Types of Tubes
Before we get into some of the tubes out there lets cover the types of tubes you may find in your amplifier.
Preamp Tubes
These may be the most important tubes in your amp if your chasing a particular sound. These are the small tubes in your amplifier. What exactly do any tubes do? As the signal enters the amplifier they boost it - certain stages such as the tone controls can diminish the signal strength so a preamp tube is used to boost the signal back up before it enters the next stage. Its not uncommon to see effects such as reverb have an preamp tube buffering their signal. Also - in most amps the first two preamp tubes are linked to each other to make more gain. This is where we can make some changes to really improve the sound of the amp. Some hybrid amps use a tube in the preamp to achieve their gain while having a solid state power section so these amps can benefit a lot by putting a high quality tube in. As you turn up the volume on the amp these preamp tubes are driven harder creating distortion. Getting an idea that this is a critical point in the sound chain? Common types are 12AX7, 12 AT7, 12AU7. A good way to identify these tubes is that they are usually covered with a small metal cover to minimized noise.
Rectifier Tubes
These are somewhat mysterious as most amps out there now use solid state rectification but in tube rectified amps the tubes basically boost the voltage up before reaching the power tubes. Their role is to take the already boosted voltage coming from the power transformer and boost it and somewhat smooth it before this power hits the power tubes. The actual tone signal doesn't travel through this tube(s) so I don't recommend doing anything with it. I do recommend you have a very high quality tube in this socket however. These tubes last a very long time and since they don't affect the sound a little degradation of the tube itself doesn't affect the sound. If you have your amp on the road a lot carry a spare. If this tube fails you'll have no sound whatsoever or a very poor sound if its in its death throws. Common types are the 5Y3 and GZ34 and sometimes even another 12AX7 preamp tube might be in this spot. Also - there is a very good chance your tube amp uses solid state rectification so this tube won't be in your amp. Remember, this tube isn't part of the sound signal path so solid state rectification brings a lot of reliability without changing the sound of the amp. A quick way to identify this tube(s) is that its a small tube with no cover on it.
Power Tubes
This is where the power rating of your tube amp comes from. Manufacturers use a formula with the ratings of the power tubes and its transformer to come up with a rating for an amp. One cool story is that the original Sovtek Mig 50 amps produced in Russia were found to have a weak transformer so their output was really about 30 watts. Rather than replace the transformers the labels on the amp were changed to Mig 30. How did they sound? Turns out they sounded incredible - the sagging transformers actually helped the sound.
Many different types abound of power tubes - EL34, 6L6, EL84, etc. As these tubes work at very high voltages - 400 watts or more is not uncommon - so they tend to wear out quickly depending on use. The power tube is the last thing to affect your sound before it is sent to the speaker. When driven at higher volumes overtones, harmonics, and distortion are created by the tubes. The qualities of these harmonics and distortion can be surprisingly different between different types of tubes. Don't skimp on tube quality here. High quality tubes will last longer and have less noise in them.
So what can we do to change the tone?
Preamp: This is the most critical area for overall sound. Your volume, tone controls (mid, treble, bass), reverb, effects loop, etc. can all have a preamp tube associated with them. I've seen five in a Fender amp so more doesn't necessarily equal more distortion. That said, amps like the Peavey 5150 have six preamp tubes specifically to create more distortion. It all depends on the design of the amp and the type of tubes in the amp. Tubes locations are labeled with the letter 'V'. V1 and V2 are typically going to be the tubes you should swap out first. While tube shopping be careful to read the description of the tubes for the amount of distortion that tube creates vs. how much noise it creates. Some tubes offer high distortion with medium to low noise. Others can generate low distortion with little to no noise. Generally the lower the noise the more expenssive the tube but don't skimp on these tubes. This is the heart of your sound. You can go a little cheaper for tubes that are used for reverb or other effects (generally V3 or higher) - low distortion low noise tubes work well in these other sockets. Research your amp to see which tubes are associated with tone - you may need to replace up to V3 to get more distortion.
What if you are chasing a cleaner sound. Lots of amps come with 12AX7 tubes in V1. Fender amps often were designed around the 12AT7 so why not try that in V1. You could even try a 12AU7 to see what that sounds like. These tubes need to be cold when you replace them - no need to burn yourself. Also preamp tubes do not need their bias adjusted. This makes changing them a simple plug and play affair. There are 9 pins so be careful to insert it correctly. If you don't its not going to damage the amp - let the tube cool down and try again.
Rectifier Tubes: I don't mess with these much. Generally they work or don't so carrying a spare is a good idea. Try to buy a very high quality tube for this spot. I've seen preamp tubes used as rectifiers but generally amp makers use different tubes here. The 5Y3 has been around since the dawn of guitar amps and nowadays most amps use a GZ34. There isn't much benefit messing with this circuit unless you are trying to increase the output of your amp. As the power output is an interplay of the power transformer, the rectifier, and the power tubes its not as simple as changing one component and wham your 50 watt amp is now a 100 watt amp. I don't recommend modifying an amp unless you have a complete package that is designed for this purpose - the right power transformer, rectifier, and a method to bias the tubes correctly if the power tubes used fixed bias. Also make sure the amp is off and tube has cooled before swapping. Do not play with the socket - this is a high voltage area. You'll notice that these tubes can only be inserted one way to make sure you don't mess up with the tube swap.
Power Tubes: The beautiful sound of an amp driven to extreme levels. That's the effect of the power tubes on the total tone. Those harmonics and bit of distortion make the sound so sweet. This can be the sweet sound of Stevie Ray Vaughn to the distorted sound of Jimi Hendrix. Amps either are bias adjustable or not. What is bias? The amount of voltage reaching the tube. Hopefully your amp is bias adjustable. Recently I rebiased a Bogner amp that had Mesa Boogie 6L6 tubes. The change in sound was unbelievable. Generally I find that fixed bias amps have too low of a bias for the tubes in them - an effort to make tubes last longer. You may have to have a bias kit installed in these fixed bias amps to allow bias adjustment. The power tubes are the biggest tube in your amp chassis. I'll generalize some tone suggestions with the tube types. You'll see that if your chasing a clean sound a 6L6 is probably not your best bet. Want a super distorted sound? A 6V6 isn't going to easily get there. I'll cover some of the more common tubes you'll encounter but remember that even in every tube type manufacturers can change designs to try and gain certain sounds. Don't mess with power tube sockets - lots of voltage is stored there even when an amp is turned off.
6V6: The most common tube in classic Fender amp designs. Clean sound that can be driven into nice overtones without creating much distortion.
EL84: A great tube for clean sound. Easily driven into overtones. This is a popular tube with many boutique amp builders for its great sound. With the right preamp section this tube can reach some nice distortion. Many low power amp designs in the 15 to 50 watt range feature these.
EL34: The tube in many classic Marshall amps. Pretty good clean sound and wonderful distortion. Another fav of boutique amp builders.
KT88: When EL34 supplies got short Marshall changed to KT88. Similar sound to the EL34 with more distortion on tap.
6L6: Capable of very high distortion levels. Clean sound can be a little brittle when driven hard. This is the modern distorted sound while the EL34 is the classic sound. Found in high output amps generally each 6L6 equals 25 watts of power. Four 6L6 tubes is the common design for 100 watt amps.
Conclusion
I hope you've learned a little about your amp. Tubes aren't cheap but getting that tone you seek is a wonderful experience. Remember that even though an amp features certain tube types it may surprise you to its tone output. Before you change any tube make sure the amp is off and the tubes have cooled down. I generally change the power tubes in my amps once a year. At the same time I clean the pots (knobs) and sockets. Knowing a little about tubes will lead you to an amp that has a better chance of nailing that sound you are looking for. Making sure those new tubes are biased right makes sure you get the most out of them. Usually this means a pro should do it but a handy do it yourselfer can handle this job. Soon I'll cover biasing an amp in a future article.
If you are still not happy with your sound research your specific amp to find specific mods that may help you achieve the tone you are looking for. Changing out a few resistors can go a long way to a great sound and even if you are going to pay someone to do the mods at least know whats involved so you know what to expect with the finished job.
Lastly, if your amp can't reach those great overtones and you have four power tubes try pulling the two outside tubes. Most designs allow for this to work and you've cut your power effectively in half. This trick won't work with a two power tube design and some four power tube designs won't work with this trick either. If you can do this trick you can raise your volume to get to better tone. If this works for you keep in mind that the resistance for your speaker is now doubled. If your amp put out 8 ohms before it is now putting out 16 ohms - set your speaker(s) accordingly. This can make a 100 watt amp into a 50 (or close to) but even more importantly it can turn a 30 watt tube amp into something like a 15 watt amp. Now we're talking about that small amp that can reach great tone easily.
