Two guitarists of consummate skill and daunting backgrounds have come together to share a tour. They are John Etheridge and Gary Boyle. A writer of unremarkable intelligence questions their purpose. He is Paul Colbert.

What exactly is jazz and how do you catch it? Is it a germ borne on the air? Victims are always talking about 'blowing' so could it be sprayed from the bells of uninoculated saxophones?

In the search for a possible cure, or at least a deeper understanding, we spoke to two carriers of the virus who even now are in the middle of a tour of this country. Their jazz is about as infectious as you can get, thus for the sake of hygiene and privacy, we promised not to reveal their names.

So, John Etheridge and Gary Boyle, what do the doctors tell us?

"Jazz, if it's anything to do with anything," says John, "means people can get together without saying, 'Oh God, what are we going to do, we'd better write something quick'. It's being able to get in the car, ask 'What shall we play' and say, 'Well, we'll just wait till we get there'."

Come on, there must be more to it than that. Any rock band could arrive at a gig without having decided what to play. Problem is, when they got on stage, they'd then sound as if they hadn't decided what to play. Somehow jazz players manage to pull it off (or at least look as if they are) and that's what puzzles us rock types... do you learn to do jazz, or do you contract it at birth?

"It's temperament," replies the long-scale Etheridge stretching two snakelike arms across the table. "I have terrific problems playing things completely as written. I'll never be a really good session player because to play a melody exactly as it's written... I find it irritating. It's a failing. I'm a jazz player by temperament rather than from liking any particular style."

At the far end of the arms and the table Gary Boyle nodded. It was a point they'd already agreed on. Neither of them had come to jazz because of be-bop, or trad, or any of the other boxes of appreciation. Nor was it the ancient definition: 'you're a jazz musician if you sit down to play'.

"It was a weird process of learning for me," confessed Gary. "I started on the guitar when I was at school and my parents couldn't afford lessons so I had to wait until I started work. It was weird because I was a pro for about six years before going to music school. I was studying with Peter Ind, the bass player, then I went on tour with Brian Auger and when I came back I rang up for another lesson and found Peter had got a job with Leeds College of Music. So I gave Brian about three and a half minutes notice, which was terrible, and went to college for two years."

Gary claimed he left mentally capable of playing all the session material placed in front of him, but not keen on sticking to its styles.

"It works the other way round," Etheridge interjects. "Session players can't go out and blow. Even Lee Ritenour... it never sounds quite right. There always seems to be a red light on." Eth lifts his head from the table to mime Ritenour's search for the studio recording lamp. "Session players never go 'wahahwhwaaa'. They're too careful because of their training."

Caution and habit are complications that can set in when jazz has been caught, and good players spend time avoiding them. Most dangerous is the affliction of mind and fingers slipping into the same routine, difficult to diagnose in its early stages. Both John and Gary conclude those habits are almost impossible to spot when you're actually playing, backed by the band and if you're lucky, an enthusiastic audience.

The time to catch them is when you listen back to a recording. "You listen to a live tape and all the solos seem to last forever and you hear the same bits again and again." Gary's grin of embarrassment grows wider. "And you think 'I'll probably finish here', and you don't, you go on for another chorus, and another, and you think... 'God, did I subject people to this last night'."

Top flight jazz players who deny copping an ear to anyone else are usually pulling plonkers. Such is the popular vote around the table. "Everybody listens to everybody," quotes Eth, "and Bowie's a perfect example of someone who absorbs things. You don't have to worry too much about putting your own character on it because unless you're a real sponge, it will always come out sounding like you."

The mention of character and identity instantly shifts the conversation to a guitarist much admired by Boyle and Etheridge, one Robin Ford...

Boyle: "A fantastic sound and so emotive..."

Etheridge: "335 and a Mesa Boogie..."

Boyle: "Yeah, but it doesn't have that sweetness of Carlton. It's gutsier, a hundred times better, street level playing, rare for the West Coast."

Etheridge: "It shows that he's not part of that high-tech West Coast session system.

"Robin Ford has a great touch though... his notes are... em... quite ordinary. If you wrote down a Robin Ford solo it wouldn't look great compared with one by, say, Larry Coryell. He has a phenomenal mind, but there's always a problem with his electric sound.

"In rock guitar playing the 'brain' is on the fretboard and in the pickups. With jazz it's played in the mind. Sometimes you listen to jazz players and think, how can they play with such a bad sound. But the thing is, it's all going on in their heads. Coryell is like that — incredible improvisational ideas spilling out, but not always well articulated. With rock it's out there (stretches fingers) instead of in here (grabs bonce)."

Given that rock players may interact with their sound more than jazz guitarists, where does that leave the 'temperamentalists' in their search for the right tone and the appropriate equipment?

"I've got quite a lot of Yamaha gear," announced Etheridge to wry smiles around the table. He is after all a Yamaha endorsee and one of their occasional demonstrators at various music fairs. "I have a love/hate relationship with my sound — sometimes I want a clean guitar sound when it suits my mood and sometimes I want aggression... it's the difference between thought and emotion and I've never managed to synthesise the two. All my life I've been like that, changing my mind in mid gig."

To assist in this schizophrenic task he has a Yamaha SA2000 (double cutaway, semi-acoustic job) and one of the few Yamaha combos fitted with a 15in speaker (a G100 — 1 x 15mk2). "It's great and it has a lot to do with that speaker. Yamaha tops are very clean. Most amps with 15in speakers are too muddy but these go well together. Sometimes I use a Polytone but I'm not sure about that, it seems to be getting a bit jaded. In the right room with the right guitar the Polytone can produce a very nice, dry sound. And with the closed back cab, the notes go 'fmp,fmp,fmp' (waves hands to stress percussiveness). The 15in gives a smoother, more legato sound."

There are pedals to match including chorus, flanger and a "mid-seventies phaser, plug that in and you go straight back to jazz rock... oh... nostalgia, young again".

The current Boyle box is a copy of his cherished Guild, made by a young guitar builder called Steve Pickard who approached Gary at a guitar demonstration. "His work looked really nice, but I didn't know what he could do for me. I thought he might make a good backup for the Guild, but to cut a long story short, he did such a good job setting it up and everything, I didn't want it out of my hands." Steve Pickard wound the original pickups himself, but sweet as they were, they were too low powered for Gary's requirements so he took one of the humbuckers from the Guild and splashed out £40 on a Seymour Duncan PAF for the other position. The Pickard creation also has a brass nut, the first Boyle guitar to boast one. Brighter, aren't they, says the owner, which spins Etheridge off on another direction concerning the Kahler trem fitted to his SA2000. It's stainless steel.

"The original one was brass, but this seems to suit the Yamaha much better. It's much cleaner and it rings a bit. I like the feel of the Kahler, it's very smooth and stays pretty much in tune... like all these things it likes to be warmed up first."

Etheridge then proceeds to tell the story of how the first Yamaha semi-acoustics, such as the 2000, came about. He claims there were no suitable, western guinea pigs at the early design stages, so the Japanese creators estimated the proportions along the lines of 'Europeans haff big hands, soh we build big necks'. Hence the fullness of the 2000 compared to later, scaled down versions such as the 1800, closer, he says, to 1958 335s.

On a recent tour Gary Boyle had found himself growing steadily disenchanted with his amplified tone. The climax came when he was making an album with a guitarist/producer. 'I know what you're talking about', came the voice from behind the console, 'You'll 'ave to go stereo'.

"I was pissed off," remembers Gary, "every guitar player in Europe is in stereo, but it works. So now I've got a Marshall 50 and a Peavey Bandit. The Peavey is nice. Normally I don't like transistor amps, but it's a good balance with the Marshall."

Ah, that old valve/transistor chariot race. Now everybody's got digital/analogue to argue over instead, maybe they'll settle out of court. "I've had enough of valve amps, really," opines Eth. "The way I play, you get a much smoother response with transistors. So many valve amps get terribly broken up."

What about Mesa Boogies, the guitarists chorus, almost in time.

Boyle: "I've had a few tries but everyone says you've got to have one and play it for ages..."

Etheridge: "...and then you find it's fantastic. I know several people who swear by them, but they are ridiculously heavy and ridiculously expensive, and I object to paying that amount of money to an American company.

"I suppose Pat Metheny opened up the idea of using effects on a jazz guitar sound without destroying it. What's attractive about his sound is that the centre of the note is there and then there's a halo around it. The overdriven rock note doesn't really have a centre, it's more fluid, the note has length instead.

"For overdrive I've got these two fuzz boxes — one's a valve fuzz box, with a valve inside it, by Guyatone. It's great but it's really messy. Then there's the Boss Super Overdrive which is really clinical. I got that because I saw Jeff Beck using it."

Everybody listens to everybody, see what they mean? How can there be a cure when musicians want to contract this ailment. "Please excuse Gnasher from the Axemaster's rehearsal tonight. He has jazz."