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Old 07-25-2012, 01:03 PM
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Question 12AT7 Testing Settings -- They Change the Voltages?!

Hey guys,

So I bought a lot of ECC81 (12AT7) tubes for funsies, and now I want to test 'em. I crack open my book and, surprise surprise, that good ol' Eico data is all jumbled up and does not make any sense to me.

I checked through all the different booklets and I'm seeing the same variation, but different numbers.

Can I just choose one set of grid and plate settings and go along testing away?

Here's my problem:




The Tone Lizard page on the Eico 666 has some great information on testing tubes and comparing the different versions of the manuals to one another (see here: http://tone-lizard.com/Eico666.html). Tone Lizard has found that the 666-04 settings seem to run the 12AT7 tubes more realistically.

So, I'm assuming here that I know things, and what I think I know is that both sections of a 12AT7 tube are the same, which means that I shouldn't have to change the voltage settings when testing the two sections.

Would I be within my right to, say, keep the voltage setting applied to the grid and plate, only changing the levers which assign the pins?


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Old 07-25-2012, 01:29 PM
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My guess is that they some how , transposed 12dw7 ( 7247) , grid and plate to the 12aT7 setting , would use the setting that give the best reading .
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Last edited by battradio; 07-25-2012 at 01:32 PM.
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Old 07-25-2012, 04:19 PM
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sounds like a mis print never seen that before
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Old 07-25-2012, 04:20 PM
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Originally Posted by jsixis View Post
sounds like a mis print never seen that before
The thing is is that it's in every revision--the numbers bounce around, but triode 1 test is a different voltage/load than triode 2 every time!
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Old 07-25-2012, 04:24 PM
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http://www.diyaudio.com/forums/tubes...o-you-use.html
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Old 07-25-2012, 04:28 PM
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What I would do in that regard, is find me an good/known testing tube and test
it at both of the suggestions just changing sliders for each half of the tube.

Find out if one setting does a better job than the other at a more accurate read.

Also see if there are settings for 6DT8 and 6AQ8 an compare how they are setup.

----------

Now in a pile of 12AT7's your bound to get plenty of bad/low/mismatched sections
as it's just the nature of the 12AT7, many more "bad" tubes then you get with say
a 12AU7 or 12AX7 which is one of the reasons I went to the 6AQ8 instead, they do
tend to be better lasting/testing tubes per sample size..
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Old 07-25-2012, 04:30 PM
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Quote:
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That's a very nice link for some good info on that tester..
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Old 07-25-2012, 04:37 PM
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Quote:
Originally Posted by battradio View Post
Yup, read the info there and the manual a bunch and it's been useful to know how the tester works so that I can run a quick mental check in my head whether or not the connections make sense (there was once an error with the heater pins) or I'll read that X tube is the same as Y tube but in a different bottle and develop my own settings for those rarer tubes.

Anyways, here's ToneLizard's opinion:

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Old 07-25-2012, 04:40 PM
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Sometimes my roll chart is right and sometimes the BAMA hosted reference is right. For the 12AT7 I have grid=85 and plate=90 for both sections, with the same switch settings as all the other 12A?7 types. V=2 and S=2.

FWIW, there are slight differences in the two sides of the tube, at least with capacitance, so maybe Eico wasn't completely toasted when they did the chart. But I doubt it!

My current question, also posted on DIY with no answers is:

Has anyone published decent corrected settings for the 12A*7 series of tubes, settings that don't overload the plate?

The manual says many contradictory things. It says dual triodes should be tested with voltage applied to both sections of the tube, and shows schematics for that mode. They boast of the speed of testing. Then the charts show to do every section separately! It would seem the lever settings should be 4511245 661111 (for the 12AX7 and the 667 model tester), though I'm not exactly clear on how the buttons get pushed and what leakage tests would be made.

Can anybody clarify this stuff?

edit- curious how Tonelizard got the currents- was it by calc or actual measurement?
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Last edited by ConradH; 07-25-2012 at 04:43 PM.
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Old 07-25-2012, 04:43 PM
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I don't know what you mean by, "settings that don't overload the plate"..
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Old 07-25-2012, 04:47 PM
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There's a dissipation rating on the plate, different even in the same family of 12A?7 tubes, and it appears some of the test settings exceed it. It's just P=IE, so it depends on the voltage. I probably need to do actual measurements of a tube in the tester.
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Old 07-25-2012, 04:48 PM
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Quote:
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edit- curious how Tonelizard got the currents- was it by calc or actual measurement?
I know in my case I've cut into the plate voltage supply to see how much current the tube's drawing.

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Old 07-25-2012, 04:52 PM
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Quote:
Originally Posted by ConradH View Post
There's a dissipation rating on the plate, different even in the same family of 12A?7 tubes, and it appears some of the test settings exceed it. It's just P=IE, so it depends on the voltage. I probably need to do actual measurements of a tube in the tester.
I know what plate dissipation is and how to calculate it there.
But an "normal" setting on a tube tester comes nowhere near that.

The one posted above with 20ma would but that is a major exception.
But still it's only 1.8w on a 2.5w rated tube, the other 2 are quite low.
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Old 07-25-2012, 04:56 PM
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Quote:
Originally Posted by Kegger View Post
I know what plate dissipation is and how to calculate it there.
But an "normal" setting on a tube tester comes nowhere near that.

The one posted above with 20ma would but that is a major exception.
But still it's only 1.8w on a 2.5w rated tube, the other 2 are quite low.
The example above states that at the voltages the tester is using (low, ~100V) that the current should be 3.7mA according to the RCA manual; the author states that at 100V the tester is putting 20mA at the plate (potentially damaging it?).

That's what I got out of that paragraph, at least...
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Old 07-25-2012, 04:56 PM
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I definitely need to check the real numbers! BTW thefragger, that's a really nice 667. I also notice that your line control is at the bottom right whereas mine is at the upper right.
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