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Old 5th March 2005, 06:55 AM   #11
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Point taken, though in my orginal thread into this, i was a sceptic, i tried it on my cv4068(13d3) input tube of my 805 sets, and proved to myself that something very good happened to the sound. Cleaner more ease and a sense of a blacker back ground, it`s thumbs up for me. I just wish someone could do the same test as John Swenson did using run of the mill 12--7's

Cheers George
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Old 5th March 2005, 07:05 AM   #12
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Quote:
Originally posted by georgehifi
I just wish someone could do the same test as John Swenson did using run of the mill 12--7's
In the link I placed in post #6, SY found nothing significant when it comes to demagnetising ECC88s. 12**7s are a broadly similar (IH dual triodes), but the ECC88 has a frame grid, which the likes of the ECC82/12AU7 and ECC83/12AX7 lack, for instance.

Have a look at the SY gets jiggy (5692 measurements) thread, and you can see what he used to generate those measurements - perhaps you could perform them yourself!
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Old 5th March 2005, 11:55 AM   #13
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I've still got the jig set up; if I can get a little alone-time today, I'll stick a 12AT7 in there, crank down the current, and see if magnets have any effect. My guess (and EC8010 suggested this) is that it's an effect that will only be observable on a very narrow range of tube types. I unfortunately don't have a 6AR8 laying around, but anyone who does can easily replicate my setup.

In any case, if John Swenson's measurement is replicated, it does indeed show that some types of tubes can be affected by magnetic fields. But generalizing it and correlating it to anecdotal reports of success with more common types is problematic.
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Old 5th March 2005, 02:21 PM   #14
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Quote:
Originally posted by Sch3mat1c
Now remagnetize it and complete the proof. Also, make note of the values of noise and distortion reduction, and run controls.

Tim
I'm agnostic on the whole issue of demagnitizing tubes - I just don't know enough about it. But it's a depressing thought if a single data point - without controls - is genreally accepted as "proof" of an effect.

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Old 5th March 2005, 03:24 PM   #15
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Quote:
Originally posted by Sheldon


I'm agnostic on the whole issue of demagnitizing tubes - I just don't know enough about it. But it's a depressing thought if a single data point - without controls - is genreally accepted as "proof" of an effect.

Sheldon
So am I, but no-one serious accepts it as proof nor anything general, just a really good first shot.

I'm setting up a 12AT7 test as we "speak" just to see if a non-frame-grid triode can demonstrate measurable changes, and I wouldn't bother to do this if someone else hadn't done some work first and have the confidence to put it up in public. I reviewed the raw data for the earlier ECC88 runs and didn't see any coherent changes in the noise floor, just variations within the repeatability and reproduceability of my measurements (the pattern in the noise floor change in Swenson's experiment is suggestive). I'll pay more attention to the 12AT7 noise floor measurements.
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Old 6th March 2005, 02:49 AM   #16
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I ran some 12AT7 measurements. I skipped the Tinky Winky test and moved right to the D28 magnet resting on the tube. Basically, no difference in distortion spectrum and noise floor. If a 6AR8 comes my way, I'll try the same experiment to see if static magnetic fields really have any effect that needs to be neutralized.

As a side note, the 12AT7 showed itself to be an excellent candidate for a preamp. At 10V peak-peak output, 1kHz THD was 0.007% for both tubes (four sections) tested. Conditions: plate current 2 mA, cathode voltage 1.75V, zero-signal plate voltage (mean) 148V. These tubes were old Philips/ECG of uncertain provenance, but certainly had spent time in my TF10 clone last century. Distortion and noise bottomed out after about 15 minutes, so measurements were taken at that point.
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Old 6th March 2005, 03:24 AM   #17
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Interesting. What supply voltage and load resistance was that? What is distortion at half and maximum voltage output?

Tim
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Old 6th March 2005, 03:26 AM   #18
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200V and the same CCS setup I used for my 5692 measurements, except that I shorted one LED in each string to get the appropriate current and cathode voltage.

I only checked it at this one convenient level.
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Old 6th March 2005, 04:44 AM   #19
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SY, can you tell us, was the tube new before you did the noise floor measurements? or old stock that has'nt been used for ages? or one that's been used recently?

Cheers George
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Old 6th March 2005, 05:12 AM   #20
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I've repeated the measurement with some beater tubes from Philips/ECG, Mullard, RSD, and Genelex (B739) with the same result- no difference. As I mentioned, these are old tubes out of my working stock. They've got some significant hours on them, yet they all tested pretty consistently at a similar distortion. Noise floors varied, but weren't different before and after applying the magnetic field.

I just repeated the experiment with a brand new JJ, which has a totally different looking construction than the others (looks a lot like an ECC88). Again no effect. The noise I see is much more random looking on the baseline (not including my rather bad 60 Hz component...) than John Swenson's spectra.

As a side note, at an increased output voltage (30V p-p), the JJ showed half the distortion of the others (0.03% vs 0.06%) and a disproportionately lower 3rd. That's what I've been using in the TF10 clone running my garage system and I was pleased with their performance even before seeing how well this version does in distortion measurements.
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