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Basics - Replacing Vacuum Tubes
Contributor: TC Staff    Rating: 4/5    Views: 111258

Tube Replacement Basics

There is a lot of hype floating around about how often tubes need replacement. As a result, too many good tubes are replaced unnecessarily, and not always with better quality tubes. Let's strip away the hype and take a closer look at this important subject.

Tubes fail for three basic reasons. They wear out, short, or become gassy. In the case of wear out, here's what happens: If you look inside a lit tube, you will see a glowing red stem in the middle of it. This is called a cathode sleeve. The manufacturer has coated the outside of the sleeve with a proprietary white powder, which emits electrons, making tube action possible. The sleeve is heated to incandescence by a coil of insulated wire stuffed inside called a filament. Your amplifier powers it. Heat causes the powder to deteriorate over time and emit fewer electrons. Tube performance dies with the powder. Shorts are caused by excessive heating of the elements, which causes them to warp and touch. Power tubes and certain small-signal types such as the 7199 are prone to shorts because of internal heat build-up. "Gas" refers to air molecules, which have forced their way inside. This is typically caused by a faulty glass-to-metal seal, where a wire passes through the glass envelope to the tube element. Gas is bad news because it causes the tube to conduct more heavily (run hotter), reducing its life.

By the way, it is extremely rare for a tube to "burn out." In other words, the filament wire has a break in it and won't light. This doesn't happen much because the filament is deliberately made rugged enough to outlast the rest of the tube.

Have you ever noticed how some tubes like new old stock Mullard or Amperex 12AX7s "flash" brightly when you turn on the power? The answer is rather technical so bear with me. The two filaments have unequal cold resistances. When power is first applied, the filament with the lower resistance passes more voltage over to the other filament, which makes it glow brightly (Ohm's Law at work). As the lower voltage filament warms up, its resistance rises to the hot resistance of the other filament, allowing both to get equal voltages. This effect is pronounced in amplifiers with AC filament supplies because larger amounts of current are available for the tubes, DC filament supplies light those found in better hi-fi equipment tend to be current limited, reducing the opportunity to "flash."

A few words about tube life. Most new old stock tubes had lives estimated in the 10,000+ hour range. This value assumed that the tube was run at less than half of its ratings, and was not abused in any way. To the best of my knowledge, lifetime ratings of current production tubes are unknown. As a practical matter, tube life really depends upon the operating conditions inside of your amplifier, and how much you abuse the unit. As a result, it is not easy to predict the exact number of hours you get from a given set of tubes. However, as a rule of thumb, small-signal tubes (12AX7, 12AU7, 12AT7, 5879, etc.) typically outlast two sets of power tubes (6L6, 6V6, 7027, 6CA7/EL-34, 6550/KT-88, etc.)

Replace the tubes when:

It is broken, and/or there is a white spot on the top or sides of the tube. The "getter" coating found in these areas is normally black or silver colored. By the way, a chocolate brown getter simply means the tube has had a lot of use. It still may test good.
It tests weak or shorted. Always use a "mutual conductance" type tube tester for testing. Cheap emission type testers with "good/bad" meters are almost useless. Always replace a shorted tube, even if it tests good. Shorts can damage expensive amplifier parts.
You cannot afford to lose your amplifier during a performance. Professional players minimize downtime by replacing the power tubes every few concerts, and by using several amplifiers. The price of tubes is cheap compared to the embarrassment of losing an amplifier on stage!
The amplifier starts to sound bad, especially when first turned on.
So how do you know when tubes are starting to fail? Consider the following:

Small-signal (preamp) tubes (12AX7, 12AU7, 12AT7, 5879, etc.): You may hear a crackle for a short time after the amplifier is turned on. Then it quiets down. Or the amplifier may "howl" at high volumes, or buzz on certain notes all the time. These problems get worse, and eventually you will be replacing one or more tubes.

Power tubes (6L6, 6V6, 7027, 6CA7/EL-34, 6550/KT-88, etc.): Upon turning on the amplifier, the sound may be hazy, with flabby bass and scratchy treble. After about five minutes of this, the sound starts to clear up, and the amplifier sounds fine from then on. As the tubes get weaker, it will take longer for the sound to clear up. Eventually you have to replace the power tubes as a set. For reliability we strongly encourage you to replace all power tubes at a time, even if one or more tubes are still good. Also, make sure your technician set the bias correctly after they are installed. You may keep any good tubes as spares for emergencies, though.

Rectifier tubes (5AR4, 5Y3, 5U4, etc.) are not in the signal path and do not affect the sound as much. However, if you notice more distortion than normal when playing at high volumes, the tube is probably half-dead and should be replaced.

Contributor: TC Staff    Rating: 4/5    Views: 111258

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