It's always very tense before a show starts -- for the performers, doubts about another road-audience, for the crowd, the tightness of anticipation. Preparations continue in the last minutes on amps and mikes and lights. In a blue-carpeted backroom deep within the Boston Tea Party, the Jeff Beck Group kicks around a squishy soccer ball.

Out in the dark dance hall -- too packed, obviously, for dancing -- the younger boys and girls are remarkably adept with each other. They must learn at high school to flit around casually and effortlessly.

The soccer ball bounces harmlessly off a streaking red tuned-up guitar. "Nahh. We never eat a big meal before we go on. If you're stuffed you get too self-satisfied on stage and then you can't play very well, can you? You have to be ugly inside to make it all come out." Said Jeff Beck.

The music is thundering and the light show irrelevant. Jeff Beck a streaming presence jerking, in sweat, on his guitar laying sheet after sheet of sound authoritatively down. The great Nicky Hopkins in one corner of the stage hunched over piano holding together the music with his discreet and rippling underpinning. Mick Waller on drums, his eyes fixed on Beck, face contorting, eyes glazed, his arms chopping, producing a sharp and clear rattling as each drumbeat rams more or less neatly into Beck's flying notes. Rod Stewart, the singer, a man of flinty beauty, and a fertile smile, has a tingling roughness in his voice.

THE ELECTRIC lead guitar is to rock-blues music what the violin is to classical--the supreme voice of the medium. Jeff Beck understands very well that a man who masters guitar has tapped an enormous source of energy. Having done so, he relishes in his freedom and glory much as a racing driver thrills to the knowledge that he is in control of a powerful machine, one that can destroy him if it goes out of control.

The Jeff Beck Group has a number called "Beck's Boogie" that the group does early in each show. The format is simple: Beck plays the melody of the line "Mary had a little lamb" solo, and then the rest of the players crash in with a few bars of straight percussion. Silence again, and Beck repeats the phrase. Each time, however, he does it a little differently and uses different elements of the guitar. Once he plays it entirely on the frets, once using wah-wah pedal, once slapping the strings with his open palm. The improvisation gets more and more complex and yet the melody is still recognizable, though made up of different timbres and tones each time around.

Every time there is a breahtless moment as Beck deliberately lengthens the melody, playing furiously, leaving the audience overhanging, teetering until the rest of the group climaxes with a rumbling flourish.

Quite apart from the fact that this number is artfully constructed for its dramatic moments, "Beck's Boogie" serves to establish in its astounding variety the basic premise: I, Jeff Beck, have such resources at my command.

In addition, Beck has learnt his blucs from B.B. King and like the great master he loves to accumulate tension until a song builds up to such a pressure as to appear to burst apart from within.

This is not to say however that Jeff Beck, for all his domineering guitar activity, would be anything without the consistent and helpful backing that he gets from the rest of his group. Leaving aside Nicky Hopkins for the moment, it is clear that Waller, Stewart, and Ron Wood, who plays bass, are above average as performers in their own right, and superb in combination with Jeff Beck.

There is much improvisation, and this would not be conceivable were the others not equally musicaly aware. At any session any one of them is free to give the others a progression to pick up and explore. More often than not, however, Beck provides the ideas. It is the nature of the electric guitar to be the lead stimulus.

One of the more memorbale instances of the group's easy relationship in performance came during an instrumental number called "Mother's Rice Pudding." Ron Wood, who says that he usually sticks to the "basic bass progressions on the blues stuff," suddenly opened up and played a bristling rock-bass solo. This has since been included in the song, and the number which started as a jam one night in San Francisco continues to grow.

Sometimes, however, the improvisation can be precarious. Beck has admitted that it is quite possible for there to be two different basic progressions going at the same time, but he and Hopkins both agree that their predominant guideline is whether something "sounds right at the moment. We don't worry about its purity or anything."

Nevertheless such frenzied and soaring musical improvisation -- in the presence of an insistent and mercurial guitarist like Jeff Beck -- cries out for a disciplining influence. And so Nicky Hopkins, ex-studio musician.

In retrospect, it is clear that last summer when the Jeff Beck Group took the country by storm, the group was basically unformed and quite rough at the edges, although always vital and exciting. With the addition of Nicky Hopkins the group has entered a period of maturity and has taken on a new dimension. If Jeff Beck did not have Hopkins, he would have had to invent him.