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In Reply to: Highest Gm 6GH8A's???? posted by vrogers3 on September 07, 2003 at 12:13:18:
I may have missed something; wouldn't EIA/RETMA standards dictate that the gm be consistant? I don't mean within 1 micromho, but within a tolerance. I don't think any manufacturer would knowingly make a tube with a lower gm than a 'standard'. Say the mu for the triode is 46; if one manufacturer made theirs with the triode having a mu of 40, rather than label the tube a 6EA8 or a 6KE8, they'd reject it.
I don't know; maybe there is something to it, but I also think there's something you're not telling us. Are these all NOS tubes?
Follow Ups:
I tested 25 NOS RCA tubes on a recently rebuilt and calibrated B&K 747 tube tester. The pentode sections varied between 74 and 108 and the triode sections between 90 and 110. This is all within the green "good" on the scale, but the tubes measuring over 95 on both sections sound noticably better in my equipment.If my understanding of this test is correct, an AC reference signal is placed at the grid, and the resulting voltages off the plate are indicated by the meter. I would think that a tube with higher voltages coming off the plate for the given AC ref. would have slightly higher gain and higher reading on the meter.
I don't think I misunderstand the test, but please let me know if I do!!!
While the triode sections of these tubes test relatively high and tightly, some of the pentode sections seem to be a little weak. If there is another brand out there with pentode sections that are a little stronger overall, I sure would have less rejects. Out of these 25 tubes, I got 8 good ones, and 17 rejects that have strong triode and weak pentode sections. I did match and try some of these 17 "rejects." Pentode sections measuring under 85 truly robbed dynamics and detail. Sections measuring above 85 went from okay to outstanding as their test measurements rose.
Thinking that I was "biasing" myself by knowing the test results of the tubes I did blind comparisons with several people. All chose the higher testing tubes as making a noticeable improvement, or the lower measuring tubes as "flattening" or "compressing."
The concept of mutual conductance refers to how a change in GRID VOLTAGE affects the DC PLATE CURRENT of the tube. The plate current pulsates in step with the signal applied to the tube grid, and the magnitude of the plate current changes reflect just how much the signal is amplified. I wanted to clear this up, as plate VOLTAGE does not change on the tube. The amplified signal is represented by plate CURRENT pulsations, which becomes the AC component of the plate current. When this pulsating current is applied across a load (resistor or transformer) an AC voltage develops which is the amplified signal.When we look at NOS tubes today, it is unreasonable to expect them to all test the same. Tubes certainly varied when they were manufactured. Plus--many people believe that some stashes of NOS tubes that have been "discovered" were tubes that had been set aside, precisely because they DID test low. One common NOS tube that may often test low on some tube testers is the 12AX7.
Tube testers are a collection of compromises that result from trying to make one machine capable of testing many (sometimes thousands) of tube types. Not all testers are created equally and not all do a good job with all tubes. In fact, most testers will do a poor job with some tubes. For instance--the early Hickoks do not do a good job with the 12AX7--the 5 volt grid signal is too high. Plus most Hickoks do not test filamentary triodes well--due to the signal voltage being at the same frequency as the voltage heating the filament of the tube. These are just examples that come to mind--but almost all models of testers will have tubes that they do poorly with.
Tube testers were designed to find bad tubes that caused equipment failure. Your B&K seems to be working in this regard, as it is detecting significant difference between tubes. Your tubes are all testing in the good range on the tester, and all of the tubes work in the equipment. The difference in sound between tubes would be difficult to account for, and be circuit related. The tubes that have the weaker pentode sections are not really rejects--as they still test in the green area. Besides--when people match dual section tubes--they are usually talking about tubes with two ALIKE sections--like a 12AX7.
Tube tester oscillation was mentioned. This is a sitation where a feedback path exists in the tester resulting in a tube reading higher than it should. The tube that causes the most problems in this regard (my experience) is the 6DJ8 family. When a tube goes into oscillation, the reading on the tester will go up and up--often off the scale. The tube may burn up if allowed to continue oscillating. It can be very difficult to diagnose and cure this problem. Sometimes placing your hand on an oscillating tube will stop the oscillation, and the meter reading will go down to normal. This is a quick diagnostic test.
Hi,Also,
The Gm (or S) of a pentode is set by cathode - input grid and screen grid. As screen grid is a thin spaced wound wire it may deform quite easy but as far as I found it will reform itself when the grid has been heated for a while...
Or atleast this is what I suspect. I have tried to "over stress" the G2 on a few smallish PCF (tele set small signal triode pentode) and after a while of stressing and following cooling down they tests a lot better...
If one don't "miss the tubes like crazy" om may try to lower the plate voltage a bit and leave them running so to stress G2 a little and see if that may help...If you demand life time guarantee on this I'll have to kill you :-)
mvh /Pär
... Nope; I think you have it right on the nose. I like the B&K 747; it is one of the better testers out there. Not perfect, but darn good. The only thing I can think of is that there is a difference, or there is something strange going on. The triode sections look to have a good tolerance, but those pentode sections are all over the place. And since they are all NOS, something is amiss. Maybe the tester has a parasitic oscillation that affects the reading. I really believed in EIA/RETMA until now.
Oh; there is a really interesting website link at the bottom of this post. What it means to me is that if the oscillations are coupled back to the input grid of the tube, there goes the readings on your fancy tube tester! Some testers use a 60Hz test signal (typically the low voltage filament winding on the power transformer), whilst others use a 5kHz oscillator. I'm not sure which is 'better'. But this is all very interesting, and might support the use of tube dampers.
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