Part 1: De-Magging
I have a tape bulk eraser. It’s quite a meaty thing. About the size of a Tom Clancy paper back novel. It has a spigot at one end to mount 10.5” diameter reels of tape, allowing you to rotate the reel evenly over the coil to wipe everything clean.
A while ago I sold some previously recorded Maxell XL1 quarter-inch tape on 10.5” diameter reels on an auction site. Not wishing to infringe copyright by selling the music on the tapes, I rotated them a few times on my Tom Clancy bulk eraser and sold them on. A few days after the sale the feedback came in: “Top seller. Fast delivery. The music on the tape was unexpected but worth a listen.”
So much for my “bulk eraser”!
The eraser does work, in case you are wondering. Except if you are planning to successfully wipe a tape with it. A few turns of the reel won’t be enough. You really have to fry the tape!
And that’s the rub: Those new to this format often treat their tapes like a newly completed jigsaw. Carefully moving them around in case any of the little magnets of sound shake off. Of course, best practice is not to be lazy in how you look after your tapes: In 35 years of working with reel-to-reel I have lost count of the hundreds, perhaps thousands, of reels of tape I have seen stored in damp basements and outhouses or stacked up, like a collection of dinner plates, on top of a speaker. But here’s a thing. The majority of them, subject to the tape itself not suffering mechanical degradation (which is another issue entirely) all played well. No dropouts, no hf loss, no funny phrasy stuff you can’t quite define. In truth it’s quite hard to wipe the contents off a tape.
Just look at the erase function of a tape machine to see this in practice. A 100kHz plus signal at 2 to 4 times the amplitude of the signal the record head sends to the tape, imparted into the tape as it slides over the erase head, with no other tape in the way to protect it. And even then, depending upon the format of the heads and how the machine is aligned, may not entirely wipe the tape. That’s how tough a job the machine has to do to blast a tape clean!
To de-mag or not to de-mag: That is the Question.
When asked “How often should I de-magnetise my heads?” it’s a little like asking me “How often should I pray to my chosen deity?” You see, I don’t know! Starting a conversation like this is a little philosophical. For others? Well, they just like to pull the pin on a discussion grenade and walk away! Yes, I do de-mag the heads of machines. Of the hundreds I have worked on, not one do I know its operational history and the care in ownership it has had, so I am not going to risk putting my expensive alignment tapes, of which I have spent thousands on over the years, on an unknown machine. Others I know, who have a lifetime of experience in the format, say it’s snake oil and you shouldn’t bother. My position is: It can’t hurt provided the de-magging process is done properly and with the right tool for the job. The actual technique for de-magging the heads of any tape-based recorder from 2” multitrack machines to a micro cassette is exactly the same, just the scope and size of de-magger changes. There is a video on the Reel Resilience YouTube channel titled “Machine Preparation” detailing the technique I use. It can be found at this link: https://www.youtube.com/watch?v=gt-7nvjAPro
Why do heads get magnetised?
The consensus seems to be that, unless there is a fault in the electronics (more on this aspect later) there seems to be no reason the de-mag the heads of a machine. If you consider the magnetic recording and replay process, it’s only, by definition, that the record head is like to suffer any residual magnetism. It is the one generating the magnetic flux that is imparted into the tape. The replay head is just picking it up off the tape. This flux level is so small anyway that if the replay head did get magnetised in any way by the flux coming off the tape, then that imparted into the coil of the head would be so small that the reproduced signal would be in audible.
The most likely cause of head to be magnetised is that a DC voltage has leaked into the coil of the head and caused a permanent magnetic “charge” within in it. And for it to do this means the Record or Replay amplifiers have developed a fault.
To allow the AC signal voltage swing within an audio amplifier to have equal level in both positive and negative directions, the centre of the signal has to sit at the midpoint between the two extremes of the amplifier supply voltage. If the amplifier was supplied with both positive and negative supply voltages of equal level this mid-point would be 0v. However, such positive/negative supply rail design increases the complexity of the circuit and the power supply. The easier solution is to supply the amplifier with a single, usually positive, voltage supply and then design the circuit so the AC signal sits at the midpoint of this supply rail. When the signal leaves the amplifier “blocking capacitors” are used to stop the mid-point DC voltage from leaving too and effecting subsequent amplifier stages. Take a look at the circuit schematics of any tape recorder audio amplifier and you will see the symbol of a capacitor as the first component the signal encounters when entering this stage and the last thing it passes through as it leaves it. This is especially important with a record amplifier that passes its signal to the record head of a tape recorder. If the blocking capacitors fail, or partially fail, (known as going “leaky”) then the mid-point DC voltage will escape into the head and impart a magnetism into it. This is more likely to happen in the record head as the amplifier design final stage will have a signal sitting at a DC mid-point. In a replay amplifier the signal is just coming of the head and being fed to a circuit stage which will then sit at a DC midpoint.
How do magnetised heads manifest themselves?
The effect of a magnetised head is the presence of high frequency noise on the signal. Some say that de-magging the heads of a tape recorder will “restore the highs” (frequencies) of your machine. It won’t. But a magnetised head exhibiting this noise will mask the high frequencies. De-magging won’t restore high frequencies of your audio, it just pulls the veil of noise off of them.
To my recollection, in 30+ years of supporting the tape format I have only encountered magnetised heads twice. The effect can be quite subtle. In one instance the audio appeared unaffected, but the measured noise figure on ONE channel was awful: Easily 20dB worse than the other one. Replaying a blank tape found the tape noise to be equal on both channels. But on Record the magnetised head was imparting this noise into the recording. It probably was affecting the recording, it was just so high, other amplifier stages were filtering it out, but that’s not the point. It was still affecting the recording. Also note it was on one channel only. The DC in the head was on one track coil only, it wasn’t spilling over and effecting the other head coil. That is how small the flux level is that causes a head to become magnetised.
De-magging the head caused the coil in the head to lose its magnetism, but it returned within a VERY short time as the failed blocking capacitor let the DC signal through and “charged” the coil up. Re-capping the board (another area outside the scope of this article) solved the issue.
A magnetised record head does not seem to effect pre-recorded tapes. Of course, the magnetised coil of the head does represent a permanent magnet, but for the same reasons outlined in the beginning of this article on my attempts to bulk erase a tape, its effect is so small it had no impact on the material on the tape. A magnetised head will not act like a Whylie Coyote’s cartoon horseshoe magnet, with anything ferric nearby, and not bolt bolted down, sticking to it!
I mentioned the Reel Resilience YouTube video on de-magging, and this is where I agree with those that say de-magging carries a risk greater than what it is designed to eliminate. I.e. an incorrect de-magging technique can impart a magnetism into the head and make things a whole lot worse. Follow the best practice outlined in the video. Have a practice with the de-magger switched off just to make sure your technique is ok.
What de-magger is best for my machine?
I have around 10 de-maggers of various types: From the cassette-based ones and small devices designed to access heads in such decks to what I call my “Looney Tunes” de-magger that will take on a 2” head and win. Provided the correct procedure is observed (see the YouTube video) here is nothing wrong in using such a device on a cassette deck, although accessing the actual heads may be an issue. But the converse is not true: You need a hefty de-magger for a large head. The other thing to note is make sure the metal tip of the device is covered. You do not want risk scratching the surface of the head.
Reel Resilience sells new de-maggers that are ideal for heads up to those using quarter inch-tape. These seem to be the only ones readily available now and before I stocked them, I made sure they could do the intended job. This involved using my large de-magger in the wrong technique to magnetise a worn-out head. I checked it had been magnetised by placing a compass near it. I then used the same model of de-magger I now stock to successfully “clean” the head. These de-maggers can be found in the Reel Resilience shop at this link: https://www.reelresilience.co.uk/shop-1?page=2
For de-maggers larger than this I recommend the Handi-mag. They are not made now but do come-up for sale on auction sites. Just make sure:
1. It is designed to operate on the mains voltage of your country
2. It works.
3. It safe!
How often should I de-mag my heads?
As I said, I don’t know! An equally useful answer could be “When they need it.” There are schools with the audio fraternity who are adamant that “analogue is best” and “there is a warmth in vinyl.” I’m not getting into this debate; all I will say to these statements is “it depends upon what kind of distortion you like.” It’s the same with the frequency of de-magging. If you think it needs doing every month to maintain the sonic purity of tape, then so be it. Remember heads on an unused machine will not magnetise themselves. They don’t draw it out of the ether like a sponge. If you are a sporadic user of your machines, then save yourself so effort and ease off the de-magging cycle. That said, if you are a regular user (which carries the impact of increased head wear, which is far more detrimental to the whole recording/replay process) then perhaps you want to increase the regularity? Whatever path you choose, just make sure the process is carried out correctly.
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