Because of this, a correspondingly large voltage now appears at the plate. A portion of the amp's electronic circuitry, the grid bias control, adjusts the proper voltage setting of the grid. When the grid bias is properly set, the tube is balanced to the circuit, and therefore produces a clean, powerful signal. The plate is connected to an output transformer, which matches the impedance to that of the speaker.

What is a tube? A tube is an electronic device consisting of a minimum of four active elements: a heater (filament), a cathode, a grid and a plate. All sealed in a vacuum glass enclosure to prevent parts from burning. Once heated, the cathode begins to emit electrons, which flow from the cathode (which is negatively charged) toward the plate (which is positively charged). The grid's purpose is to control this flow, in effect, acting as a valve.

How do Tubes Work? When the guitar's pickup produces a small voltage (the result of the string vibrating in the pickup's magnetic field), this signal is applied to the grid, which causes a large current flow from the cathode to the plate.

How do Tubes Distort? As the signal emitting from the plate approaches its maximum potential, the tube gradually begins to react less and less to the original input signal. This results in a type of compression of the signal, and the signal becomes cut off or "clipped." Tube distortion ("clipping") occurs gradually, producing low order distortion which compliments the original signal, creating a warm sound. This is also why it's easy to move between clean and distorted tones.

Analyzing and Selecting tubes.

We literally go through thousands of tubes, rejecting more tubes than we accept. We put every tube through a rigorous performance test, one at a time, in real amp circuits. The preamp tubes are rejected for adverse microphonics, low output or high noise. Power tubes are more involved, and they go through additional analysis. Here's what we're looking for:

1. Grid Leakage. Causes tube to burn out quickly, and worse yet, to short out and wreak lots of havoc. We throw these tubes out.
2. Weak Vacuum. Tube will act deceptively normal, then just burn out on you (usually at the worst time!). These tubes don't make it past the first round.
3. Gain to Distortion Ratio. The center of all Groove Tube magic! This measurement tells us if the tube will distort early or later in its power range. Since the tube distortion is rich in harmonics (That's why we love 'em!), the tonality of each tube will vary according to this ratio. Only Groove Tubes sets have been measured and matched to insure longer sustain, wider frequency response and a more musical amplifier!

Rating and Matching Tubes.

We translate this gain to distortion ratio into a performance scale that can range up to 100 points, then match up the tubes to within 1 point of each other. To simplify things, we reduce the scale to 1-10 before the tubes leave our shop. All tubes are exactly matched into duets, quartets, sextets and even octets.

More than just Great Tone.

The Groove Tubes Process produces many welcomed improvements:

1. Dramatically Improved Sustain by eliminating phase cancellation (which occurs when dissimilar tones coming from unmatched tubes are combined and cancel out certain frequencies.)
2. Improved Harmonic Balance- All the notes you play will have the same amplitude. (No more dead spots or dull notes!)
3. Longer Life Span by 1st) eliminating faulty tubes. 2nd) Our balanced sets reduce stress that occurs when a stronger tube is working too hard to make up for its weaker partner, and 3rd) because our sets drive the output transformer evenly it actually runs cooler!

Why do Tube Amps all Sound Different?

1. Types and quality of tubes vary, some tubes amplify more than others.
2. The amount of gain a tube produces varies with the each amp's circuit design, some tubes amplify more than others under similar conditions.

Why Should You Replace Your Tubes?

Glass, metal, lots of little parts... tubes are subject to mechanical problems and are not meant to last a lifetime. Actually, the harder you work your tubes, the faster they wear out. Which means that from time to time you will need to replace your tubes. More reasons to replace your tubes:

1. When one starts to go, it drags the others down with it, decreasing the over all efficiency of the amp.
2. The better the tube, the better the tone. Simply put, the best reason to replace your tubes with Groove Tubes is that they'll make your amp sound better.

The 4 amp stages:

1. Preamp Stage - Amplifies an incoming guitar or mic signal for tone shaping.
2. Signal Processing Stage -This is where you add the effects, reverb, tone controls, etc.
3. Power Amp Stage -The real workers in your amp. Most amps over 10 watts use an alternating push/pull power amp design- when one tube is on, the other is off. Your amp may have two, four, six or possibly eight power tubes.
4. Rectifier Stage -Converts AC wall electricity to the DC electricity inside your amp. Most modern amps have sold state rectifiers.

Bottom Line:

Pay attention to your amp's performance. When you hear the sound begin to deteriorate, it's time to change your tubes. Your amp will last longer. Your tone will always be at its best. Think about it, you don't wait until the strings break before you change them, do you? Look for these signs that Your Tubes May be Deteriorating:

1. Loss of highs or lows, Muddy chords
2. Poor balance in the output of various notes
3. Lacks punch, Makes funny noises
4. Amp starts sounding weak
5. Power fading up and down
6. No sustain, or fast decay

Courtesy of www.groovetubes.com