What is a vacuum tube?

The tube is basically an electronic valve that controls the flow of electrons. It consists of an envelope (bulb, usually glass) from which most air and other gasses have been removed. Inside this near vacuum are two systems. One is called the heater. This is in the center of the tube and is the portion you will normally see glowing orange (some tubes may have more than one heater). The other system consists of the cathode, grid(s), and plate (also called the anode). The plate is the largest metal structure you see inside the bulb. All of this is held to correct locations by thin disc spacers made of mica or ceramic.

More than a century after its invention, the vacuum tube remains the preeminent choice for high quality sound reproduction. It is the ubiquitous and essential presence in the recording studio, mastering suite, music amplifier, and home music room. The reason is simple: it is the most linear amplifying device yet created. And, being the uniquely finest amplifying device in existence, it is the cornerstone of all VAC designs.

VAC uses vacuum tube technology almost exclusively. This may seem old fashioned at first glance. However, the triode vacuum tube remains the most linear (accurate) amplifying device ever invented. It remains at the cutting edge of sound technology after almost 100 years! Still not convinced? Consider the following:

  • Virtually every high-quality recording studio uses at least some vacuum tube electronics
  • Linda Ronstadt, Barbara Streisand, Garth Brooks, and countless other artists record their vocals with vacuum tube microphones.
  • Eric Clapton plays only through vacuum tube guitar amplifiers.
  • Vintage vacuum tube equipment (Neumann microphones, Fairchild compressors, etc.) sell for 10x to 100x their original price and are highly sought after.
  • Every Intelsat satellite launched today incorporates vacuum tubes.

What importance does the signal path have in audio quality?

Now, it is not sufficient merely to have a vacuum tube in an audio. There is an alarming trend for some companies to include a tube or two in the signal chain and claim that it is a tube product. This can be misleading for two reasons:

  1. The signal path may also include transistors, so you are not hearing pure tube sound.
  2. The tube may be operated in a deliberately non-linear fashion to sort of “warm up” the sound. This is not the case with VAC electronics, which are designed for the most faithful sound possible. Regardless of price, VAC components are genuinely high performance; you are in for a “Ferrari” experience.
How many channels of sound do I need?

It all depends on your priorities. There are two basic types (sound only and sound and picture), each of which can take one of two forms (stereo and multi-channel). Let’s briefly examine each:

Option 1. Sound-only stereo
This is the classic “audiophile” system. It is appropriate for the music lover. At this time, almost all truly high fidelity music is in two-channel (stereo) format. Sources include CD, LP records (which can actually sound better than digital CDs), and live FM broadcasts. This type of system is capable of astonishing levels of musicality and satisfaction. Frankly, this is my favorite application. Soon there may be higher-fidelity two-channel music on the new DVD-Audio discs and the Sony/Phillips “Super CD.”

Option 2. Sound and picture stereo
The two-channel sound system (above) easily integrates with your television to provide and excellent home theater experience. All you need to do is connect the stereo outputs from your VCR or DVD player to your stereo. Now, many people will argue that home theater must be surround sound; however, I have found a high quality two-channel system to deliver superior enjoyment and dialogue intelligibility to the best Dolby surround home theater systems. This is probably due to the compression techniques in Dolby Digital--it throws away sound information that it thinks the average person cannot easily hear. Try it. You may be surprised.

Option 3. Sound-only multichannel
Over the next several years, music programs will become available with up to five channels and more. Today, 5.1-channel audio is available as compressed audio on movie-spec DVD and DTS discs. Soon there will be higher fidelity audio-only DVD discs, and a new Sony/Phillips “Super CD” disc. Playback of these discs will require a “surround sound processor.” When the specifications wars settle down, VAC will release a surround processor fine-tuned for the best sound performance. Not every home will accommodate five or six speakers in their optimal positions. Frankly, if you don’t put the extra speakers in the correct positions, you wind up with more, but not better. For this reason, it is good to know that multi-channel discs will still play on two channel systems; the extra channels are “folded” into the channels you have. This means you can also “grow” your system at your own pace: can start with left and right speakers, then add a center speaker, then add the rear ambience speakers.

Option 4. Sound and picture multichannel
Really, this is simply the Sound-only multichannel system with a TV or video projector added. VAC makes products that fit into any of these system types.

Is multi-channel worth it?

It depends upon your priorities and your listening environment. There is an old saying that sometimes more is not better--it’s just more. At VAC, we believe quality is more important than quantity. There are several issues at work here:

First, if you have a choice between two excellent speakers and amplifiers or five mediocre speakers and a compromised five-channel amplifier (and there usually are some horrible compromises to fit five channels into one box), choose the two excellent channels. It’s a bit like food: Would you rather have five bags of potato chips or one fine entre for dinner? ‘nuff said!

Second, you have to be able to position the speakers in their optimal locations, or the sound of the extra speakers can make the overall system sound worse. Regrettably, speakers often sound poor when located hard against a wall.

Third, what will you hear in the rear speakers? Most of the music now mastered for surround-sound release sounds very much like the failed quadraphonic system from the '70s, trying to place the listener in the center of a group of musicians. That may be fun for a few minutes or good for a discothèque, but it is a very unnatural way to hear music and even a bit disorienting. Do you really want guitars that sound 20-feet-wide or to feel like you're sitting in the middle of the mouth of a gargantuan singer? Listen carefully, and ask yourself, “Does this really seem more natural? Do I really enjoy this more? Am I able to feel the emotion of the music as well? Can I tell if the singer is enjoying herself or sad?” Regrettably, exaggerated and unnatural presentation in multi-channel will be common, at least at the beginning, as record companies try to grab your attention. Well, it must be remembered that 40 years ago many engineers engaged in an exaggerated form of stereo that was derisively called “ping-pong” stereo.

There are ways to use the extra channels to convey natural ambience and thus enhance the musical experience, at the same time improving clarity and reducing distortion, but only if artists and recording engineers have the will to use it with discretion. We think it will be quite rare.

How do I safely remove a vacuum tube?

Be Careful - IT’S HOT

Before replacing tubes, all power must be turned off. Unplug the power cord from the wall, and remove all audio interconnect cables.

Current tube technology requires high internal operating temperatures. As a result, the glass part of the tube can reach temperatures as high as 250 degrees. Always allow your amplifier to sit switched off for several minutes before you touch the tubes.

Typically the hottest tubes are the large output tubes (such as KT88, EL34, 6L6, 6550, 6CA7, KT66, KT77, EL84, 300B). Smaller tubes normally do not get as hot (such as types 12AX7, 12AU7, 12AT7, 6DJ8).

Do not bend or force the metal pins coming out of the base of the tube. This could break the vacuum seal and ruin the tube. The same is true of any sharp mechanical shock. You can usually tell if the seal breaks, as the silver deposits that coat the inside of the glass will turn to a white powder.

Tubes with plastic or metal bases should be handled by the base--not the glass. Such tubes have a “key” pin that ensures proper alignment in the tube socket. Do not use the tube if the key is broken off.

Nine-pin miniature tubes (such as the 12AX7) have no plastic base. Locate them in their sockets by noting the space where there is no pins. Do not wiggle these tubes excessively, as you might damage the socket or the glass seal.

I think I've got a bad tube. How do I check this?

The simplest way to debug a tube unit is by the “process of elimination.”

  1. Switch the left and right input cables at the tube device. If the problem moves to the other speaker then the source component or cable is at fault.
  2. Switch the left and right outputs cables at the tube device. If the problem does not move to the other speaker then the tube unit is not at fault.
  3. If the tube unit is at fault, begin switching tubes one by one between corresponding positions in the left and right channels (follow proper cautions for changing tubes as outlined above and in the instruction manual). If the problem changes channels on one of these swaps then you have found an unsuitable tube.

If I replace one tube, should I replace them all?

The best answer depends on how much the tubes have been used. As a general rule of thumb for VAC electronics, if the tube set has been run for less than 3,000 hours, just replace the bad tube. For sets with more hours of use, replace the entire set and keep the good used ones as spares.

How do I safely install power tubes?

Bias should be checked whenever a power tube is installed or moved to a new socket. New tubes may experience substantial change in idle point during their first 100-200 hours of operation. During this time you should check and readjust the bias periodically. Thereafter you should check the bias every one to three months. Of course, this does not apply to amplifiers that employ forms of automatic bias.

Signs of trouble:

  • the plate glowing orange means the tube is improperly biased and is running too much current.
  • a very strong blue glow in the tube indicates improper bias and excess current, or else an excessively gassy tube, which may not hold a stable bias point.
  • powdery white patches on the glass indicates loss of vacuum.

What’s that silver stuff on the glass?

The silver deposit is called the “getter” and is there to help increase the vacuum in the tube. Its color may vary slightly. Sometimes the getter will flow with use, even to the point of becoming evenly and thinly deposited over the entire envelope. The edge of this flow may have a brown color. None of this is important as long as the tube biases correctly and stability.

If you see the getter receding leaving a whitish profile the tube is loosing vacuum and should be removed from service.

What About Noise?

Tubes may be specially selected for low noise in critical applications, such as moving coil phono input. Such tubes may pick up noise as they age. There is no way to predict this by pretesting. If a tube becomes too noisy for your application contact your dealer.

What about microphony?

If you strike any tube it will emit a slight “tink” or “ring” through the loudspeaker. This is called microphony. In extreme cases this may become excessive, and the tube should be replaced. Rough handling, poor chassis construction, and even air shipping can encourage microphony..