Is tape head degaussing a myth after all?

[rob aylestone]

Why do we try so hard to debunk historical practices that seem to have worked very well. Preventative servicing. Sure, loads of tape machines have had their tape path degaussed and rotary head machines have had their heads cleaned when they didn't need it, but in my early days I was taught to do this as my routine. I can't remember how many machines had an alignment tape run through them and no adjustment made - until you came to one that needed it! VT bays full of machines on rack slides with no top case, and a box of wipes and Colclene. The physics supports degaussing because heads are magnetically permeable, and therefore can be magnetised. Valuable tapes passed through magnetised heads and paths can be affected. Most do not suffer, and very few heads get magnetised to the point that level and HF start to go off - but it does happen. Or are we saying the whole thing is a myth? I may not be totally convinced of the benefits of some mega priced preamps in the sonic department, but good maintenance based on established practices can never be pointless?

 

[miroslav]

I think the "myth" is the need to demag regularly...almost daily/weekly.
I use to do it almost every other day back in my 4-track days...but the more I recorded, and moved up in decks, the less I saw a need for that regular demag...or alignment.
My current 2" deck has been fully aligned once and I think demagnetized twice in three years. Plays/sound great.
I've been meaning run an alignment check on it before I break out the next new reel, and start a new session, and I'll also demag at that point...but I won't wprry about it if I don't.
Cleaning the tape path...that's still a regular thing I do with each session.

 

[jaddie]

Why do we try so hard to debunk historical practices that seem to have worked very well. Preventative servicing. Sure, loads of tape machines have had their tape path degaussed and rotary head machines have had their heads cleaned when they didn't need it, but in my early days I was taught to do this as my routine. I can't remember how many machines had an alignment tape run through them and no adjustment made - until you came to one that needed it! VT bays full of machines on rack slides with no top case, and a box of wipes and Colclene.

The issue is that demagging can also increase the residual if not done exactly right. The only way you can know if it's been effective is to test for symptoms before and after. Just simply demagging may or may not do anything good or bad. You just can't know without testing.

The physics supports degaussing because heads are magnetically permeable, and therefore can be magnetised.

Basically true, but not in the specific application. The materials chose for the tape path have a neutral magnetic state, and have very low retentivity. Very old machines didn't have materials that were completely magnetically neutral, and the residual field was often their natural state, so you could demag, leave the machine off in the corner, and it would require a magnetic polarization. The myth, if any, is that passing tape over a path component will magnetize it. The flux on tape is so incredibly low, and AC besides. Passing tape over a path component would, if anything demag it. But the flux is too low to do anything.

Valuable tapes passed through magnetised heads and paths can be affected. Most do not suffer, and very few heads get magnetised to the point that level and HF start to go off - but it does happen. Or are we saying the whole thing is a myth?

Not at all. I'm saying, test for symptoms, if found, isolate the cause. It's not "normal" for anything to acquire a magnetic polarization. If symptoms indicate a magnetized component, figure out why it has become magnetized rather that routinely put a band-aid on it.

 

[radardoug]

So Jaddie how do my screwdrivers in my workshop get magnetised? As far as I know, I never put them near a magnet. They get magnetised strongly enough to pick up screws. Is it my magnetic personality? I know what I experienced. The cure was demagging.

 

[Lt. Bob]

So Jaddie how do my screwdrivers in my workshop get magnetised?

they are magnetized by the manufacturer.
Are you saying they weren't and somehow just became that way sitting on your workbench?

Not possible ...... go to any store selling screwdrivers .... a good portion of them come magnetized exactly for the purpose of picking up and/or holding screws.

There may be arguments to be made for your position on this but your screwdrivers are not one of them.

 

[jaddie]

Every ferrous material has a natural magnetic state. For some, it's strongly polarized, for other less, and for those used in tape paths, hopefully, it's relatively neutral.

Your screwdrivers have a natural magnetic state that is strongly polarized. Demagging will likely only help temporarily.

 

[mdainsd]

The engineers who designed and produced these machines know far more about it than I do. I follow their recommendations. Somehow I think they wernt responding to a myth.

 

[mdainsd]

And I just went and dragged out the manuals for final generation pro tape machines from 3 different manufacturers that state "Clean and De-magnetize daily..."

 

[Lt. Bob]

personally, I come from that era and I demag tape machines as a regular part of their maintenance.

I've never done it daily but I would if I ran my machines all day long like a busy studio might.
I don't see any way it can hurt and it's what I've always done.
Yes, if you demag wrongly you can make things worse but if you bother to read about the process then you won't demag wrong.
If you're too lazy to bother to learn how to do it right then you deserve whatever you get.

 

[Bobbsy]

Sigh. You're all correct...at least partially.

It depends on your machine. Older ferrite heads definitely DID need regular degaussing. I've seen a 2 or 3 dB drop in frequency response at 15k and above as judged with a manufacturers alignment tape. It can even get as silly as the balance between how much you record vs. play back since the process of recording does some basic degaussing.

Newer types of heads like Permalloy or amorphous don't need degaussing much, if at all. I've even heard of some playback machines where degaussing can actually cause problems though I don't have any personal experience to know if that's true.

So, basically there's no one correct answer to the original ancient question. It depends on your machine.

 

[RFR]

Straight from the Tascam msr 16 manual.

"We can't stress the importance of cleaning and demagnetizing too much."

"Clean up and demagnitize at least every day before you start to work with the MSR-16"

Now to be fair it does say; "Also the heads may become magnetized" Not will, but may.

But they do stress doing it, so it must be important. And an msr 16 is a later model, not an early tape machine.

To my understanding, this lineup of machines were part of the last tascam made before moving away from tape formats.

 

[jaddie]

The engineers who designed and produced these machines know far more about it than I do. I follow their recommendations. Somehow I think they wernt responding to a myth.

And I just went and dragged out the manuals for final generation pro tape machines from 3 different manufacturers that state "Clean and De-magnetize daily..."

What you are actually saying is:

1. The engineers wrote the manual (not true)
2. The engineers designed a machine with ferrous material with a highly polarized natural magnetic state (not true)
3. The engineers designed a machine with electronics that would magnetized heads in normal use (also not true)
4. The engineers, as smart as you may say they are, are so inept as to design a machine that needs daily repair to compensate for poor design. (sadly, sometimes true, mostly not. Perhaps the MR16 is one where 4 is true)

I've already explained why the manuals say what they do. I've already explained how to determine the need for demagnetizing.

All magnetic materials have the property of coercivity (how much external field strength it takes to reorient it's magnetic state), and retentivity (how much magnetism remains in the material after the magnetizing field is removed. AC fields cannot magnetize anything unless they are asymmetrical. That means the field on a recorded tape, and the bias and erase signals, as well as the audio component.

I've already noted several times that I'm not saying "Don't demagnetize", I'm saying, if you must do it, do it with testing before and after. Before you start, look for an elevated noise floor with virgin tape, look for even-order harmonic distortion in recorded signals. Look for a gravely sounding noise component in recorded signals. Look for high frequency erasure (play 5 second loop of 15KHz tone, and watch for gradual lowering of signal. 25 plays and no change, you're good). Then demag, and test all those things again looking for a change, either better or worse. You will find that sometimes you make things worse, sometimes you make things better. That should tell you that just randomly demagnetizing is a crap shoot.

Years ago when I researched residual magnetism in tape recorders, one of the experiments I did was to test the effectiveness of standard degaussing methods. I had a ferrous guide component from an older machine (I think it might have been from a 3M/Mincom). Using a Bell Gaussmeter and hall-effect probe, I measured it's magnetic state, found it to be almost immeasurable. Then I attempted to magnetize it using a rather strong permanent magnet. Getting that piece to accept magnetism wasn't easy. I found I could not magnetize it by passing a pole of the magnet across the guide perpendicularly. Not at all. I had to use the old science class brushing technique, brushing the magnet in a single direction repeatedly in line with the guide. I never did get the thing up to 15 gauss, but I took on something like 8 or 10 gauss. Then I tried to demagnetize it with a tape head degausser. I used my best technique, but my first attempt did very little, reducing the field strength by a gauss or so. My second try actually increased the field strength, and only after several tries, measuring with the gauss meter each time, was I able to neutralize the guide again. That tells me that no matter how good you think you are at demagnetizing, you can actually make it worse just about as often as make it better. You need to measure the result to know. For a ferrous guide, you could use a gauss meter, but not for heads. The field is concentrated in the gap, and no magnetic measurement device, not even my tiny hall-effect probe, can get into that field. So you're left with measuring the effects of the field, as mentioned above.

BTW, I did magnetize the guide and leave it on the bench over night. The next morning, it was almost demagnetized. Had elves invaded my shop and degaussed it over night, or did it return to its natural magnetic state? I'm guessing the latter, and that was probably why 3M chose the material.

Taking a look at a few tape machines, the Studer B67, A80, A810, A820 (I stopped dealing with them after that) don't have a single ferrous component in the tape path except for the heads. Some machines may have steel guides, but I'll bet not a whole lot. Apart from heads, what would you demagnetize?

Consider heads for a second. The erase head has a huge ultrasonic AC signal applied to it when operating, which is stronger than the field at the tip of a head degausser, and that erase current is demagnetizing the head all the time. If the erase current is ramped up and down, as it should for punch-in/out, there's no single polarity transient to magnetize it. The same is true of the record head, the bias must be free of any DC component, or it will have lousy noise performance and high even-order distortion. If the bias is ramped up and down, again for punch-in/out, the head is self-demagnetizing. The play head should have no voltage applied at all, so there's no way it should become magnetized by the electronics, unless something is broken. Moving tape presents an AC field, and an very weak one at that, to all tape path components. All AC fields demagnetize, they do not magnetize.

So, why would a head or a guide become magnetized? Only two reasons. One, it's a manufacturing defect, two, there's been an electrical failure and a DC component has been applied to the head, magnetizing it.

Ever seen an error in an equipment manual? I have, and not just a few, many. Ever see warnings and cautions in manuals that are clearly there to cover the manufacturer's legal tookus? The manual may suggest daily degaussing, but a responsible tape operator would determind the need, then the effect of degaussing, especially since the very act can end up magnetizing instead.

All magnetic materials have the properties of coercivity and retentivity. I haven't studied tape formula enough to know if one tracks the other, but they seem to. However, it seems some forum members also have these properties, only they appear to have very high coercivity, and very low retentivity.

And finally, I don't care if one guy insists on randomly magnetizing and demagnetizing his heads and guides every day, with his demagnetizer. Go ahead, knock yourself out. I'll just make sure not to give that guy any of my masters to work with.

 

[RFR]

Hey, no need to insult my machine. I've had 2 msr 16s that have performed flawlessly and recorded great sounding stuff.


Back on topic.

In hard use, daily with min 5 hours a day, I've only demagnitized the machine about 2 times with no noticable difference. This is in a years time. But cleaning was religiously done twice a day on average. That was noticable.

Regarding manuals. Ive seen plenty with mistakes. Japanese products were notorious for bad translation to english.

I can also see manuals having "cover your ass" things printed in them.
Anything by a corporation that goes out in print has some kind of legal dept approvals.

I'm not all knowing about this degaussing stuff. But, I do know cleaning is VITAL.

 

[rob aylestone]

Jaddie, while I respect your opinion, I simply choose to believe my own training. I failed to get one job because I got interviewed by the chief engineer of a large corporation who was in charge of training new technical ops, and he grilled me on tape lineup. That corporation believed degaussing was good practice, so I always did it! Bias current does do what you say, but, from memory the problem was it cuts off instantly when you press stop or do a drop-out, and cannot do the gentle ramp down you do with a degausser. Indeed, many trainees did degaussing badly, and left the heads and guides worse than they started. Unless bias current is stopped at zero crossing point, then there is some evidence that bias current is one of the causes of head magnetisation.

 

[Lt. Bob]

I find the discussion interesting and I think it's entirely possible that demagging isn't needed to the extent we used to believe.

However ...... just like my preference for vinyl, I'm indifferent to others beliefs ..... I do what feels right to me and since I've always done it that way and it's always worked fine .... i will continue in my old fashioned ways.

 

[jaddie]

Unless bias current is stopped at zero crossing point, then there is some evidence that bias current is one of the causes of head magnetisation.

If bias is hard-switched, it's on and off point is random, changing every time, with the average being neutral. Bias cannot be hard switched with the tape in contact with heads without a transient being printed, so there's typically a soft switching element in there somewhere. The bigger issue is bias signal asymmetry and "purity", lack of distortion. If it's asymmetrical, that is a DC field component, and will magnetize, even if soft switched. Asymmetrical bias and erase current are also defects that need to be corrected. The symptoms are elevated noise relative to virgin tape, and elevated even harmonics. Start and stop pops could also eventually show up.

 

[jaddie]

Jaddie, while I respect your opinion, I simply choose to believe my own training. I failed to get one job because I got interviewed by the chief engineer of a large corporation who was in charge of training new technical ops, and he grilled me on tape lineup. That corporation believed degaussing was good practice, so I always did it! Bias current does do what you say, but, from memory the problem was it cuts off instantly when you press stop or do a drop-out, and cannot do the gentle ramp down you do with a degausser. Indeed, many trainees did degaussing badly, and left the heads and guides worse than they started. Unless bias current is stopped at zero crossing point, then there is some evidence that bias current is one of the causes of head magnetisation.

See my above comments re: bias current.

I totally understand the company position re: degaussing. I worked for one company that had us doing it routinely without checking results. They also had a Magnetometer "for checking head magnetism" (it can't do that). The company had machines by Scully, Magnecord and Crown, so perhaps the policy wasn't totally off base, it's just that we never checked the result. Then I changed jobs to a company with all Studer, two Nagras, and a Mincom or two left over. They had a "do not demag" policy. I questioned the policy, and took on the research project I've been talking about. We rented the Bell Gaussmeter for a month to do the project. Turns out, their policy was right, aside from every copy of the Studer manual clearly suggesting routine demag.

IF you don't question things, you'll never have the real answers.

 

[miroslav]

I'm not saying "Don't demagnetize", I'm saying, if you must do it, do it with testing before and after. Before you start, look for an elevated noise floor with virgin tape, look for even-order harmonic distortion in recorded signals. Look for a gravely sounding noise component in recorded signals. Look for high frequency erasure (play 5 second loop of 15KHz tone, and watch for gradual lowering of signal. 25 plays and no change, you're good). Then demag, and test all those things again looking for a change, either better or worse. You will find that sometimes you make things worse, sometimes you make things better. That should tell you that just randomly demagnetizing is a crap shoot.

This is really the answer to the question...."Is degaussing necessary or a myth?"

There's lot of "hand me down" info and practices that people just follow along because, well, that's what everyone's been doing for the last 60 years, and because of that, it's sometimes seen as an absolute.

The reality is that degaussing IS necessary...only WHEN it's needed.
If folks just want to do it by some sort of set schedule, without concerning themselves if it's really needed by doing testing and monitoring...that's a personal choice.
Poor degaussing technique and/or using small/weak/cheap degaussers will either do little, or it can even make things worse.
So if you're going to do it regularly, make sure you have the right tool and follow the correct process.

There have been some tests done by audio engineers to see how much audio quality degrades without degaussing, and over what period of time...and there was little to none, even over long periods of non-degaussing...which is why it may be a waste of time doing it all the time, daily/weekly.
(Very old-manufacture machines with easily magnetized metal parts in the transport are a particular case).
Ideally...you should find what works best with the least amount of degaussing for your machines and your use. It's not going to be some set schedule for all decks and all uses.

It's like the difference between taking aspirin every day so you don't get a headache VS taking aspirin when you have a headache.

I'm going to now go take a couple of aspirin...

 

[mdainsd]

What you are actually saying is:

1. The engineers wrote the manual (not true)
2. The engineers designed a machine with ferrous material with a highly polarized natural magnetic state (not true)
3. The engineers designed a machine with electronics that would magnetized heads in normal use (also not true)
4. The engineers, as smart as you may say they are, are so inept as to design a machine that needs daily repair to compensate for poor design. (sadly, sometimes true, mostly not. Perhaps the MR16 is one where 4 is true)


And finally, I don't care if one guy insists on randomly magnetizing and demagnetizing his heads and guides every day, with his demagnetizer. Go ahead, knock yourself out. I'll just make sure not to give that guy any of my masters to work with.

I typed what I actually said. Dont need your interpretation, or your condescending commentary. Do whatever you like, just dont dog others who do it differently.

Also I didnt see anyone here asking to do anything with your masters.

 

[jaddie]

I typed what I actually said. Dont need your interpretation, or your condescending commentary. Do whatever you like, just dont dog others who do it differently.

Also I didnt see anyone here asking to do anything with your masters.

Sorry if my sarcasm (and just about everything else) was lost on you.