Tubes Amps Explained for Dummies - Like Myself
By Matthew Toledo - 3/8/2004
So you bought a tube amp. Congrats! Many people believe that a tube amp's sound is "warmer" and just plain better than transistor-based, solid-state amps. The tone of a tube amp tends to "break up" nicely, meaning the distortion comes in gradually with increased gain where in many solid state amps, the distortion curve is very steep. The key to the tube amp's sound charactersitics lies in it's reliance on a piece of early 20th century technology: the vacuum tube, sometimes called a valve.
Tubes are basically glorified light bulbs. There are plenty of other web sites on the "inter-web" dedicated to explaining how they function. This article was written for musicians like myself. I recently had to change some tubes and knew nothing about the process. The information in this article covers things I learned while I taught myself about maintaining my tube amp. Hopefully, this story will also clue you in on how to maintain your tube amp.
Warm Up
If you have a pure tube amp an not a tube/transistor hybrid, you may notice that your amp has two sets of tubes. There are large tubes and smaller ones. The larger set tubes are usually the power tubes while the smaller set are called the preamp tubes. Tubes function better when they are hot. Heat is essential to their performance. Lightbulbs don't give off light unless the filament in them is very hot, likewise tubes don't give off electrons very well unless their filament is hot.
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You may notice how some tube amps have a "standby" switch. When you get to a gig, put your amp on stage, plug it in and turn it on in standby mode before you do any other unpacking. This will turn on the heaters in your tube--yes they have heaters. It's best to let your amp warm up in standby mode for at least two minutes before playing so that the heaters can bring your tubes up to a proper working temprature. It's better if you let them warm up even longer. Faliure to do this will simply make your power tubes work harder, thus shortening their lifespan. You could even break a fuse. If you ever have to play a gig outside in the freezing cold, let your tubes warm up for a very long time first.
Cool Down:
Have you ever noticed that if you shake a hot lightbulb, the filament will break easily. The heat makes the filament softer and weak until it cools down and becomes relatively stronger. Likewise, moving a hot vacuum tube can damage the tube. It's best to let your amp cool for at least two minutes before carting it off stage. Pack up your pedals, your guitar, heck, even help the drummer with his gear. Leave your amp for last.
Symptoms a Tube is Dying:
Amps usually come with fuses. When a tube reaches the end of it's lifespan they become ineficient and need to draw more power from the wall in order to keep up. Eventually, they draw so much power that you'll most likely blow a fuse. If you change your fuse and it breaks again, chances are your tubes need replacing. Sometimes one or several tubes will pop and die completely when the fuse breaks. Putting in a stronger fuse can dammage your amp so don't do it!
Sometimes your amp will develop a low rumble or high whistling noise that gets worse as the volume is increased, even if nothing is plugged in to an input jack. Eventually, the squeal will get so loud that it will overpower your playing. That noise is called "microphony" and it's a sign that your smaller, preamp tubes need to be changed soon. The constant low rumble occurs when your power tubes succumb to microphony. The low rumble occurs very rarely, usually they start blowing fuses first.
The good news about tubes is that they usually fail in sequence. If your first preamp tube starts to microphony, you may be able to temporarily fix the problem by changing the order of the preamp tubes. The first tube in the chain bears the brunt of your playing more than the others. This fix is purely temporary and you should still get new tubes as soon as you can.
If a power tube has a slight blue or purple glow it does not mean that the valve is necessarily faulty.
Removing a Tube:
This may sound like a stupid tip, but since I thought tubes were basically lightbulbs that emit electrons, I thought they would twist out of their sockets in the same way as a regular lightbulb. Nope, to remove a tube, pull straight up on them. First, make sure your amp is cool, turned off, and unplugged. If you are only changing one tube, make sure not to touch the other ones with your bare hands. The oils on your hand could cause the tubes to explode when they get hot. Obviously, that would be bad.
How does oil from your skin cause a tube to explode? The oil wicks away heat from the glass, so the spot you touched will be cooler than the other parts of the tube. Ever heat a test tube or thin glass cup really hot and then dip it in cold water? CRACK. The same principle applies. It's best to use a small towel or t-shirt to yank out the tube. Although tubes are very strong, yanking on a very stubborn tube the wrong way could shatter it and a towel will help protect your hand from the glass as well as protect the glass from the oils on your hand.
When you remove a tube, take a look at the socket it came from. If it has a burn mark, you have a problem. This is a sign that your amp had been left on without a speaker cabinet plugged into it. In general, you should never turn on a tube amp unless a speaker cabinet is plugged into it. A car can over-rev and blow the engine if you press on the gass while in neutral. The same goes for your amplifier. Without some sort of resistance, you can "over-rev" your amp. If you see a burn, take your amp to a pro for repairs.
Editor's Note 12/19/2006 - You can also over-rev your amp if your speaker cabinet is missing a speaker, or the speaker is blown. I played a gig a few months back where one of the speakers in my 4x12 Marshall cab was blown. I started smelling burning plastic, it was the circuitry in my amp getting ready to blow up.
Life span and Replacement:
The average life of a power tube that is used three to four times a week should be approximately 18 months to 2 years. The average life of a pre-amp tube if used three to four times a week should be approximately 3 to 4 years. Of course, that estimate will vary from amp to amp and from playing style to playing style.
Your tubes will have a numbers and/or symbols on them, use those numbers and symbols to find out what make and model tube you need to buy. I bought my tubes from thetubestore.com. They were cheap, and their web site was very useful when I had to identify my current tubes. In general, you should never mix or match different brands of tubes.
Though not necessary for all amps, many amp manufacturers like Marshall recommend that you replace all the power tubes at the same time, even if only one power tube is dying or dead. Since tubes draw more power as they die, putting in one new tube will cause the new tube to burn out quicker. I guess you can get away with it, but you're throwing money away if you do so.
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It is interesting to note that you can change the tone of some amps by putting different tubes in them. Please check with your amp manufacturer BEFORE you attempt this. In general, it's only safe to decrease the power and pre-amp tubes to lower level tubes. Best to check with a pro before you attempt this. This trick can reduce the gain of some amplifiers thus making the distortion sound more melodic and less like marbles rattling around in a tin can. It will also lower the total power of your amp, but then again, when was the last time you had all the knobs on your amp turned up to 10?
Balanced Pairs, Quads, and so on:
When you replace your power tubes, you should do so in pairs or quads. You usually can't buy just one tube from a store, most times they come in balanced pairs. When a tube is part of a balanced group, their power curves closely match each other. If you don't have a balanced set of tubes in your amp the tubes can end up "fighting" against each other for power. When it's time for your tubes to go to work, the ones with a steeper curve will try to drag along the slow-poke tubes with the less-steep curve. This can reduce the life span of your tubes and possibly even damage your amp's circuits if your tubes are way out of balance.
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Bias Your Amp:
Once you get your balanced set of tubes, you need to bias your amp. Marshall recommends that you bias your amp every time you change your power tubes. They are usually the larger set of tubes in your amp. However, Marshall states on their web site that the smaller, pre-amp tubes don't have to be biased. Thetubestore.com also states that the smaller pre-amp tubes like the 12AX7 do not need to be biased
Biasing your amp is the trickiest step of the bunch and can involve inserting or removing resistors into your amplifier while it's ON to adjust the amount of power going into your tubes. This is something you don't want to do yourself! Vacuum tube equipment operates on very high voltages. Typically, voltages anywhere from 350-450 volts or more can be found inside your tube guitar amplifier. THESE ARE LETHAL VOLTAGES.
Why do you need to bias an amp anyway? Like most of the preventative measures listed in this article, biasing your amp helps to extend the life of your tubes and prevent damage to your amp's circuits. Proper biasing also improves the tone of your amp. If your amp is biased too low or "cold" your tone will suffer. If your amp is biased to hot, you'll burn through tubes and possibly meltdown your amp. You should note that when you buy a new amp, they often bias the power tubes a towards the cold side. This is safer for the manufacturer, since it extends the life of the tube and makes your amp seem like it's built better than the competition. The drawback is, the tone isn't as good as it could be. When I got my new power tubes, the folks at Insea set the tubes closer to the hot side and my amp sounds much better than it did when I first bought it. So, if you get a new amp, you may want to get it biased.
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Biasing, in my opinion, is a giant and expensive pain in the amp :-). Still, you NEED to do this last step. I personally know someone who spent $300 on tubes for their boutique Rivera combo but they skipped the biasing. They ended up having to buy new tubes a couple months later. So even though it's a pain, take your amp to a certified tech once you get new power tubes.
How to Avoid Biasing:
The next time you have to buy replacement power tubes for your amp, buy a larger balanced set than what you need. For example, if your amp has 4 power tubes like mine, buy a balanced set of 8 power tubes and keep the extra 4 someplace safe. When your tubes need replacing, swap out the 4 old ones and put in the 4 extra ones you bought. Since they are all part of the same balanced set, you will not need to re-bias your amp, thus saving some hassle if not some money.
In Conclusion:
You now know as much as I do about tubes, amps, and how to live with them. If you have any tips, please feel free to comment below or send me an email.
Web pages of interest:
- diyguitaramp.com tons of technical info on tubes, biasing, and even how to build your own tube amp.
- Machine Gun Amps Lots of tips on living with a tube amp, more troubleshooting advice, and covers all the stuff discussed in this story in much more detail. They even have a "Strange Amp Problems" page with funny fix-its to uncommon amp problems.
- Marhsall - Since my amp is a Marshall, I visited their site first to find out more about the tubes in my amp.
- The Tube Store - I bought my replacement tubes online here. Great price and they even Fed-Ex them if you need them the next day. They also helped me identify the make and model of the tubes I needed.
