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.
Well, 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.
