Is tape head degaussing a myth after all?

Next, gauss meters and magnetometers were mentioned....


......they are sensitive to the earth's magnetic field. You need to find a position where the device reads a null in the earth's field, then swing your tape machine parts into that position. It's really hard to do. Again, ok on large guides, no good on heads.

Glad to see you came into this thread to readdress some of the points that have come up. I myself don't demag as a blind routine, but will do it as part of a major setup or between long periods....like maybe twice a year...and I have NOT seen/heard any issues with my deck or my tapes for lack of regular, routine demags. I don't record every day...so maybe as you mention, whatever magnetizations does accumulate after a couple of sessions, it goes away on it's own during my down time, but I will run the same tape over and over for days sometimes during tracking, and I still don't hear any HF loss or any noise/issues increasing. It seems that after the initial 2-3 passes of a virgin tape, it just settles into a steady state, and it doesn't change form there...to my ears.

AFA the gauss meter thing...yeah, I have one of the finer calibraated ones, and yes, it's not easy to get it right up against the head gap, and yes, the earth's magnetic field does have an effect on it.
I usually spin the meter until I get the lowest "earth" reading, and then keeping it in that position, I bring it right up to the heads (pretty close). I also do that with the rest of the tape path, which I can actually touch with the meter, so my thinking is that if the meter is not deflecting much (less than 0.25 Gauss) at ANY of the parts, even if I can't really touch the head gap with it, I'm confident that the heads are no worse off than any of the other parts for that reading.
 
AFA the gauss meter thing...yeah, I have one of the finer calibraated ones, and yes, it's not easy to get it right up against the head gap, and yes, the earth's magnetic field does have an effect on it.
I usually spin the meter until I get the lowest "earth" reading, and then keeping it in that position, I bring it right up to the heads (pretty close). I also do that with the rest of the tape path, which I can actually touch with the meter, so my thinking is that if the meter is not deflecting much (less than 0.25 Gauss) at ANY of the parts, even if I can't really touch the head gap with it, I'm confident that the heads are no worse off than any of the other parts for that reading.

If that's a "Magnetometer", the field it's calibrated in has a 6" cross section, which is never found on a tape machine. Nearly everything you read is still the earth's field, especially down below 10 gauss. If that's a gauss meter with a detached hall-effect probe, the reading is showing you have no problem. Residual fields don't even begin to show symptoms until way above 30 gauss.

If you found residual magnetism by symptoms or direct measurement, the question to ask is "Where did it come from?", rather than to just accept it as a result of normal operations.
 
Note to self....file under "everything you've been led to believe for the last 15yrs is bullshit." Just like acoustics. sheeeezus..never ceases to amaze me.
 
I've never trusted magnetometers myself for the low residual magnetism we're talking about as found on parts of a tape path, so I don't use them. The best test for demonstrating the phenomenon is the one I shared earlier in the thread from Jim McKnight of MRL. And it works. There's no question that a tape path will retain some magnetism over time and thus the general rule is to degauss every 10 hours of rec/repro time. In a home or project studio it can take a while to reach those 10 hours, but in a busy commercial studio it's not unusual for degaussing to be done every day, if not before each project throughout the day. If you grew up like I did doing it all the time it's no big deal. It's as easy as cleaning the tape path. I can do it in my sleep. ;)
 
Lots of people have a story about how demagnetizing fixed their machine. Well, sorry to say, it didn't "fix" anything, it just hid the real problem temporarily. Hey, that might just work for you! In my world, I like things to work properly, and not have issues that sneak up on you. I'd fix the machine, not just keep demagnetizing it.

Anyone and everyone is welcome to disbelieve what I say. I just would encourage you to experiment for yourself first, before sticking to widely accepted dogma.

No, not at all correct. Degaussing fixes problems inherent to magnetic recording the same as cleaning the tape path will. It's the nature of the beast. It's a nice thought, but the perfect tape machine you're describing could only exist on paper. In the real world degaussing is a reality, from budget semi-pro machines to the highest quality Studer. Sure it's fun and all to think you're bucking the system by exposing what you think is dogma or some grand conspiracy. But in this case it's not blind dogma, any more than regular cleaning of the tape path is blind dogma. Degaussing is a well established practice for good reason. If it wasn't necessary I'd be the first to hang up my degausser.

:spank: ;)
 
The best test for demonstrating the phenomenon is the one I shared earlier in the thread from Jim McKnight of MRL. And it works.

Yeah...I've been planning to try that test and see how it works, but here's a thought...let's say that there IS residual magnetism strong enough to do damage to the HF in the manner he described...logarithmicly dropping the HF level with every pass.
So to take that out a little further...will it KEEP dropping with every pass 'til there ain't none???

That's where my experiences with my deck have left me to believe that it's not really accumulating any significant magnetism. As I said...I'll RW and play a recorded tape over, and over, and over, and over during tracking...and I've not noticed that the HF changes from first to last play. Only with a fresh/virgin tape do I hear a slight change from 1st to 3rd-5th play...but once that tape has been used a few times, if I re-record on it...then there is NO loss of any kind. (Mind you, I can't hear 20k...but then who really can. :D)
It's like fresh strings on a guitar...after a little bit of play, they change slightly, but then they settle in and stay pretty consistent for a long time. I know there are guys who are addicted to the sound of fresh strings during the fist couple of hours...and there's some who will change them daily!!! :rolleyes:

Anyway...I'll try his test and see how it goes. By that logic, right now I should have substantial magnetism since I haven't demaged in months!!! :)
I'll post back...will probably try the test this weekend.
 
No, not at all correct. Degaussing fixes problems inherent to magnetic recording the same as cleaning the tape path will. It's the nature of the beast. It's a nice thought, but the perfect tape machine you're describing could only exist on paper. In the real world degaussing is a reality, from budget semi-pro machines to the highest quality Studer. Sure it's fun and all to think you're bucking the system by exposing what you think is dogma or some grand conspiracy. But in this case it's not blind dogma, any more than regular cleaning of the tape path is blind dogma. Degaussing is a well established practice for good reason. If it wasn't necessary I'd be the first to hang up my degausser.

:spank: ;)

Let me ask a few questions here, the same ones I asked my self.

1. What is the cause of the residual problem in the tape path?

2. Have you ever tested for symptoms before and after you degauss? What were the results? What specific machine?

3. How did you establish the 10 hour degaussing interval?

4. What tape are you using, and what flux level does it take to begin erasure?
 
Question:...

Will a residual head magnetization dissipate or bleed itself off simply with the passage of time?

F/E, if I used my recorder a year ago but not since then, would it need degaussing?
I have recorders that may not have been used in many years.

Thank you,:eek:;):spank:
 
.... The best test for demonstrating the phenomenon is the one I shared earlier in the thread from Jim McKnight of MRL. And it works.

More questions:

Do you perform the McKnight test every time you degauss?

Do you do a before and after using the McKnight test?

Again, what specific machine?

You seem to be getting the point that I'm saying "never degauss". If I'm leading people to that, I'm sorry, that's not my point at all. My point is never degauss blind, because if there is a problem it seems to fix, you need to find the cause.

There are many machine defects that can cause residual. Asymmetrical bias is one, leaky caps in the record amp is another, and a defective head is yet another. These are real, but can be fixed.

Degaussing blind is like changing your car's spark plugs once a week because "spark plugs foul with use". True, they do foul and carbon up, but that's a symptom of a problem, not a product of normal engine operation. If you drive a 1959 Chevy, it might be necessary for real, but if you drive a modern car, you might sell it before it needs new plugs. Yet, if they did foul often, would you begin changing them routinely, or find out why? And wouldn't you know they fouled by the performance loss?

If you have a vintage machine (which they all are now, right?) then anything is possible. But if you know that because routine degaussing has been traditionally recommended as a cautionary measure, then assume that the reason is that the normal use of a tape machine causes residual magnetism by nature, you are making a connection on your own of two unrelated premises. The connection is not actually stated. Rather, the procedure is recommended because of the consequences of a defect: destroyed masters and test tapes. It's a precaution.

What I'm suggesting is, monitor the symptoms, find the cause and fix it, then continue monitoring for symptoms. Otherwise, how do you know that once a month/week/year is enough? What if the problem comes back in 24 hrs? What if you induce a temporary field by sloppy degaussing? How would you know for sure you're not making it worse? That's why I'm saying that blind degaussing is not recommended.

Here's the wake-up call. In my research I found that even though I thought my degaussing technique was perfect, it wasn't. I could easily increase residual without knowing it. I had followed the recommended procedure, slow movements, not turning the degausser on or off near the machine, etc. Monitoring the results taught me how to properly degauss. It was hard to learn, because I had to deliberately magnetize the deck so I had something to degauss. If you have a hard time with not routinely degaussing, just try taking a big permanent magnet to your play head!

Now consider tracking down the cause. Will playing tape magnetize a head or guide? Tape has, by design, a very high coercivity level. That's how strong a field it takes to begin to magnetize tape (record). It takes a very strong field to get past that coercivity level, and that's what bias is about. Pushing you way past the non-linear region, into the levels above which tape will take on a more or less linear representation of the signal you are trying to record. The same high coercivity is responsible for the tape being resistant to stray fields during storage, like the earth's field for one. If the field strength is too low, the tape just resists it, nearly completely. Try recording with no bias; it just doesn't happen. Yet, the actual recorded flux on tape is very very weak, barely inducing a current in the play head which takes a huge amount of amplification to recover. So, lots of field strength to record, and the result is a tiny bit of recorded field.

All magnetic materials have a certain coercivity level below which they ignore an attempt to magnetize, above which they may take on a polarized state. Materials also have retentivity, or the ability to hold a magnetized condition. Tape has very high retentivity, and coercivity, heads have very low retentivity and relatively low coercivity, or distortion would be a problem, and playback wouldn't be possible. But, that means they are hard to magnetize with a condition they hold on to. The field moving tape presents to stationary parts is alternating, which is a degaussing field. The flux on tape is so weak it can't magnetize parts even if it's stationary. So, playing tape won't magnetize anything in the tape path.

Recording involves very high bias currents, again, AC, which tend to degauss. The audio signal mixed in is much lower, but still AC, and therefore degaussing in effect. Erase currents are basically bias, and designed to degauss tape, and will degauss the erase head when it does.

Heads must, to operate properly, have very low retentivity and coercivity. If they didn't, they would be a cause of distortion and non-linearity. And the signals applied to record and erase heads will degauss them. There should be no electrical signal on a play head other than that induced by the moving tape, which is, again, AC, degaussing if it were strong enough.

So, that's why I say there is nothing in the process that should magnetize parts. There are defects, however, than can. DC on heads, asymmetrical bias, head materials with a polarized natural state (these are defective, should be replaced), high switching transients sent through heads (power up/down, mode switching) for the big basic ones. The presence of any of these represents either a serious design flaw (like that of an extremely primitive or low cost machine) or an issue requiring repair.

Let me pose a final question:

What is the cause of residual magnetism in a tape path if it's not a defect in the machine?

Sorry for the long post, there's just no way to talk science without at least some detail.
 
Will a residual head magnetization dissipate or bleed itself off simply with the passage of time?

F/E, if I used my recorder a year ago but not since then, would it need degaussing?
I have recorders that may not have been used in many years.

Thank you,:eek:;):spank:

Can't say without knowing how it got magnetized in the first place. Since there's a defect causing it, then even if the head itself returned to a neutral state (which it should by design) in the course of a year, the cause will no doubt be back when you turn on the machine again. Turn it on, do the tests we've talked about, then you'll know the whole story.
 
Sorry for the long post, there's just no way to talk science without at least some detail.

I'm glad it was a long post rather than a short answer....some interesting points to consider. I have to say, most people that do talk about routine degaussing have often given rather "adopted" answers...that it's something you just "should do" and something they and others have always "done"...but rarely does anyone really get into the meat of the subject like you have done.
There's certainly a lot of food for thought there and I would love to hear some as-detailed counter-arguments. It's an interesting subject for anyone working with tape.

It would be cool if your points could also be addressed by someone like Eddie Ciletti/Tangible Technology - http://www.tangible-technology.com/ - since he is considered somewhat of an analog/tape deck guru and has covered a variety of tape deck technology subjects for various audio magazines over the years.
 
I can tell you that demagnetising IS necessary. Many years ago I was called in to a 'professional' studio to look at their Ampex ATR102. The problem was as follows.
You could record one pass and it would play back clean. The next pass and succeeding passes would have whistles on them. This whistle would vary in frequency, and the more passes, the worse it got!
The ATR100 uses steel bearings as guides on either side of the transport. This machine had never been demagged. The bearings had residual magnetism, and would print onto the tape as they rotated. The combination of two magnetised bearings made an interesting noise!
I demagged the machine, the problem went away.
Thats why you need to demag regularly and do it to ALL ferrous metals in the tape path.
 
Can you degauss a thread to prevent necromancy?

Or does that only work on ferrous threads and not permalloy ones?
 
Just to be safe, I'm going to demag my hard drives too. I don't want bad things happening to all my digital recordings, family photos and such...
 
I can tell you that demagnetising IS necessary. Many years ago I was called in to a 'professional' studio to look at their Ampex ATR102. The problem was as follows.
You could record one pass and it would play back clean. The next pass and succeeding passes would have whistles on them. This whistle would vary in frequency, and the more passes, the worse it got!
The ATR100 uses steel bearings as guides on either side of the transport. This machine had never been demagged. The bearings had residual magnetism, and would print onto the tape as they rotated. The combination of two magnetised bearings made an interesting noise!
I demagged the machine, the problem went away.
Thats why you need to demag regularly and do it to ALL ferrous metals in the tape path.

So, your ATR102 had steel bearings magnetized with an AC field? Cool. If you can replicate that with a larger magnet, you can build a passive power generator and beat the cold fusion guys at their own game.

Back in the real world, things get magnetized and produce a DC field only, with which it is impossible to produce an AC field that would record itself on a passing tape without some sort of physical movement of the pole as well. Not saying your experience was invalid, it's just highly unlikely that demagging alone was the cure.
 
Jaddie, why dont you think about your answer before you leap into print? The bearings were magnetised. Exactly why, I couldn't say. But I do know the studio never demagged their machine. The owner didn't beleive in it, (or maintenance in general). When those bearings rotate, they print to tape.
This effectively produces the a.c. If you knew much about tape recorders, you would know that on very cheap recorders they used a magnet for erase. The tape movement produced the erase effect. No ac involved, just movement of the tape. I will admit, it took me a while before I figured it out. But once I did, it was very obvious. And demagging the machine was all it took to solve the problem.
 
Jaddie.
Also looking at the math. Take a standard alignment tape at 185 nWb/m. According to the web, 1 Wb/m2 = 10 to the 4th power gauss. The field for the noise level on tape will be approx 185nWb/m divided by 1000 for -60 dB signal to noise. So thats .185 nWb/m. Thats 1.85 by 10 - 4 gauss. The gaussmeters seem to have scales of 25 or 50. So the needle wouldn't be off the end mark for the typical fluxes involved at the noise floor.
There are some real luminaries in the tape recording field. Jay McKnight is one. The people at Ampex, Studer et al were not idiots. They say demag. How many years experience do you have with tape recorders? I have 50 odd. But Jay McKnight has probably forgotten more about tape than I ever knew. Jay has spent his whole life playing with tape.
Unfortunately the Internet encourages uninformed speculation without needing any proof, and presents it as fact.
 
Jaddie, why dont you think about your answer before you leap into print? The bearings were magnetised. Exactly why, I couldn't say. But I do know the studio never demagged their machine. The owner didn't beleive in it, (or maintenance in general). When those bearings rotate, they print to tape.
This effectively produces the a.c.
Yes, and last time I had hands-on with an ATR-102, I recall those guide bearings being around 1/2" or so, though I admit it's been a while. So, what's the rotational speed of those bearings? Well, a full rotation would take about 1.6" of tape, so at 15ips, a full rotation would have a period of 0.106 seconds. If the bearing were magnetized such that one side of it were N and the other were S (highly unlikely orientation, BTW), that would print a 9Hz signal on the tape...if the magnetic field on that guide exceeded the coercivity of the tape. Which, if it did, it would be printing waves of noise, not a "whistle", if we assume a "whistle" is something at 1KHz or higher. To do that with a rotating bearing guide, the thing would have to be 0.005" in diameter. And, BTW, the coercivity of most tapes means you need a field somewhere around 25 gauss just to get outside the tape's hysteresis loop and actually record something on it. The recorder itself uses a monstrous bias signal to give the tiny audio signal something to ride on outside the hysteresis loop, so we can at least a kind of linear relationship on tape. But, backing up, a guide roller won't print a whistle magnetically, because it rotates too slowly. And, a guide roller in any tape machine will never.. and I mean NEVER just simply accumulate a residual field of 25g or more. That's a magnet, not a guide.

There's' nothing in an ATR-102's path that can print a "whistle" magnetically. However, there are mechanical means to do it if you have "binder breakdown", which is most likely what you experienced. The fact that it went away after demag was likely a very confusing coincidence.
If you knew much about tape recorders, you would know that on very cheap recorders they used a magnet for erase.
I owned one of those in 1963.
The tape movement produced the erase effect. No ac involved, just movement of the tape.
Movement past a magnet held in contact with the tape, yes, that did erase it, and also raise the noise floor quite a bit. So? It never recorded a whistle.
I will admit, it took me a while before I figured it out. But once I did, it was very obvious. And demagging the machine was all it took to solve the problem.
I have no doubt you solved your problem. I highly doubt it was residual magnetism, for all the reasons above.
 
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Jaddie.
Also looking at the math. Take a standard alignment tape at 185 nWb/m. According to the web, 1 Wb/m2 = 10 to the 4th power gauss. The field for the noise level on tape will be approx 185nWb/m divided by 1000 for -60 dB signal to noise. So thats .185 nWb/m. Thats 1.85 by 10 - 4 gauss. The gaussmeters seem to have scales of 25 or 50. So the needle wouldn't be off the end mark for the typical fluxes involved at the noise floor.
The problem with your "math" is, that's not how tape recording works. You don't apply a particular flux density to a tape and then get that flux back off. There are two factors: coercivity, or the amount of magnetic field strength it takes to actually magnetize at least a few oxide particles and Retentivity (reminance), the amount of magnetism left on the tape after the magnetizing field has been removed. You've figured none of that in, your math is wrong. Coercivity alone creates the need for a bias signal thousands of times greater than the audio signal, but it's also responsible for tape being resistant to erasure by stray fields, like that of the earth.

I've actually measured the magnetic field it takes to raise the noise floor, increase distortion, and erase high frequencies. It corresponds well to the secified coercivity of tape, and nothing but a seriously broken or badly designed machine can have that sort of residual field on anything in the tape path.
There are some real luminaries in the tape recording field. Jay McKnight is one. The people at Ampex, Studer et al were not idiots. They say demag. How many years experience do you have with tape recorders? I have 50 odd. But Jay McKnight has probably forgotten more about tape than I ever knew. Jay has spent his whole life playing with tape.
Unfortunately the Internet encourages uninformed speculation without needing any proof, and presents it as fact.
Yeah, I have about 50+ years too, but they weren't odd. Very educational. You've mentioned the experts. I've spoken with Jay, yes some years ago. We did not disagree about this.

Manuals say "degauss" as a matter of course, largely because in the really old days, they didn't have good magnetically neutral materials with which to make heads and guides, and electronics often had enough asymmetry and DC issues that they became magnetizing forces on their own. Modern, like 1980s+, machines had solved all of that, at least in the pro world. Having worked with countless Studers, Ampexes, and others, I'll tell you that testing for residual magnetism symptoms is the only way to know. And a reall hall-effect gaussmeter is nice too, but useless at the heads.

I even did an experiment on a Studer B67 where I tried deliberately to magnetize heads and guides. The best I could do was a temporary increase. The next morning, it was gone.

Degauss if you feel you have to, but evaluate the results and indicators that it either was necessary, and/or you actually did accomplish it.
 
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