Tube Frequently Asked Questions

This article is written by Doug from Roccaforte Amps and was
posted at the
Harmony-Central Forums.
For more info check his site.

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.

Tube Matching
You don't have to read many tube dealer ads to discover that "matched" versions of their power tubes are available at slightly higher prices. Yet, most players and technicians really don't understand what matching is all about, and what they get for those extra bucks. Let's take a look at matching.

Please note that we are talking about matching power tubes such as 6L6, 6V6, 7027, 6CA7/EL-34 and 6550/KT-88. I can tell you from long, hard experience that matching power tubes affects the sound of your amplifier more than any other tube. Compared to random tubes, an amplifier with matched tubes will provide slightly more power and a reduced hum level. In most cases, the improvement in sound quality will be slight, but worthwhile. It is up to you to try matched tubes to judge for yourself. Personally, I install matched power tubes in the highest quality amplifiers, and always in amplifiers that demand them.

You should be aware that most musical instrument amplifiers have a single bias adjustment, or none at all. Use matched tubes in these amplifiers because there is no way to adjust out bias differences between tubes. Typically, if you put random tubes in one of these amplifiers, one tube will run hot, while the other will run cooler. As a result, the life of the hot running tube is reduced, and you have to pony up for a new pair sooner than expected. By the way, it is especially important to use matched tubes in amplifiers like the Fender Deluxe‘. This unit runs a pair of 6V6 tubes far beyond their voltage ratings (430 volts, 25 mA for you tech types), and it doesn't take much of a mismatch to smoke both tubes!

On the other hand, matching both sections of small-signal tubes such as the 12AX7, 12AT7 and 12 AU7 has proven to be a silly idea. I discovered the performance improvement, if any, was measurable, but not great enough to hear. We don't offer matched small-signal tubes for this reason. Save your money if you wish to buy from others!

By the way, always replace your power tubes as a set. This is true even if only one goes down. If you don't, you are gambling that the tube you didn't replace will last as long as the new one. This is highly unlikely, and you will end up taking your amplifier in for service much more often. I know professional players who routinely replace all of the tubes in their amplifiers after 4-5 performances simply because they can't afford to have their amplifiers quit on stage! Let me repeat: always replace your power tubes as a set.

So what do you get when you buy matched tubes? They will be matched only in terms of bias current. In other words, if you put two perfectly matched tubes in an amplifier, each tube will draw the same current. This is called "single point matching." Mismatches of 5% to 10% in current draw are acceptable in most amplifiers. This is the best you can expect for the low cost "standard match" we offer.

Be aware that tube aging affects matching. The first 100 hours of playing may change matching enough to become a problem. After about 500 hours of operation, there is much less change in the match. For maximum tube life, we recommend checking the bias settings in your amplifier at those times.

Tube Biasing
One of the key factors which affects the tone of your amplifier and its long-term health is something called the bias setting. Unfortunately, there is a lot of misunderstanding among players and technicians as to how "bias" works and what "setting" it really does. Let's correct that situation here and now!

Basically, your amplifier contains analog circuits which work best only over a narrow range of conditions, or "operating points." Most amplifiers have a hidden screwdriver adjustment (bias pot to you tech types), which sets the operating point to the correct value. It does this by adjusting a DC voltage applied to the tubes. Tube action translates the DC voltage into tube current draw, which is how the operating point is expressed.

Please understand that we are talking about biasing power tubes (6L6, 7027, 6CA7/EL-34, 6V6 6550, KT-88, etc.) here. Your small-signal types (12AX7, 12AT7, 12AU7, 6EU7, 5879, etc.) are self-biased for optimum performance, and no adjustments are needed.

So what happens at different bias pot settings? Consider this:
If the bias control is set too high, current through the output tubes will be high. As a result, the tubes will run very hot, and need frequent replacement. The sound will probably be clean at first, and then turn distorted. If the metal structure (plate) inside the tubes starts to glow cherry red, turn the amplifier off immediately and take it in for repair! You can cause extensive damage to the tubes and other amplifier parts if you let this condition continue.

If the bias control is set just right, current through the tubes will be at a proper value. As a result, the sound will be good, there will be lots of power, and tube life will be long. To the amplifier manufacturer, proper biasing is a compromise between good sound and reasonable tube life.

If the bias control is set too low, current through the tubes will be low. As a result, the sound will be distorted on loud notes. The output tubes will run cool. Tube life will be longer under this condition, but other parts (capacitors) in your amplifier may be stressed by the higher voltages which appear on them.

So how is the bias set? Technicians monitor the tube current draw with a special adapter unit, while adjusting the bias pot for a certain current value on the adapter's meter. The volume control(s) must be set to zero for this adjustment. The correct current value is found in the manufacturer's service literature, or under "Typical Operating Conditions" in a tube manual.

Let me leave you with these thoughts: If you buy a set of tubes and choose to install them without setting the bias, you are gambling the original adjustment is close enough. For best results after replacing the output tubes, always have the bias adjusted by a competent technician. Due to tube aging, it is also recommended that you have the bias checked and adjusted, if necessary, after about 100 and 500 hours of playing. This simple step maintains sound quality and extends tube life.

© Doug Roccaforte

This article is written by Doug from Roccaforte Amps and was posted at the Harmony-Central Forums and was used for this site without his explicit permission.