|Do it Yourself Tube Installation Guide
Amplifying vacuum tubes have been with us approximately 90 years and they still are popular although semiconductors have taken over in most standard electronics applications these days. As a result, many people no longer understand what a vacuum tube is, how it behaves, or how to install tubes and maintain tube electronics.
Well, don’t get excited, just relax! Changing a tube is not very different from changing a light bulb except you don’t unscrew it, just lift it out gently! This guide 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 which are fragile.
Be Careful – Tubes get 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.
Before you do anything with the tubes please UNPLUG THE POWER CORD!
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). In Ham Radio amplifiers, the final output tubes are often the hottest and are also in a shielded cage that can get very hot. So be careful and let them cool off before you work on them!
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 (inside the tube) – this means the tube has lost its vacuum and is no longer good! Hope you have a spare set.
Tubes with plastic or metal bases should be handled by the base, not the glass. Such tubes have a center "key" pin on the base that ensures proper alignment in the tube socket. Do not use the tube if the key is broken off unless you know how to line up the pins with the correct socket pins. A broken pin tube does not effect the inside of the tube, only the alignment of the pins in the tube socket.
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. The most popular miniature tubes are 7 or 9 pin tubes. If a pin get crooked, gently straignten the pin out and it should be ok.
Installing Power Tubes
Bias should be checked whenever an audio 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.
For Radio Frequency (RF) amplifier tubes, you may have to “neutralize” them for proper operation. Many Kenwood radios or 6146 transmitter circuits had neutralization capacitors that had to be adjusted when a new set of final tubes is installed. Make sure you always use a new driver tube (like a 12BY7, 6CL6 or 6GK6 to name a few) when you install the final tubes.
Signs of trouble:
- the plate glowing orange means the tube is improperly biased and is running too much current, and is in danger of being ruined if it operates too long in this condition
- 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.
WE TEST ALL OF OUR TUBES PRIOR TO SHIPMENT SO THEY SHOULD BE READY FOR INSTALLATION WHEN YOU RECEIVE THEM
The silver deposit is created by the "getter" and is there to help increase the vacuum in the tube by burning off any internal oxygen 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 the audio sounds good or the transmitter has full output power.
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 it time for a replacement.
What About Microphonics?
If you strike any tube it will emit a slight "tink" or "ring" through the loudspeaker. This is called microphonics. In extreme cases this may become excessive, and the tube should be replaced. Rough handling, poor chassis construction, and even air shipping can encourage microphonics as well as loose internal tube elements.
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.
You can also use the “process of substitution” wherein you start substituting one tube at a time until the problem goes away. The last tube substituted is the problem child. Always replace dual or quad output tubes all once even if only one seems to be weak or bad.
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 if you play a lot of gigs or use your electronics a lot or 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.
Have fun with your new tubes!