Dynamic Mutual Conductance Test - vrogers3 - Tubes Asylum

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Dynamic Mutual Conductance Test

In Reply to: Re: Highest Gm 6GH8A's???? posted by Mr. Droopy-Drawers on September 7, 2003 at 12:24:15:

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."

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Follow Ups Full Thread

Topic - Highest Gm 6GH8A's???? - vrogers3 12:13:18 09/07/03 (10)

The Mighty 6GH8....... - MrTanner 04:40:02 09/08/03 (4)

Re: The Mighty 6GH8....... - Mr. Droopy-Drawers 15:14:29 09/08/03 (0)

Re: The Mighty 6GH8....... - vrogers3 11:31:51 09/08/03 (2)

ESRC - MrTanner 02:49:10 09/09/03 (1)

Re: ESRC - P-90 16:18:14 09/09/03 (0)

Re: Highest Gm 6GH8A's???? - Mr. Droopy-Drawers 12:24:15 09/07/03 (4)

Dynamic Mutual Conductance Test - vrogers3 16:08:26 09/07/03 (3)

Re: Dynamic Mutual Conductance Test - Chris Haedt 03:28:02 09/08/03 (1)

Re: Dynamic Mutual Conductance Test - Pär 05:01:10 09/08/03 (0)

Re: Dynamic Mutual Conductance Test - Mr. Droopy-Drawers 16:52:50 09/07/03 (0)

Follow Ups

Dynamic Mutual Conductance Test - vrogers3 16:08:26 09/07/03 (3)

Re: Dynamic Mutual Conductance Test - Chris Haedt 03:28:02 09/08/03 (1)

Re: Dynamic Mutual Conductance Test - Pär 05:01:10 09/08/03 (0)

Re: Dynamic Mutual Conductance Test - Mr. Droopy-Drawers 16:52:50 09/07/03 (0)

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