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Old 7th November 2007, 09:32 AM   #1
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Join Date: Oct 2005
Default Testing tube for noise


I need some EF83 (or EF86) for my tube preamp. When i bought this small pentodes before, i noticed that some of them were very noisy and needed some testing.
This time i found another seller who offers testing so i will ask him to test noise (and maybe microphonics) for me.

I would like to know more about how this test; how it is done, can it be done with the tube tester etc...
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Old 7th November 2007, 10:01 AM   #2
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The noise test is done by taking a sample from the anode load resistor (or CCS) and amplifing the output to a speaker (or eye tube).

Some testers have settings for this test that more resemble actual operating conditions, rather than "full throttle current" as typically done for life-test.

That simple

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Old 7th November 2007, 10:10 AM   #3
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Thanks. I expected simple procedure.

But what are the "values" of noise for this kind of tubes? Datasheet for EF86 only tells noise voltage at certain frequencys...Should i ask him to simply send me tubes that showed lowest noise?

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Old 7th November 2007, 10:40 AM   #4
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with a simple amplifier setup and an RMS voltmeter measure the A.C. output with the input open and shorted -- calculate the ratio take its logarithm multiply by 20 etc. gives you the voltage signal to noise ratio in dB.

Good discussion of tube noise here:

if you were doing this commercially you would probably want to bandwidth limit the measurement -- say from 22 Hz to 22kHz -- or ITU-CCIR486-ARM or A Weight the measurement. Beyond the scope of this discussion.
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Old 7th November 2007, 12:13 PM   #5
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Location: Leverkusen
Hi rotation,

Here is a Link to a DIY tube microphonics tester, in German language, though.

Regards, Tom
If in doubt, just measure.
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