than 50% (if you can afford it). My Hickok 752A gives a transconductance figure (minimum) of 3000 for a 6L6, however, you must remember that is for the tester's settings and not the application's requirements. If you really wanted to get fancy, you could make a test circuit with the proper settings to test transconductance at the amplifier's settings but that would really be gilding the lilly.
<P>In the case of an audiophile who "thinks" he/she really needs to know such figures ( too much information can be a dangerous thing in the wrong hands), they need to get a good transconductance tester (or find someone with one) and research what the proper readings are for their particular equipment and how to use the tester properly. The old drug store/super market testers were quite unreliable and were known to be set on the low side to increase tube sales. I think that in most cases, a person should rely on their own ears to tell when their amplifier needs servicing and then take it to a reliable source for checkup (be that a technician in a repair shop or someone who "knows" what they are doing). If their amplifier does have meters for monitoring bias, tube current and whatever, the instruction manual will give them readings at which time the tubes should be replaced. These days, changing tubes on a whim or a fancy can end up costing you a lot of unnecessary money, if you can afford it, so be it but, in the end, it drives up tube prices even more because of unnecessary demand.
<P>You'll also notice from the tube curves, that just about every parameter of a tube varies with the operating point and in the case of an audio amplifier, for example, it would be highly difficult and complicated to pin down all the actual variables during operation. That is why certain design principles came into being during the development of tubes and are still used today even with computers available to run calculations. Spice simulation of tubes, in my opinion, still leaves something to be desired but making progress.
<P>As far a tube testers go, there are only a few models I'd be interested in, the Hickok 752A which I have is one of the better units, I'd be careful about military tube testers as they were generally specific for a certain group of tubes and were limited in scope. The TV-7 is a pretty good machine but I've found the price tended to be a little high because of demand for them. If you'd like, I will look around for a tester for you.
<P>I've probably went into more detail than you were expecting and much more detail would probably cloud things even more. Most of your customers who are fussing about numbers do so because they don't really know what they NEED to know and are going by hearsay (so what's new?) from people who know even less about the subject.
<P>Hope you have a good weekend.
<P>Best regards,
<P>Edwin
<P>Pettis Engineering
<P> I'm running out of space and time. I'll toss in another two cents worth next time. Remember, all ears hear slightly differently, and reasonable people will disagree. <A HREF="mailto:[email protected]">Send me e-mail</A> with some ideas for the next article!</H2><P><BR>
<P><A HREF="http://www.geocities.com/SiliconValley/Hills/3262/ ">Click here to read the last issue of "Speaking of Tubes..."</A></H3><BR>
<STRONG><P><A HREF="wbdj.htm">Listen to WBDJ while you browse!</A> RealAudio format radio show.</STRONG>
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<H4>Copyright © 2000, Brent Jessee Recording, letter content courtesy Edwin Pettis.</H4>
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