All practical audio amplifiers require some type of amplifying device, whether in the form of new school transistors or old school vacuum tubes. Transistor amps are durable, efficient and in lesser forms, comparatively inexpensive. Tube amps like power, they eat it like a college student making a fifth run at the "all you can eat buffet” while being fragile compared to their solid state cousins. Let’s not forget to mention the heat generated by tube amps. Your studio rarely needs that radiator to work on those cold nights while you are honing your craft.
So, why put up with the heat, the inefficiency and the expense of replacing tubes? Because you finally found that tone, the one that you were searching for that seems only a tube amp can produce.
There are as many myths and opinions about tube amps, as bad and usually unsolicited advice on how to maintain your investment. Some of this advice can actually damage your amp. Let’s dispel some bad information and give you a solid and simple advice on how to extend the life of your tube amp.
What is Going on Here?
A vacuum tube, sometimes called a “valve”, modulates power from a power supply to match your input signal to output a stronger or amplified signal. To do this it needs a filament, think incandescent bulb, to motivate this action, controlled by a grid, think gate keeper, to control this action between the cathode and the plate. Without proper operation between the cathode and grid, the tube can run wide open, simply burning up or can run cold by not allowing not enough through.
Pre-amp tubes, typically triode types like 12AX7s or the more rare 12AT7s, do not need to be re-biased in the amplifier when they are changed. When changing these you can simply plug in the new tube and you are ready to go.
For power amp tubes like EL34, 6L6, EL84, and 6V6 to operate correctly they typically need to be biased. These are usually in a push pull AB amplifier topology. To correctly operate the grid needs to “shut off” when the signal is switching to the other tube in the circuit. This is what the “push pull” term means. These are usually in even quantities because one half is sharing the work with the other half. There is an optimum bias DC voltage that needs to be applied to function correctly to reduce distortion and optimize power output.
Make sure your tubes are correctly biased, too high and hot, means they will die prematurely. Too low and cold and your sound will actually sound cold and lifeless. When working with a new rig, your life is simple because it should already be biased correctly. When using an older amp with non-specified replacement tubes, take it to a technician and have it re-biased with new tubes.
Note – Use the same tubes as the originals when replacing expired ones. It is always best to have your amp re-biased when replacing the power amp tubes. Again the pre amp tubes do not need to be biased. Note some matching systems claim using the same matched numbered tube does not require re-biasing. This is true in some cases and will work, but it is still best to check the bias. Only qualified persons should bias a tube amp. There are very dangerous high voltages present that can kill.
12 Habits to Extend Tube Life
Now that the basics are covered, time to cover a dozen good tube tips:
- When bringing your amp in from outside (especially the cold) let it assume room temperature. This covers a lot of ground, from dealing with condensation, to handling temperature induced differential expansion, things that can lead to premature failure of components.
- Turn on the standby switch then main power and let the amp warm up. THEN take the amp off the standby for half a minute to allow the preamp and power-amp tubes come to optimum temperature before being put to use. This step by itself will greatly increase tube life.
- Keep ALL liquids and beverages away from your amp. A spill is disaster waiting to happen and could not only kill your amp but could take you down in the process.
- Always use a quality cable in good working order. If your cables are producing crackles or pops when moving, replace them. Shorts will reduce the life of the system.
- Take care when plugging in your amp to the speaker cabinet. Do this before firing things up, but if hot and hopefully in standby, always plug in the cabinet first then plug in the amplifier side. There is a slight short produced when plugging in and out and the cabinet presents some impedance. The same principle works well for signal lines. Plug the cable into the source or instrument first, and then plug into the amp.
- Use a proper load. Impedance and loading are very critical for most tube amp designs. Always make sure that the amplifier has the correct impedance to match the cabinets being used. Low impedance situations and miss-matched loadings are very hard on amps.
- Do not move the amp around when it is running. The filaments and parts in a tube are physically weaker when hot. Shock can cause immediate failure. Remember that old flashlight that wouldn’t light up again after you dropped it in the basement?
- Allow the amp to cool down before loading out. You are less likely to do any damage when the tubes are cold.
- When shutting off, no need to go to standby. Just shut it off. If after cool down, you put the standby on you will not have to remember to turn standby on when you power up next time.
- Don’t drop it. Common sense right? We have all seen a solid state amp light right up after being dropped, rarely have we seen a tube amp do the same. Keep bumps, drops, and shocks to a minimum even when protected by a soft case.
- When cold and stowed - blow out debris with a can of electronics “blow-off” to remove cobwebs, dust and the fur from the drummer’s cat and wipe tubes with a lint free cloth. Dust keeps in heat and excess heat will age the components in your amp. This is also a good time to make sure all your tubes are firmly seated in their sockets.
- Always store your amp in controlled environments. Long periods in damp or overly humid conditions like cars, sheds or garages will age your amp. These conditions are also not very good for speakers, they can lead to surround rot, contact corrosion and make paper cones brittle.