Tube Replacement? When and Why? How?

Glass, metal, lots of little parts... tubes are subject to mechanical problems and are not meant to last a lifetime. The harder you work your tubes, the faster they wear out. Plus, there are a number of other factors that make tubes one of the easiest and least expensive “fixes” for your tube amp.

  1. Tubes are subject to vibration damage. The inside of a combo tube amplifier is a super-abusive environment for tubes because you crank up the volume, create a huge amount of sonic vibration, directly couple the tubes to the chassis of your amp and then stick the chassis inside the speaker cabinet!

  2. When tubes operate, the metal parts on the inside of the tube begin to glow. That means that they are really hot, like molten metal. When metal is heated enough to glow, it gets soft. Excessive vibration can create a situation where the metal becomes fatigued or stressed.

  3. If you are at a gig with your amp, you run it hard and then quickly take it outside and throw it in the back of your car while the internal components are still warm and soft, the bouncing around while you are driving can cause damage to the tube.

  4. The reason that it is called a “vacuum tube” is that the air is sucked out of the inside of the tube when it is manufactured. This creates a “vacuum” inside the tube and this lack of oxygen allows the metal parts to heat up without actually burning. If the tube develops a leak, or the seal is broken air will leak back into the tube and the components could smoke, smolder or burn.

  5. Tubes are inexpensive compared to transformers, speakers, etc…and a lot easier to replace!

  6. When one tube starts to go, it drags the others down with it, decreasing the overall efficiency of the amp, which adversely changes the tone and reduces sustain.

  7. You can keep spare tubes with you in case something happens at a gig. With Groove Tubes you can even make sure that you have a spare power tube set with the exact same rating.

  8. Many manufacturers have started producing small, inexpensive tube amps. While many of these are really cool, sometimes a manufacturer will cut corners on the tubes that come in the amp. Groove Tubes preamp and power tubes can really punch up the performance of these amps.

  9. With the easy-to-understand descriptions of GT preamp tubes, you can experiment with different styles and types to achieve a wide variety of different gain structure and tone.

  10. Audio troubleshooting. Change tubes if your amp experiences any of the following:

    1. Loss of tone, clarity, sustain and harmonic richness.

    2. Poor output-level balance.

    3. Lack of midrange punch and definition.

    4. Rattling, whistling or humming.

    5. Feedback or metallic sound on certain notes

    6. Weak sound and power loss.

    7. Or, if your want to experiment and change amp tone characteristics.

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Tube cross reference basics

There have been a myriad of tubes types and styles made over the years for industrial, military and commercial use. Many of these use European nomenclature. A cross reference is included here:

Groove Tubes Industrial European
GT-12AT7 6679 ECC81
GT-12AU7 6189 ECC82
GT-12AX7 7025 ECC83
GT-5AR4 - GZ34 / GZ37 / U54 / U77
GT-5U4 - GZ32 / GZ31 / U51 / U52
GT-5Y3 6853 GZ30/U50
GT-7025 7025 ECC83
GT-6CA4 - EZ81
GT-ECC83 7025 ECC83
GT-6L6 5881 KT66
GT-6V6 7048 / 5871 -
GT-6550 6550 KT88
GT-EL34 - 6CA7/KT77
GT-EL84 - 6BQ5
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Introduction to Vacuum Tubes

Electric guitar and bass players over the past 50 years have been linked by a single critical piece of equipment: The tube amp. With all of the advances in transistor and digital technology, professionals still prefer tube amps as part of their musical arsenal from studio to stage. There are many reasons for this, and we will explore many of these factors in plain language, throughout in this guide.

Tubes have provided the foundation for musical and audio circuits that have produced much of the worlds’ greatest rock, jazz, blues and country music during the last century. Ironically, tube-driven circuits are not necessarily a perfect “reproducer” of sound. They are actually a “producer” of part of the musical output by adding definite, subtle, tactile and sonic characteristics to the signal path of instrument amps.

So, why do tube amps sound and feel different?

First, tubes are imperfect. They each have unique performance characteristics and designs. They respond to signal input in a softer, slower and less precise manner than digital circuits. This provides the soft feel, or touch that many players enjoy in great tube amps. This is due to the natural “compression” that tube inefficiency creates. Unfortunately, they also degrade over time and use, which is why many professionals re-tube their amps before each tour and maintain spares for road use.

Second, tubes tend to distort in a very non-linear way. If you look at tube circuit distortion on test equipment, you would see a slightly rounded waveform as the tube starts to distort. This is different from a solid-state or digital waveform which tends to appear square, distorting all frequencies at the same time.

In tube circuits, low frequencies tend to distort first, and higher frequencies stay clean longer. Because of the way we hear sounds, this is more pleasing, less harsh. The harmonics that tend to distort in a tube circuit are also more musical and are the reason that when you “grab” a note on a guitar at high volumes and the amp distorts, it will feed back in a musically useful and pleasing way.

Finally, even though tubes are made of glass, and have little metal parts inside, they are actually pretty durable, and with proper testing, rather predictable. Over the years, there are many specific amp designs that have been associated with an “American” sound, a “British” sound, etc… Many high-end boutique products use these reference sounds from tubes in their circuit designs to achieve signature tones.

What is a Tube?

A tube is an electronic gain producing device consisting of a minimum of four active elements: a heater (filament), a cathode, a grid and a plate. These are 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”. This is one reason that tubes are called “valves” in the UK.

What is the difference between preamp tubes and power tubes?

When the guitar’s pickup produces a small voltage (the result of the string vibrating in the pickup’s magnetic field), this signal goes into the “preamp” part of an amp circuit. It is applied to the grid, which causes a larger current flow from the cathode to the plate. Preamp tubes are usually the main tone generators in an amp circuit and you can experiment with the gain and tone of you amp easily by trying different preamp tubes. GT makes it easy by providing clear, intuitive descriptions of these tubes.

Power tubes provide the major horsepower in an amp circuit, and will have their greatest impact on tone as they distort. This means you have to run amps pretty loud to get the maximum benefit from the tube distortion in the power amp section. That explains the popularity of small, lower power tube amps which distort nicely at lower volumes. Keep in mind, there are several styles of power tubes available. You must use the specific style (EL34, 6V6, 6L6, etc…) that the circuit was designed to accommodate.

What is “biasing”?

Bias refers to the adjustment (generally in the power tube section) that controls the voltage of the grid of a tube. When the grid bias is properly set, the tubes are balanced in the circuit and produce a clean, powerful signal. It is like matching the engine RPM to the proper gear in a race car to achieve optimum performance for the desired speed.

If the bias is not set correctly, the amp may not perform properly, or the tubes may wear out more quickly. If you install a set of tubes with very different outputs or strengths from each other, it may not be possible to adjust the bias to achieve maximum performance from all power tubes. You will get a compromise on all tubes, and none will perform to their proper potential or specification.

Fortunately, many amp manufacturers use power tubes that fall in the same specification as “Medium” (4-7 fine scale) of Groove Tubes as their stock tubes. The amplifier’s bias control is adjusted at the factory for tubes in this power range. Tubes in this range are widely available at all times from Groove Tubes.

If you want to change the performance of your amp to distort more quickly, or stay clean longer, you can use Groove Tubes in the Low (1-3 fine scale), or High (8-10 fine scale). If so, you should have a qualified service tech re-bias your amp for the new tubes. The good news, is that once this is done, you should be able to replace your power tubes with Groove Tubes of the same type and rating without bias adjustment.

Preamp circuits and many tube power amp designs today feature an “auto’biasing” circuit. You can check with the amp manufacturer to see if your amp has “auto-bias”. If so, no further adjustments are needed.

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The Groove Tubes Warranty

Preamp tubes
Warranted to be free of burn out, excessive noise or microphonics for 180 days from date of purchase.

Power tubes
Warranted to be free of burn out for 90 days from date of purchase. If one tube fails, the whole set will be replaced to insure proper matching.

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contact our support team

For Groove Tube questions relating to tubes, GT Branded amplifiers, microphones, compressors, Hi-Fi receivers etc., please contact Groove Tubes Product Support at 800-856-9801, or via email sent to consumerrelations@fender.com

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