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Golden Gear: Studer Revox PR99

Open-Reel Stereo Tape Recorder By Hugh Robjohns
Published October 2021

Golden Gear

The final Revox tape recorders left the factory almost 25 years ago, yet the enthusiasm and support for these classic machines remains as strong as ever.

There will be few ‘baby boomer’ studio engineers who won’t go a little misty‑eyed at mention of the legendary Revox A77: one of the first all‑transistor, high‑quality, open‑reel tape recorders that non‑professionals could afford. And, of course, those of slightly younger years will undoubtedly have much the same emotion for its successor, the elegant B77. However, it is this latter descendant’s close cousin, the PR99, that I really want to reminisce about here! Forty years ago the PR99 was launched to span the gap between the Swiss manufacturer’s Revox consumer machines and their more expensive professional Studer recorders.

Revox Background

Although the A77 (and later B77) was designed fundamentally for the 1970s (and ’80s) domestic hi‑fi market, it has been suggested that most of them were bought for professional or semi‑professional applications. They certainly became familiar sights in musicians’ home studios, educational facilities, and in radio production offices, as well as making regular appearances in professional recording studios and location recording trucks, too!

The simple reason for this enormous breadth of service was that, despite their notional consumer origins and relatively affordable prices, these were extremely well‑engineered, rugged and reliable machines which, when correctly aligned and maintained, achieved remarkably good technical specifications and gave surprisingly little away to pukka professional machines. These Revox recorders could also accommodate ‘professional’ 10.5‑inch tape reels and run at 15 inches per second, when most comparable domestic machines could only manage 7‑inch spools and 7.5ips!

Revox’s A77 was constructed around a strong aluminium die‑cast chassis, just like its Studer cousins, to ensure a stable and accurate platform supporting the transport motors, tape platters, guides and heads. And instead of sharing a single motor and relying on belts, pulleys and cogs to drive the reels, the A77 featured three powerful direct‑drive asynchronous AC motors, with the capstan motor being electronically controlled via a tacho‑sensor for absolutely speed accuracy — most consumer machines of the era relied on a synchronous motor whose speed depended on the (varying) mains frequency. In the A77 everything was controlled electronically, using relays and solenoids, allowing ‘light‑touch’ transport buttons and versatile remote‑control options, while the audio electronics were arranged on modular plug‑in circuit cards for easy access and servicing.

Studer Revox also designed and built — to exemplary standards — their own ‘Revodur’ tape heads, which allowed their recorders to deliver a significantly flatter frequency response than most of their contemporaries could manage. Moreover, head life was maximised through gentle tape paths with modest wraps, resulting in less tendency to develop flats and grooves in the heads. One amusing Revox A77 advert from the ’70s stated that the company guaranteed four parts for just a year: the capstan, the pinch‑roller, and the record and playback heads... while the remaining 842 parts were guaranteed for life!

...from 1967 to 1977, the A77 model evolved through four Marks, and around 450,000 machines were built!

From A77 To B77

In its 10 years of production from 1967 to 1977, the A77 model evolved through four Marks, and around 450,000 machines were built! In 1977 it was succeeded by the B77, which built upon the great successes of its forebear while maintaining the same fundamental design concepts, mechanical engineering and build quality. The audio circuitry was largely derived from the earlier model, but the B77’s transport control system was completely redesigned principally to use digital logic instead of relays. The B77 achieved slightly better technical performance across the board, and most say it sounded it a little better. It was certainly more generously equipped with features, including sound‑on‑sound via an internal track dubbing facility.

Around 220,000 B77s were built altogether, from a catalogue of 61 different model variants, and at the height of production, Studer’s factory in Regensdorf, Switzerland could churn out around 175 new machines every day! Different model options started with selecting the head format. The ‘professional’ options included mono full‑track, two‑track (with a wide 2.0mm NAB‑standard guard band), or stereo (with a narrow 0.75mm DIN‑standard guard band), using Studer’s acclaimed ‘butterfly’ heads to minimise magnetic crosstalk. These three formats ran the tape in a single direction, of course, while the standard ‘consumer’ option employed the stereo quarter‑track head format to record a stereo track in one direction, whereupon the tape could be turned over and a second interleaved stereo track could be recorded in the opposite direction. Additional options applied to the erase head, which could span the full track, or be split to erase each half‑track separately, or the two erase gaps could overlap to ensure the guard band was wiped as well.

Another key decision when specifying a new machine was the preferred equalisation standard. The A77 featured a switch to select NAB or IEC (CCIR) replay equalisation (although the record format was fixed at the factory). However, the B77’s record and replay EQ was determined during manufacture so the appropriate NAB or IEC standard had to be specified on the order. Another factory specification set the two operating speeds: any adjacent pair could be provided between 15/16‑inch and 15ips. Other model options included a playback‑only version, machines with deleted controls, rackmounting rails, added sync and remote‑control facilities for slide projections, and much more besides.

Enter The PR99

Despite this plethora of factory configurations, some of which were obviously already aimed at professional users, Studer Revox announced the PR99 model in 1980. This was an obviously ‘more professionalised’ version marketed primarily towards the rapidly increasing numbers of local radio stations which were appearing across Europe at that time. These new broadcasters tended to have relatively low budgets, yet still needed workhorse tape machines in great numbers. Studer Revox met that need with the PR99 which was promoted as an ‘affordable Studer’, neatly bridging the capability gap between the domestic B77 and the nearest professional equivalent at the time, the Studer B67.

The PR99 also featured transformer‑balanced input and output XLR connections to integrate properly with professional mixing consoles...

The most obvious difference between the B77 and the original PR99 is the latter’s raised transport chassis, bringing it flush with the control panel to give much easier access to the heads for rapid lacing and tape editing. Less obvious was the addition of a second damper arm in the tape path on the exit from the headblock (the B77 had only one damper arm on the entry to the headblock). The PR99 also featured transformer‑balanced input and output XLR connections to integrate properly with professional mixing consoles, and front‑panel buttons selected calibrated or adjustable I/O alignments. A Tape Dump button was also added to disable the take‑up spool motor, allowing unwanted sections of tape to be ‘played off’ during editing without the take‑up spool spinning wildly. Another useful feature was the appearance of a pair of Sync buttons, which replayed audio from either channel of the record head rather than the replay head, enabling synchronous recording or overdubbing onto the other track. Standard PR99s came with 19‑inch rackmounting rails, but there was also a version mounted horizontally in a wheeled trolley, and another in a flightcase with built‑in monitor loudspeakers for portable location recording applications — although this is probably the rarest version to leave the factory.

In most other respects, though, the PR99 and the B77 were still very close cousins, and this frequently created ‘operational issues’. In particular, the profusion of front‑panel controls with options for input selection, track dubbing, I/O level adjustments, monitoring modes and so on, often lead to embarrassing user errors! Remember, these machines were typically being used by non‑technical radio presenters and broadcast journalists rather than experienced studio managers or engineers.

Many options could be specified when ordering a PR99. This unit, for example, lacks the mic preamps.Many options could be specified when ordering a PR99. This unit, for example, lacks the mic preamps.

Outside Influence: ASC

ASC, a small British pro‑audio manufacturing company with close ties to the UK radio broadcasting industry, picked up on this problem and, in 1983, launched a much‑simplified version of the PR99 in which all the ‘non‑essential’ controls were removed, with a new grey panel fitted to hide the missing knobs and buttons. But ASC didn’t just take things off the PR99: they also installed an elegant digital tape timer to the left of the head block. This was a really useful modification, as the simple four‑digit mechanical counter carried over from the B77 was far too basic for use in a professional setting. An additional benefit of removing most of the front‑panel controls was that it created space in front of the headblock to fit a large, traditional ‘EMItape’ splicing block (similar to an Xedit ‘Editall’ for those to the West of the Atlantic), allowing easier and faster editing.

These ASC PR99s proved hugely popular in the UK, and perhaps that’s what spurred Studer Revox into launching their own Mark II version of the PR99 in 1985. This new updated model likewise incorporated a smart digital tape timer, with zero locate, address locate, and auto‑repeat functions, and a rather small but functional splicing block complete with integrated cutter to the right of the headblock. Going beyond ASC’s updates, Revox also added a varispeed facility (ranging over ±2 semitones) controlled by an on‑off button and range knob above the head block, and a Cue‑Edit function operated by a mechanical slide switch just in front of the head block. This last feature defeated the tape lifters and replay head hum‑shield when in Stop mode, and activated the replay audio circuitry to allow manual audible tape cueing (or scrubbing, as it’s often known). A less visible but still quite important improvement expanded the range of treble EQ adjustment in the replay amplifier, allowing compensation for greater head wear.

The most eye‑catching feature of the MkII PR99 was the digital tape timer.The most eye‑catching feature of the MkII PR99 was the digital tape timer.

Possibly of even greater significance is the fact that Studer Revox changed suppliers around this time and started using high performance Beyschlag and Philips components, which many claim resulted in worthwhile audible improvements.

Many claim this Mark II revision was the best version of the PR99, but four years later in 1989 Revox brought out a PR99 Mark III model, recognisable by its grey control panel. Most of the changes introduced in the Mark III were really aimed at reducing manufacturing costs, perhaps driven by a need to remain competitive as the digital audio revolution was building rapidly in the broadcast world. New digital recording systems like Sony’s PCMF1 and the then‑new ‘R‑DAT’ recorders cost a fraction of the price of a PR99 and, although impractical for editing at that time these formats were quickly gaining popularity for recording applications, not least for their longer recording times, less costly media, and more compact and lightweight physicality.

Other aspects differentiating the Mark III include the deletion of the microphone inputs, input selector knobs (in the centre of the control panel), and the Sync buttons and associated sync replay amplifiers (so it was no longer possible to overdub synchronously between tracks). Internally, the electronics were significantly reworked, too, with extensive application of FET analogue switches and op‑amps throughout the audio circuitry. On the upside, wider adjustment ranges were provided for treble EQ, bias and erase drive current, presumably to cope with higher‑output tape formulations. As a result, though, the Mark III’s electronics cards were no longer interchangeable with Mark I/II PR99s.

Still Rolling

The last B77 Mark II and PR99 Mark III machines left the Regensdorf factory in 1997, ending 46 years of Revox open‑reel tape recorders. That means that, today, even the youngest PR99 is over 25 years old, and the oldest models are around 40, with most having endured hard lives in broadcast studios. Nevertheless, there are always more Revox machines in the 'For Sale' ads than any other brand of tape recorder, and they often change hands today for considerably more than their original purchase price. Such buoyant prices only confirm the impressive longevity and continued desirability of the PR99. The most sought‑after models are good‑condition stereo half‑track machines fitted with DIN butterfly heads.

One reason for the continuing popularity of the PR99 is that virtually every part is still available.

One reason for the continuing popularity of the PR99 is that virtually every part is still available from a collection of specialist companies that enthusiastically support these revered machines. The same is generally true of its siblings the A77 and even the later 36‑series tape machines! Consequently, bringing these elderly machines back to perfect working order is generally quite feasible for an experienced technician with the right tools and test equipment — albeit at a cost; running any open‑reel tape machine is inherently an expensive business these days!

The PR99’s mechanicals are generally very robust and long‑lasting, while elements that suffer normal wear and tear like the motor and guide bearings, the capstan shaft, pinch‑rollers and brake linings are all fairly easy to obtain and replace. As for the electronics, the biggest risk by far is failure of the many electrolytic and mains‑filter capacitors, which is why most service technicians automatically replace original components as a standard part of any service or renovation — hopefully before their looming failure can do any serious damage! Trimmer pots on the electronics cards can also become intermittent due to dirt and corrosion, or just through excessive use, but again these are easily replaced. Thankfully, there are very few custom or unique circuit components in a PR99, so most electronic repairs are straightforward and inexpensive. Studer Revox’s service manuals are very thorough and easy to follow as well, so routine maintenance and alignment presents little challenge to anyone familiar with electronics and tape recorder servicing — you just need to make sure the manual covers the specific boards in your particular machine because, as we’ve seen, there were quite a few versions over the years.

Modifying or converting machines from one configuration to another isn’t usually too much of a problem, either, or obviously far more involved than routine maintenance, and there are even companies offering brand‑new front‑panel and case metal work (in fancy colours, too) as well as wooden cabinets to rejuvenate tired‑looking machines. I think it’s a fairly safe bet that there will still be Revox PR99s in use at the turn of the next century!

Revox Or ReVox?

The Swiss manufacturer’s name is often seen written as ReVox, but in this article I’ve stuck with Revox partly because it’s simpler to type, but mainly because it’s much easier on the eye! The actual company logo used small capital letters throughout, with the V in a larger type size: reVox.

My Own PR99

I bought a well‑used ASC PR99 MkII from the BBC in about 1998 — it was one of many literally being thrown out, as the BBC considered it pretty much obsolete and disposable equipment by then. It had had a hard life, but it was a half‑track stereo machine with butterfly heads and came with a full service manual, so I was confident of being able to restore it to good health. It’s still going strong today, although it has been re‑capped, had a couple of new bearings and a new pinch‑roller.

About 15 years ago it had major surgery to remove the record amplifiers and bias oscillator, install a second set of repro amplifiers, and the record head was replaced with a quarter‑track replay head. I can now switch between professional half‑track stereo and consumer quarter‑track stereo formats by using the Sync buttons. Not a standard mod, obviously, but it works well and suited my needs at the time!