VAC - A Primer on Owning Vacuum Tube Electronics

The amplifying vacuum tube is now approximately 90 years of age.  Although semiconductor devices actually preceded vacuum devices, semiconductors have taken over in most standard electronics applications.  As a result, many Americans no longer understand what a tube is, how it behaves, or how to maintain tube electronics.

You may relax.  Changing a tube is not very different from changing a light bulb.  This brief note should provide you everything you need to start being a savvy tube owner.


 
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.

 
Be Careful - IT'S HOT

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).

 
Mechanical Handling

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.

 
Installing 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.

 
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 in 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.

Please note that some other brands of electronics routinely wear their tubes out in about 2,000 hours.  Our guidelines do not apply to such units.