Don't know which valves to fit in your amp, or where to get them? Worried by the rumours about duff samples? Don't understand what all the 'valves sound best' fuss is about anyway? Aspen Pitman - originator of the 'Groove Tubes'system - explains the subject to Gary Cooper.

Good though the overdrive sound of transistorised amplifiers has become (and some of them are very impressive now-days), probably the overwhelming majority of pro guitarists still favour the sound and 'feel' of valve amps.

And yet, as non-valve 'chip' technology races onward, sources of new valves are drying-up, there are quality control problems with some of what is available, and a scarcity of replacements for several older amps. Enter Aspen Pitman, Californian creator of the Groove Tubes 'matched' replacement valve system.

'Groove Tubes' have become a cult in the U.S.A., with a list of endorsees including legions of 'name' players. Now that Groove Tubes are at last available in Britain, it seemed like a sensible time for me to corner Aspen during a recent U.K. visit and ask him what all the fuss was about.

But first a simple (and I do mean simple!) interpretation of the Groove Tubes philosophy. Basically, Groove Tubes offer replacement valves ('tubes', as our American cousins would have it) for many valve instrument amps, old and new. Output (power amp stage) valves are offered 'matched' in multiples of 2's, 4's or 6's so that they are, electronically speaking, operating identically, and are numerically graded, indicating how easily (or how early) each grade will distort. Groove Tubes also offer alternative types to manufacturers' recommended replacements in some cases, with descriptions of how these will alter your overload sound. Solid state rectifiers are on hand to replace certain valves, and there are 'high quality' pre-amp valves available too. The catch? Well, they're not cheap compared with ordinary replacement valves, which could lead to accusations of a price hype. "O.K., Aspen," I opened with when we met, "what do you say if you're accused of over-pricing your products?"

"I hear the same basic objections here, and in other parts of Europe, as I did when I started up in the States - that Groove Tubes are three times the money, and so on. But they last three times as long and you only have to listen to the sound to see where the price comes from.

"From the day I started I've always said to players, 'go and try them, see what you think. Prove it for yourselves.' Now I've got at least 750 shops selling them in the States, and we're in Scandanavia, Germany and a lot of other European countries."

Working on the assumption that Aspen couldn't be fooling so many players and getting away with it, it looks as if there is indeed something worth investigating in this re-valving approach; certainly so if sales figures mean anything. Accordingly, my next question was to ask him to explain the way in which Groove Tubes grade the individual valves they offer, and what the different grades mean.

"Really, the best way of explaining the Groove Tubes idea would be to go back and tell how it all came about.

"Originally, I found that there seemed to be too high a failure rate among valve amps, along with all sorts of stories like, 'this is a good Fender Showman, and this one isn't'. I became aware that in ninety-nine per cent of cases any problems were due to the valves themselves.

"What usually happens when a player has an amp failure is that he takes his amp along to a repairman who just changes the valves over. That's crazy, because a valve is a user-serviceable part and what he's really paying for is mostly the repairman's labour."

Fair enough, and (providing you unplug your amp from the mains first, and are safety conscious - especially with regard to capacitors, which store charge and can give a possibly lethal shock even when the amp is disconnected!) valves certainly are 'user-serviceable'. So, back to the 'matching' system.

"There's been tube matching available for years, but what people have traditionally been measuring was was just emission quality, or output, and the difference between any two tubes on that score is very small - on a pair of 6L6's, for example, just 5-10 per cent at the most. The truth is, if you've just got that sort of order of output power difference between two tubes then - and I don't care who you are - the human ear just isn't sensitive enough to detect the difference.

"Anyway, musicians don't use tubes to produce power. If all we wanted was power we'd use transistor amps every time. We use tube amps because they're tone generators in themselves.

"We found that each type of tube has its own different kind of distortion, so what we began to do was measure this characteristic. On this premise we then began to analyse distortion relative to amp sensitivity. We'd put a tube in an amp and note the point at which it began to noticeably distort. Now, while you can't hear 5 per cent of power output difference, you certainly can hear 5 per cent distortion. Say we're talking again about 6L6's - you might, for argument's sake, have one sample which goes into 5 per cent distortion at 5 watts, and another not until it delivers 15 watts. We put the tubes in a numerical order, from the earliest clipping to the latest clipping tubes. And from this we can now make duets, quartets or sextets of identically clipping tubes.

"The point of that is because all instrument amps work on a class A/B circuit principle, a push-pull circuit. To put that simply, if you know what a sine wave is, then the tube on the left is making the top half of that wave, and the tube on the right handles the bottom half. As a guitar signal, say, goes through the amp from 0 to, perhaps, 60-70 per cent power, you can have one tube clipping and not the other if your valves aren't matched. The effect of this is to create a different overtone structure between the top and bottom halves of a wave, and if they sound different enough, they phase-cancel. Now, phase cancellation has got to be the biggest 'no-no' in music - in principle it means that some notes will ring and others won't; hence you can get one amp sounding great and another, apparently identical one, awful, and it depends on the tubes."

According to Aspen, using matched valves will eliminate this effect. Are there any other advantages he'd lay claim to for the system?

"The other thing you get," he answers, "is increased reliability. That has a lot to do with the biasing current requirements. In a way, biasing is a bit like adjusting the idling of a car engine. Most valve amps that have proved themselves over the years (like Marshall and Fender) have adjustable bias controls, and these should be adjusted every time you change your tubes. With Groove Tubes, however, because of our grading system, once you've set your amp up for one number tube you don't have to re-bias your amp when you next replace them."

The question of the reliability of valve amps generally is an interesting one. Speaking as someone who has used quite a number of them down the years, I've never found them (generally) anything less than perfectly behaved, and that goes for amps from quite a variety of makers - when they have failed me, then a swift revalving has nearly always put things to rights. Transistor amps, on the other hand, when they go phut, tend to really mean it! Was that Aspen's view?

"Yes, certainly. In fact there's a rule I have - if there's something wrong with a valve amp, if that happens, one - change the tubes. Two, if that doesn't work, change the tubes, and three, if that doesn't work, change the tubes!"

On the other hand, there's not much point in changing tubes if what you put in their places are poorly made examples from dubious manufacturing sources. On that point, many of the traditional sources of supply of good quality European and American valves have dried-up in recent years. Names to have gone include Mullard, whose once universal EL34 hasn't been in production for some years.

How did Aspen obtain valves offering the sort of quality he claims to be able to supply, given this drying-up of sources, I enquired?

"Yes, it's certainly true that tubes have gotten worse. I buy from the same people that everyone else uses, I admit that, but the difference lies in the way we match individual samples. We use a lot of computer switching gear in testing our tubes, and we can now, for example, warm-up eight tubes at a time and test them within 70 seconds. This also, of course, tells us whether a tube is going to suffer from thermal shock problems. Overall, I guess we reject some 5-6 per cent of power tubes as unsuitable."

"The problem with production quality is that the tube manufacturers are using tooling that hasn't been replaced in years, because there's just no incentive for them to do so. A lot of companies have shut down because it's a tremendously expensive operation, making tubes, and the demand from the electrical industry just isn't there to warrant it.

"Power tubes aren't a problem to us, we can live with the scrap rate there, but preamp tubes are a problem. If a pre-amp tube is made loose, well, it's made loose and that's that. It's like using a bad microphone through a P.A., and we have to reject some 70 per cent of pre-amp tubes because of failures of one kind or another.

"You have to remember that 99 per cent of valves today are made to go into products which are nothing at all to do with audio; they're used in radio gear and the needs of the music industry aren't taken into account."

So will supplies eventually dry up, I asked?

"What will keep us going is that most of the world still uses tube equipment for its communications - the most important job there is, after all. Hence you have the old Mullard and Telefunken production lines having been bought-up and re-established (even re-tooled) in Eastern Europe. If you look at Russia, Africa, South America and Asia, that's where the tube is being used today - most of the rest of the world can't afford the microwave gear which we in the West use, and tubes are still being produced to cope with that need.

"If you take the Telefunken 12AX7 plant as an example, they shut that, but sold the tooling to the East Germans and the Hungarians. Seven out of ten valves from there are faulty from our point of view, maybe more; they're microphonic and so on. That doesn't matter in the applications for which they're being produced, so testing the valves before they're fitted into instrument amps becomes more important than ever, to make sure that they'll suit.

"Today there are only two valve makers left in the States - RCA has been out of business for for ten years, for example. Sure, they sell valves, but they could be made anywhere.

"However, once you do get a good preamp tube it can last for up to fifteen changes of power amp tubes. It has a very low voltage drain on it and doesn't get hot (unless it's used in an amp like a Boogie), and a properly selected pre-amp tube will last a long while. What really do need changing are power tubes and phase inverter types - most musicians don't seem to realise that."

One of the personal reasons which makes me tend to favour valve amps for guitar use is the feeling you get, when using one with a decent guitar, that somehow the amp is far more 'touch sensitive' to your picking and left-hand style than a comparable solid state amp. How does Aspen account for that? Has he noticed it too?

"Sure! As well as the tonal advantages of using valves is this whole thing of dynamic content, which makes tube amps more 'playable'. It's that transfer of acoustical touch or sensitivity, as much as the sound, which is analogue - it's like on a Steinway piano or a Martin acoustic guitar - it's properly balanced dynamically; you can feel it! It's something which you see so often on stage with a top player. He'll have his Marshall fixed to one set of tone positions every gig - no-one can change that setting! If he wants a change of tone, he'll hit the strings differently and the amp will respond.

"I find that with a solid state amp you tend to find yourself fooling with the tone controls all night trying to get the exact sound you want, because you can't vary it with your playing style alone."

Instrument retailers selling the Groove Tubes range have data sheets telling them not only which valves are suited to which amps, but also which valves will, Groove Tubes say, alter the sounds of your amp. The matched power valves will, to a greater or lesser extent depending on their numerical rating, distort-at lower or higher overall volume levels. But it goes further than that. You can even change the valve types in some cases to adjust your amp's sound (although you'd have to be careful with regard to your manufacturer's warranty in this case, I'd suggest).

Aspen's highly useful booklet, The Tube Amp Reference Guide, contains sections on virtually all of the valves found in amps, ancient and modern (unfortunately, originating from the U.S.A., it misses such museum-pieces as the dear old U.K.-built Selmers). The booklet does say that Groove Tubes don't recommend fitting different valves in place of the maker's specified types - but it goes on to detail some alternatives to standard valves. Whether you should try them, of course, is a very debatable point, and no doubt many manufacturers wouldn't like you tinkering around with their basic specifications. Where the booklet is particularly helpful, however, is in deciding which valves go in which amps. Some long-established makers change supplies over the years, and you're shown how to identify which valve suits which vintage amp.

Moreover, the booklet goes on to detail how to change your valves safely, ways of prolonging their lives, details on biasing for new replacement types, how valves work and what they do - in short, if s a fine primer on this massively confusing subject.

Whether all the claims made for Groove Tubes (better, more predictable sounds, longer life and so on) are justified isn't something which I can personally vouch for, never having used any of Aspen's 'super bottles' - although IN TUNE hopes to be testing some in the future. Either way, a great deal of what Aspen says makes complete electronic and musical sense, and the success he's achieved implies that it certainly can't all be hype, by any manner of means.

My own guess is that a set of Groove Tubes would certainly be worth a try. If you do, (while we arrange our own tests), why not write and let us know how your experiences went?

More details on Groove Tubes and your local supplier's name and address can be had from British distributor Mike Cooper of Scott-Cooper Marketing (no, he's not a relation of mine!) at (Contact Details).