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Buying A Used Tape Machine

Advice on acquiring and maintaining reel-to-reel recorders By Hugh Robjohns
Published March 2010

The sound of analogue tape is still revered, but acquiring and maintaining your own machine presents a unique set of challenges.

At least a generation has grown up without hearing high‑quality analogue tape recordings, and with no experience of using reel‑to‑reel machines, but as more people move to digital recording, the old technology is becoming more affordable, and there's a resurgence of interest in tape. There are plenty of reasons to want to use it ('Analogue Warmth' in February's SOS explores them in detail), but buying and maintaining a tape machine can be a minefield for the unwary...

Look Before You Leap!

Let's start with a reality check. If you want good results, a tape machine can be expensive to run and is likely to become more so over time: there are lots of moving parts in even the simplest machines, and they spend their time slowly wearing themselves out. No big manufacturer has built new machines for years, so some spare parts are becoming scarce and expensive. Motor bearings, tape guides and rollers, pinch wheels, brakes and tape heads are all essentially consumable parts. The electronics also need regular alignment, so recording and replay remain within appropriate tolerances. Most settings need adjusting as heads wear or if tape type is changed, and ideally should be optimised for each new batch of tape. If everything isn't aligned and properly maintained, quality suffers quickly and obviously.

Routine maintenance includes cleaning the tape path, heads and pinch roller of the oxide and other deposits that gather as tape runs through. Running a magnetised tape over ferrous objects in the tape path will also induce some magnetism in them, so you should also consider degaussing (demagnetising) the machine: how often depends on the machine's design and the amount of tape run through it, but could vary from weekly to annually. Decent degaussing tools aren't expensive, but they need to be used with care: it's easy to end up magnetising the machine quite strongly if you're unsure what you're doing, which could damage any tapes you play on it.

Electronic alignment comes down to physical head alignment, replay gain and EQ, record gain, EQ and bias. To align the heads and replay circuitry you need a decent test and alignment tape, which will also wear out eventually. This might set you back around £130, and you'll need an oscilloscope and audio level meter. To set up the record electronics, an audio signal generator will be required too. It's not hard to align a tape machine's electronics if you have the equipment and knowledge, but if you don't the machine will need to go to a specialist.

Mechanical alignment of the transport is often quite straightforward on more modern three‑motor servo‑controlled machines: little is needed other than lubrication with the correct oils or greases in the appropriate places. Older machines, though, often relied on friction brakes to control tape tension and arrest fast winds, can be fiddly to set up, and may require specialist spring gauges and other tools. The medium itself (tape) isn't especially cheap, either!

The oldest machines tend to have valve electronics, which can add an extra sonic dimension, but they also tend to have wide replay head gaps, which limits top‑end response dramatically, and they're mostly mono rather than stereo. The heads can be replaced with modern, narrow‑gap alternatives (they may have to be replaced, due to wear and tear). More modern machines tend to have better specifications, including more headroom, making them better able to cope with modern high‑output tape formulations. Better machines also have sophisticated servo‑controlled motors, providing much gentler tape‑handling and better wow‑and‑flutter figures.

Recording Formats

Paul White's Tascam 32 half‑track tape machine.

The narrowest common open‑reel tape format is quarter‑inch, generally available on five‑, seven‑ or 10.5‑inch spools, the first two with 'cine' centre sprockets and the latter usually with NAB centres (which usually require cine‑NAB adapter hubs). Domestic tape recorders generally record two tracks (or stereo) in one direction, and another two tracks in the other direction. This is usually called a 'quarter‑track' format, and the track width is the narrowest in common use. Narrow tracks mean fewer magnetic particles to record the signal on to, and hence a poor signal‑to-noise ratio: they tend to be hissy. This is made worse because such machines run at low speed, typically 3.75 or 7.5 inches per second (ips): the slower the speed, the fewer magnetic particles pass the heads. It's impossible to be precise, but machines of this type would usually have perhaps a 55dB signal‑to‑noise ratio, with high frequencies rolling off increasingly above 14kHz.

The playback heads of the author's modified Revox PR99. The record head has been removed and replaced with a quarter‑track playback head (left) to play consumer tapes. The original half‑track playback head is on the right. The different head gaps for the two formats are shown in orange.Next up is the 'half‑track' format on quarter‑inch tape. The tape runs in a single direction, capturing two tracks (or stereo) along its length. The doubling of the track width brings a reduction in tape noise, and most half‑track machines can also be run faster (typically 7.5 or 15ips), bringing a further quality improvement. Pro and semi‑pro stereo machines usually employ this format, and well set‑up half‑track machines running at 15ips pretty much defined 'broadcast‑quality sound' for half a century: a signal‑to‑noise ratio of 65dB and a bandwidth close to 20kHz is perfectly achievable.

The primo format is half‑track, half‑inch at 15 or 30 ips. These are popular high‑end mastering formats, and provide well over 20kHz bandwidth and a 75dB signal‑noise ratio. Machines are relatively rare and expensive, the Ampex ATR100 being among the most favoured for this role.

The quarter‑inch, quarter‑track format was also used for semi‑pro four‑channel multitrack recorders, with the tape only running in one direction, usually at 7.5ips or faster. Scaling this format up, half‑inch tape allowed eight channels with the same track widths and speeds, and one‑inch tape gave 16 tracks. In each case, the relatively poor signal‑to‑noise ratio was a problem when mixing four, eight or 16 tracks together, so various double‑ended noise‑reduction systems were employed. Dbx systems were very common on semi‑pro and project‑studio machines, while some later machines adopted Dolby C or Dolby S.

Professional multitrack machines tended to extrapolate from the professional two‑track formats: half‑inch tape generally supported four tracks, one‑inch carried eight, and two‑inch had 16 or 24 tracks. Sixteen tracks on two‑inch tape provides broadly the same quality as half‑track quarter‑inch and is a highly regarded format — but 24 tracks offered versatility, and in the '70s, 24‑track became the de facto standard. Again, noise‑reduction systems were commonly used (Dolby A and latterly Dolby SR being the favoured choices), and were absolutely necessary with the two‑inch, 24‑track format.

Which Machine?

Some of the accessories you may need: leader tape, a splicing block and blades for editing, a chinagraph pencil, a degausser and some cotton buds and head-cleaning fluid.

The cheapest available machines these days will be simple, two‑head, low‑speed, quarter‑track machines using quarter‑inch tape. These have an erase head and a combined record/replay head, much like most cassette recorders. They're a nightmare to align, and best avoided for serious work. On the next rung are several reliable three‑head domestic machines, the Akai 4000D, DS (Mk I and MII) and DB. The Sony TC377 or TC399 recorders were particularly popular, but there were many more from the likes of Tandberg, Philips, Ferrograph, Dokorder, Technics and others. Three‑head machines use an erase head, and physically separate record and play heads (making off‑tape monitoring possible), and are usually much easier to use and maintain. Most can't accommodate spools larger than seven inches (diameter) and use the quarter‑track format, but they're fine machines for messing around with tape loops and learning basic tape-editing skills, or for providing lo‑fi tape saturation. They're unsuitable for high‑quality recording or mastering.

Moving up a notch brings us into the realm of the mighty Revox, the domesticated half of the Studer‑Revox group, which shared much of its professional sibling's DNA. There's massive support available for virtually the entire Studer‑Revox lineage, from valve machines like the G36, through the A77 and A700 and onto the B77 and PR99s. Quarter‑track, slow‑speed Revox variants do exist, but the majority on the market tend to be high‑speed (7.5/15ips) half‑track machines, and all accept 10.5‑inch reels of quarter‑inch tape. It's possible to convert low‑speed quarter‑track machines to high‑speed half‑tracks, but it's not a cheap option.

With the broadcasting industry's wholesale move to digital recording, countless Studer (and other) machines flooded onto the market and are now easily found: models ranging from the elderly B67s, through the A810s and A80s to the A807 are all quite common. You may also find the A80's broadcast‑studio rival, the Telefunken M15, both huge flat‑bed machines ideal for fast tape editing. There are plenty of other equally high-quality machines from Sony, Ampex, Ferrograph and others, and Nagra should get an honourable mention too: the Nagra T is a stunningly good studio machine, and for transportable applications you can't beat a Nagra IV‑S, but the Nagra name inevitably attracts a high price. Other professional options include machines from Otari (MX5050 and its siblings), and the unusual but impressive Technics RS1500, with its unique 'closed loop' transport.

Perhaps the most dominant name after Studer‑Revox is TEAC‑Tascam, who produced a wide range of two‑track and multitrack machines over a long period. They're reliable workhorses, and their four‑track 3340 and 3440 introduced affordable multitrack recording into the project studio market.

I've focused largely on stereo machines because that's where it's easiest to get quality on a budget, but the same names crop up in the multitrack market. The high end was dominated by the Studer A800 and Otari MTR90 (and their forebears and derivatives), followed by various Tascam models, with a wide variety of Fostex machines popular in the home market. The Fostex machines tended to use narrower tape formats (eight tracks on quarter‑inch tape, 16 tracks on half‑inch) and slower speeds, so the quality isn't up to that of the large‑format high‑speed recorders, but they generally included Dolby C or Dolby S noise reduction, and are perfectly adequate for most semi‑pro purposes.

Other Considerations

Some accessories you may need: leader tape, a splicing block and blades, a chinagraph pencil, a degausser, and some cotton buds and head-cleaning fluid.

One thing you shouldn't forget is that the recording medium's characteristics have an impact on other aspects of the recording process. Recording to tape will inherently impose a certain sonic character (however subtle), which will change the way you work compared with DAW recordings and mixes: you might prefer to use a rich‑sounding valve mic, or a mic pre with generous transformers when recording to a DAW (to impart musical character), but when working with tape you may prefer something that sounds a little 'cleaner' or 'sharper' as a source. Similarly, you may find that when working with all‑digital audio, you choose plug‑ins to smooth the sound, but with tape you work to add air and sparkle. There's nothing wrong with either approach, but they are different, and you have to take that into account. Blending the best of both worlds can be a great option, whether that means tracking with a DAW to capture a pristine, accurate source, and selectively bouncing tracks to tape and back to introduce tape saturation or character, or recording to multitrack tape and transferring to your DAW for editing and processing — or working completely in your DAW but bouncing the mix down to a tape master.  

Good Head? Checking For Head Wear

How do you know if your prospective purchase is in good condition? One thing you must do is check the condition of the record, play and erase heads.

In a well set‑up machine, it can be tricky to assess the amount of head wear by looking, unless you know what a new head looks like.

  • The most tell‑tale sign is an obviously flat surface: a new head is generally convex and touches the tape over a relatively small horizontal area. As it wears, the surface flattens and the contact area broadens.
  • If the head is not aligned accurately, the wear pattern will be asymmetrical with respect to the gap: wider at top or bottom implies a zenith error; not centred vertically suggests a height error; and not centred horizontally suggests a wrap error. Correcting mechanical alignment will result in sub-optimal tape contact, so head may need replacing.
  • In a well‑maintained machine, wear should be modest, and the worn area nicely centred and linear relative to the gap. The head should also be clean; not covered in sticky tape residue, oxide dust or grease-pencil marks!

The Brown Stuff: Which Tape To Buy

When it comes to buying tape, there are fewer options than there once were, but still enough to cater for most machines' requirements. The easiest brands to find are ATR, RMGI and Zonal: the first two are available in widths from quarter‑inch to two‑inches, but Zonal seem only to offer quarter‑inch. Most types are 'matt backed standard play' types, but you'll also find long‑play versions. The latter are thinner, so more tape fits on a reel, for more recording time, but there's more risk of tape stretch, print‑through and other damage. RMGI produce the LPR35 long‑play tape and Zonal produce type 840. The matt backing is now standard and offers improved tape handling and winding.

The magnetic compound is formulated in different ways for different applications, producing different noise‑floor and print‑through characteristics. There are no set rules about which tape works best with which machine, so experimentation may be required. Older machines often can't cope with the high recording flux levels some modern high‑output tapes require, but those that can benefit from lower noise‑floors. Here's a few tape guidelines.

• TAPE TYPE: 'Standard Bias' tapes (normally best for old machines) include RMGI SM911 and Zonal 820. 'High output' tapes (for modern pro machines) include all ATR stock, Zonal's type 700 and RMGI's SM468. Mid‑range alternatives include the Zonal type 675 and RMGI's SM900.

• ALIGNMENT: Ideally, the recorder should be realigned to optimise performance with the tape type, and in doing so you'll normally find that one type suits the machine best. If you can't realign the machine to the tape, experiment with different types to see which allows the machine to deliver its best performance. An experienced technician should be able to advise which tape is best for your machine.

• PRICE: At the lower end, budget Zonal tapes range from about £11 to £20 per 10.5‑inch reel of quarter‑inch tape in the UK. A 10.5‑inch reel of two‑inch tape may cost £130-plus. There's still some 'new old stock' (NOS) tape available, but buying NOS tape is a gamble: heat, cold, sunlight and humidity during storage all have a damaging affect, and there are issues of binder decay that affect some tapes (notably, some Ampex), resulting in a gooey mess on heads and tape guides, and damaging the audio in potentially catastrophic ways.

• RE-USE: Tape can be re‑used several times, but repeated passing over the heads gradually wears the magnetic coating and performance declines (with a loss of high frequencies and increased noise). How tolerable this decline is depends on the circumstances: when I started in the pro audio business, tape was cheap enough to use fresh reels for almost everything (although the BBC had a tape‑recycling facility). In a project studio, the cost of tape today might make re‑use a practical necessity, You could expect to get around five re‑uses before you notice a reduction in quality.

Tape Heads

The three magnetic heads of a professional tape recorder are each carefully optimised to perform their own particular job, but basically comprise an iron C‑shaped core, the back of which is carries a coil of wire. When an electric current is passed through the winding, a magnetic field is produced in the core and bridges the gap, which is arranged to be in contact with the recording tape. If the current through the head varies with the wanted audio signal the magnetic field strength and direction also varies. The magnetic layer on the tape captures the magnetic state, and as it is moved past the head gap it moves outside the magnetic influence and retains the magnetic field at that time. This recording would ordinarily be grossly distorted on replay because of the very non‑linear magnetic characteristics of the recording particles within the tape. The solution is to add a high‑frequency ( typically about 150kHz) 'bias' signal to the audio signal which forces the magnetisation process to become far more linear. The bias signal is not retained by the tape, it merely linearises the recording process.

The size of the gap in the recording head is not particularly critical, provided a sufficient field strength can be created, so relatively large gaps tend to be used to ensure that the magnetic field is large enough to penetrate the full depth of the magnetic layer in the tape. In fact, the effective recording zone is not actually in front of the head at all, but is at the point where the tape moves out of the influence of its magnetic field.

A replay head is constructed in a similar way to the record head but operates in reverse: the magnetic field carried on the tape is focussed through the head's C‑core and induces an alternating electric current in the coil of wire. These currents are then amplified and processed to reproduce the wanted audio signal.

The replay head must be extremely sensitive to capture the relatively weak magnetic flux stored on the passing tape, which means that some care must be taken to shield it from external magnetic fields generated by the transport motors and mains transformer. Consequently, the head is usually enclosed in a protective mu‑metal case, often with a fold‑down front plate which can be raised after the tape has been laced.

The signal voltage induced across the replay head windings increase in direct proportion to the rate of change of the magnetic field on the tape, so there will be a much greater output for a high‑frequency audio signal recorded on tape than for a low‑frequency one. This is compensated for with a 6dB/octave low‑pass equaliser in the replay electronics, but various magnetic anomalies result in a slightly uneven low frequency response, often referred to as 'head bumps.' The size of the replay head gap is critical for the high frequency response because when the recorded signal on the tape has a wavelength equal to the width of the head gap there will be no net magnetic flux, and no output. This is called the 'extinction frequency'. To ensure that the extinction frequency is well above the highest required audio frequency, a very narrow head gap must be used – typically one micron or less. The recorded wavelength is related to the tape speed of course, and lower tape speeds mean shorter wavelengths and a lower extinction frequency, so less top end.

Given the very different requirements for the head gap between the recording and replay functions, professional machines are generally equipped with three heads, each optimised for its specific requirements. Some machines have only two heads, erase and a dual‑purpose rec/rep head, although this compromises both the record ability and replay performance. In multitrack machines, where it is necessary to switch some channels of the record head to replay to generate a synchronous overdubbing cue feed, the output is often noticeably duller sounding than when auditioned via the replay head.

It is worth noting that modern machines have far narrower replay head gaps than was possible in the early machines of the 1950s and 60s, and as a result can extract a far wider and flatter frequency response from old archive tapes than was ever possible at the time of their recording. Since the recording process is largely independent of the record head's construction, recordings made in the 1950s and 60s are frequently found to be of extremely good technical quality when replayed on modern machines – the recordings were often of a far higher quality than could be replayed at the time!

The erase head may be a single unit covering the entire tape width, but is more usually split to allow independent erasure of the individual tracks in a dual or multitrack machine. Erasure is achieved by sending the same bias signal as was used to linearise the record process into the erase head's wire coil. The tape is not capable of storing this high‑frequency bias signal, but it does shake up the magnetic particles, leaving them in a random arrangement, thus erasing any previous recordings and guaranteeing a clean tape for the new recording.

Additional Background Information

Practical magnetic audio recording first appeared in the 1930s but the form we know it in today was developed by the Germans during the Second World War. Recording tape is, essentially, a sophisticated rusty ribbon, with the shape and depth of the 'rust' particles providing the required recording properties. The first recording 'tape' was paper‑backed and made by BASF in Germany, but later polyester designs proved more durable, and the magnetic coating (a metal oxide or metal alloy) improved dramatically over the years, reaching peak performance in the 1980s. The mechanics and electronics of the recording machines also improved steadily over the years, again reaching a zenith in the mid 1980s... and just as digital recording started to become cost effective and largely took over.

Magnetic tape recording is inherently imperfect as various harmonic and dynamic distortion artefacts are an integral part of the medium. However, these characteristics are so familiar to us all that many feel the indisputably more accurate digital medium lacks 'warmth' in comparison.

The way recording tape works is that a strong and flexible polyester backing tape is coated with a metallic compound capable of storing a varying magnetic pattern. This magnetisation is varied in strength and polarisation to represent the air pressure variations of the wanted sound as captured by a microphone.

'Standard Play' open‑reel tape is typically around 50 microns in thickness, allowing a full 10.5‑inch reel to run for just over 30 minutes at a speed of 15 inches per second. 'Long Play' types are typically about 35 microns thick and are used wither where extended recording times are required, or where the recorder has a particularly tight tape path or restricted spool size capability is restricted – such as in small portable recorders like the infamous Nagra machines.

All modern tapes are 'back coated' with a dull matt and rough‑feeling surface (as opposed to the shiny, slippery nature of older tape types). This matt back coating provides more friction between layers which helps to give more consistent and stable handling and smoother fast winding characteristics because less air gets trapped between the tape layers. Neat, even winding reduces the risk of edge damage which is the most common cause of drop‑outs and poor head‑to‑tape contact. On a well set up machine, the tape shouldn't touch either spool flange – it should sit neatly between them. If the tape spool is bent such that it rubs against the tape as it revolves, it should be discarded and replaced with new straight one to avoid causing edge damage and potential dropouts.