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GTE 'Studio Tube' Bass Pre-Amp

On the tube train - do valves still have a role in bass amplification? Gary Cooper tries America's latest tube

Once upon a time (and not so long ago, at that!) a bass player would count him or herself pretty well-off if they had a 50 watt combo, when their guitarist was playing through a 30 watter. Boy, how times change! Since the mid-1960s advent of the 100 watt valve guitar stack, bass guitarists (who can always reckon on needing around double the output power of a guitarist just to make themselves heard) have been struggling for audibility.

In the late 1960s, a 200 watt version of the valve stack did the bass job adequately, but although these gave the volume, the bass player (who was increasingly being called on to take a more lead instrument orientated role) needed extra clarity too. In the 1970s, when many guitarists began to move down towards ultra-loud combos bassists still had a problem making themselves heard - especially if they were playing near-lead roles.

Thus the concept of the bass 'rig', comprising a pre-amp, power amp, complex equalisation (tone) controls, and super quality speakers, was evolved. These weren't just louder than 100 watt valve bass stacks - they were more audible too. During the 1980s the bass set-up grew to become increasingly like a smaller version of the band's PA system. Pre-amps fed power amps, low frequencies went from these (via crossovers) to one speaker enclosure, highs to another, so that each speaker enclosure had only the appropriate frequencies fed to it. From there, twin power amp ('bi-amped') systems have developed, where each enclosure is driven by its own power amp, the appropriate frequencies being fed to that via a crossover.

For a variety of technical (and cost) reasons, all the electronic components in the typical modern bass rig are based on transistorised circuitry - the argument running that where power and tonal flexibility are the most important considerations, valves just can't cut it. This is all well and good, of course, but, as most of us will realise, many professional guitarists have never been over-happy with the fundamental sound obtained from transistorised amps. The theory is that solid state devices sound hard and dry compared with valves - which is why Marshalls, Mesa-Boogies, Laneys and Ampegs are so popular amongst pro guitar players. But what about the bassist? Would a valve amp help him?

Los Angeles-based GT Electronics obviously think valves would sound better for bass, so they've just released a brand new bass pre-amp, using all-valve technology, and aimed directly at the top professional bass player. With the GT Electronics 'STP-B' the idea is that the pre-amp generates the basic valve 'warmth', which is then amplified by a normal (almost always transistorised) power amps; the result being a superior, more 'musical' sound than you'd get from solid state preamps.

In theory, GT Electronics certainly should know what they're doing with valves - they're the same people who produce those much-vaunted 'Groove Tubes' valves (see IT Issue 1). To put that point to the test then. I've recently been trying one of the very first STP-Bs to reach these shores.


Clad in a black steel casing, the GT Electronics bass pre-amp fits any standard 19" rack system. It's toughly made, fitted with metal carrying handles, and weighs just a few pounds.

Starting with the back panel, the STP-B has a fixed mains lead with a wide range of input and output features, comprising jack sockets for effects loop in and out, a balanced 600 Ohm output with a switch offering line or direct levels. A crossover high frequency outlet is matched by a crossover low socket, you have a foot-switch socket for equaliser in/out, a full-range pre-amp output, a standard fuse holder and an extra AC mains outlet. This latter device is not acceptable on gear intended for the U.K. market and should be removed from the specification, as it always is with exported U.S. gear - a point which the makers should attend to as soon as possible.

Equally well equipped as the back panel, the front end offers two inputs (high and low gain), followed by a pre-amp gain control next to which is a red LED to indicate clipping/overload levels, a limiter control with a variable threshold adjustment alongside which is another LED to indicate the onset of limitation, a control for the on-board 'Aural Enhancer' circuit, bass frequency pot, treble control and then a variable graphic equaliser operating on four bands. A crossover control comes next (suitable for using the STP-B with two power amps in a bi-amped set-up) which provides feeds ranging from 100 Hz-1 kHz. Finally, you have a main master volume control, a headphone jack socket, and master volume output


A really excellent handbook comes with this unit which not only tells you how everything works, but also gives you complete details of the numerous ways in which you can connect and use this remarkably well equipped pre-amp. Each control has its own section, as do all of the output and input options, and no owner will be left in any doubt about how to suit this pre-amp to the rest of their bass gear.

Space limitations being what they are, there seems little point in explaining the obvious functions on this pre-amp, but I'll take a few lines here just to detail the more obscure features, so that you can see what does what.

On the front panel the inputs, gain control, limiter, bass and treble pots are quite straightforward, but the 'Aural Enhancer' and the graphic Eq. do warrant some explanation. The Enhancer circuit is intended to do much the same job to bass guitar sounds as the Aphex studio unit does for other sounds on recording. It aims to emphasise the fundamental tonal qualities from your instrument, putting back some of the losses which are inevitable in even the best amplification systems, thus giving you a sound closer to what you'd get if you were recording you bass in a good quality studio. According to the makers, this circuit gives a sound which is 'more live and transparent. It will be especially noticeable with the slap style of bass playing'.

The graphic equalisation system comprises eight controls with a slider for volume (with a ±15dB range) and a rotary frequency select pot positioned beneath each of the four band sections. A very useful chart in the GT's handbook compares the relationship between the frequencies available on the pre-amp and the actual notes on your bass. It goes on to explain some fascinating experiments you can undertake, which allow you to play around with the fundamental frequencies of your strings, as well as the overtones and harmonics of your individual strings. 40Hz being the fundamental frequency of a bass guitar's open 'E', the controls can be manipulated to enhance the first overtone (80 Hz) as well as the second harmonic, at 160 Hz. Using the potential to adjust these frequency characteristics can let you 'tune' your sound to extract the maximum effect from your speaker enclosure (they all differ, and often quite markedly), counteract 'dead spots' (thank you Leo!), as well as making for some extremely precise tone controlling.

Features on the back panel which may require some explanation include the 'Balanced Output' and the 'Active Electronic Crossover'. The former is a true balanced XLR output at 600 Ohms with a two-position switch which allows it to be set to either 'direct' or 'line' feed. As a 'direct' output it functions as a 'direct box', and can feed either PA or recording mixers. In this role the front panel controls have no effect on the output signal. The opposite is true when this function is set to 'line' (ie all the controls work), whence it can feed direct into a tape recorder or studio mixer as well as a normal power amp fitted with balanced inputs. The 'Active Electronic Crossover' is fairly self-explanatory, in that it feeds out in accordance with whatever settings you've made on the front panel controls.

Specifications quoted by the manufacturers (for the technically minded) are as follows. Total Harmonic Distortion is quoted as less than 0.05%, transient distortion is claimed to be 'negligible', noise levels are rated at 'less than 2 microvolts, unweighted', the variable graphic operates at ±15dB on 40Hz-160Hz (Band 1), 80Hz-320Hz (Band 2), 200Hz-800Hz (Band 3) and 400Hz-1600Hz (Band 4).


It would have been tempting to try the GT Electronics Tube pre-amp with an active custom bass - after all that's the sort of instrument which anyone who can afford one of these is likely to be using. But what I really wanted to do was hear how it sounded 'au naturel', so I opted instead for my Tokai 'Jazz' bass - a good, simple instrument but not one endowed with bags of active Eq. What, I wanted to discover, could the STP-B do with it? The handbook is very explicit on how to set the STP at first. Fortunately, it's extremely simple, which is always a plus point with any equipment destined to be used on stage. Having decided which output option to use, you simply plug in to either the low or high gain outputs, then set the first gain control so that the red LED indicator lights on the highest powered notes. Needing to hold my power back, I also opted for the Limiter switched in - again a simple matter of rotating the appropriate pot so that the red LED for that function illuminates on peaks.

To be strictly logical about it, I ought really to get onto the 'Aural Enhancer' function next, as this follows on from the gain and limiter controls, but as it's a bit mystical I'll pass first to the Eq stages and make a mental note to get back to the A.E. in a while.

The two basic tone controls - just marked bass and treble - have a pretty massive effect. Technically speaking, they swing the tone around by ±15dB each, on fixed frequency bands. Nothing so special about that, of course; except that they do work very effectively.

The key to setting your sound comes from the carefully thought-out individual graphic Eq. section. Here you find four rotary controls with four sliders set above each of them. The rotaries govern frequency centres and the sliders decide how much cut or boost you give each frequency. The handbook suggests that when you first try the circuit, you set the tone and level controls to their flat (centre detented) positions and set the frequency controls fully anti-clockwise. This gives you zero cut or boost on the basic frequency ranges of 40Hz, 80Hz, 200Hz and 400Hz. From there you slide the level control up to maximum (ie 15dB gain) and next play an open 'E' (the fundamental of the string being 40Hz). The effect is very marked - the whole tonal character of the sound of an open E changes as you emphasise the first overtone and then the second harmonic, the former falling at 80Hz, the latter at around 160Hz. Again, this isn't a revolutionary idea, but it does show anyone who's never really followed what a good quality graphic can do how to work with one. Used with care and a bit of experience, you can certainly counteract 'dead' or 'flat' spots (are you Precision owners listening?). You can also negate the influence of hollow stage floors, feedback and some inherent weaknesses in the 'tuning' of speaker enclosures. To be truthful, that doesn't make the STP's graphic so very different from quite a few others, but, once again, it works remarkably smoothly, with no circuit induced noises. Combined with the use of the two tone controls, you should certainly be able to get a mammoth amount of tonal variety out of any bass with this system. The key to its effectiveness really lies in the choice of the frequency bands; 40-160Hz, 80-320 Hz, 200-800Hz and 400-1600Hz. They're perfect for bass and give impressively accurate control over your sound.

Having thus set myself up with the Tokai it was time to try and assess whether or not the use of the Groove Tubes 7025 valve made any noticeable difference over what you might expect from a similar (and probably considerably less expensive) tranny pre-amp. Speaking purely personally, my own tastes have always definitely inclined me towards valve amps for guitar work. I wouldn't knock the best solid state types (Sessions, Pro-Amps and the like) but; given the choice, Td always opt for a valve guitar amp, for any number of reasons. Until about ten years ago I might well have said the same for bass - in fact I stuck religiously to an old Vox 250 watt KT88 powered bass head for years in preference to tranny amps. In the end, though, the superb tonal flexibility, quietness of operation, available power and onboard effects changed my mind conclusively in favour of solid state bass.

One of the biggest problems with valve bass heads was always noise. The first tranny bass amps were hissy, but they got better and better until many a valve bass head sounded like an old steam train by comparison. Almost supernaturally, however, the GT pre-amp is astonishingly quiet. Even at high power it doesn't hiss, buzz, crackle - yes it deserves its 'studio' tag. What's more, the traditional valve bass amp has very little tonal control. Forget that! The STP-B has every bit of the tone control that a modern transistorised bass amp has - more so than in many cases - and delivers it without unwanted noises.

Being a pre-amp, of course, the problem of obtaining enough power from valves disappears. You can link the GT straight into a tranny power amp (two, if you are bi-amping your rig) and let the trannies do what they do best - provide clean muscle to simply enhance your output power. A good quality power amp won't alter the essentially 'warm' sound from the pre-amp; all it'll do is raise the output from the pre-amp, keeping the same valve effect sound. It's an ideal solution to the problem of keeping a valve-driven warmth and touch-responsiveness whilst not relying on huge transformers and 'bottles' from which to gain your power. So, does it work? My answer (and this is surely going to be a very personal thing) is yes - and how! What you get is that intangible effect wherein the harder you hit the string, the more the amp seems to deliver the fast-rising transient in your attack, yet it does so without harshness or dryness. It's an effect which 'slap' players will definitely appreciate - especially if they've got the limiter set so that the pulled notes don't induce unwanted overload distortion. You can also, by virtue of the limiter's exceptionally smooth functioning effect, keep your fundamental pre-gain drive high enough to get a full sound. The same effect works on bass chords too. Here you can whack-out a driving chord and get the corresponding 'lift' in your sound - the limiter holding back overloads, the tube giving a touch-sensitivity which no solid state amp I can think of can equal.

Of course, on bass (unless you're after an early Jack Bruce/Cream sound!) you're not likely to be looking to get valve distortion from this amp. You could have it if you wanted (after all, you've got a pre and post gain), but what you're really after here are the other (less widely acknowledged) qualities of valves - a warmth which is hard to put into words, a transient attack which is equally intangible, and an ability to emphasise the even order harmonics from a note, leading to a sweetness and feeling of 'space' in a note which seems all but impossible to get from transistors. And this pre-amp delivers the lot! Yes, it's all there, from a tonal versatility which would do justice to a solid state unit to the qualities which still lead many bassists towards Hi-Watts, Marshalls, Ampegs and the few other remaining valve bass amps. What's more, it's packaged in a form which lets you amplify that pre-amp sound any way you want, with a sackful of output options, enough to satisfy even the most versatile pro player.

Ah yes, mental note time! I promised earlier that I'd go into the 'Aural Enhancer' control - and I'll attempt to do that now. It certainly does seem to do something unusual. The effect becomes increasingly noticeable as your volume is raised, and somehow it seems to add an extra vibrancy, 'ballsiness', a richness or a 'punch' to your sound. I wouldn't like to guess how it works, but I know that you'll hear it too if you try one of these pre-amps, so you'll have to pick your own way of describing the effect, won't you! When the original Aphex appeared quite a few engineers claimed that it was a gimmick and little more. Don't make that mistake with this control, because (hard though it may be to describe its influence in words) it really does make a useful sound difference.


Well, there's no getting away from the high price of this pre-amp. It's certainly not a unit which very many bass players can afford; probably only top professionals being able to justify one. Nonetheless, it does have similar qualities to those which make so many guitar players pay countless sums for Mesa Boogies and their ilk. For myself I've never managed to get anything like the sound out of my Tokai that I did with this product (and so easily, too). Pro bassists who feel that the/ve lost something since they switched to all-transistorised amplification should certainly take their courage (not to mention their wallets) in both hands and try one of these new pre-amps, which are just becoming available in a handful of specialist dealers throughout the UK.

I'm not going to even try and assess the GT Electronics pre-amp on a value for money basis - how can you? An Aston-Martin may well be the best car made today, but is it value for money? The point is that if you can afford the best and you're good enough to appreciate it, then you'll have one, and damn the cost! Much the same considerations apply with this pre-amp. At just over £600 it's one of the most exciting, best-sounding pieces of bass gear that I've ever tried. Not every player (even among those who could afford one) will agree that the gain in sound from one of these is worth the price. Others might even argue that there isn't a gain at all. But for those of us who like the valve sound, it's a hugely impressive and dangerously tempting piece of gear. For the bassist with money and a hankering for a warm, natural sound, it defies comparison.

RRP £612

More details from Scott Cooper Marketing Ltd., (Contact Details).

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In Tune - Copyright: Moving Music Ltd.


In Tune - Jun 1985

Donated by: Gordon Reid

Gear in this article:

Preamp > Groove Tubes > Studio Tube Bass Pre-Amp

Review by Gary Cooper

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