The main difference in these preamp tubes is the gain characteristics of the two. The preamp tube is extremely important in the signal path. This is because it is the first point the signal from your guitar is amplified.
If you want a preamp tube that is suitable for clean sounds, then it is important to consider low gain tubes which have more headroom to them and are harder to overdrive. There's something about the intensity of the sound coming from preamp tubes that allow you to obtain your desired tone without losing any of the original essence and character of what made your guitar sound so good to begin with.
EL34 tubes are often loaded into British amps such as Marshall, Orange, and Vox. They are therefore often associated with the “British sound”. This means the mids on EL34 tubes are often scooped. Scooped mids are common amongst heavy genres as well as being preferred by many blues artists.
In most cases, EL34 tubes are easier to overdrive than 6L6 tubes. This is not always the case. You’ll need to take a look at the output of each tube to figure that out on your own. You can check out the reviews of the Starbright Music Collectives’ favorite EL34 tubes if you’re not sure which to get.
6L6 tubes are well known for creating the sound of American Rock’N’Roll. You will find them in a large variety of Fender amps as well as others that emulate that distinct sound. So, if you’re thinking 6L6, think Fender tone. Simple.
6L6 tubes are available in a wide variety of wattage and resistance. They are able to create an awesome crunchy tone in some models but are also capable of a crystal clear clean tone. This fact has made them popular for other uses such as stereo amplification too.
There is no “better” when comparing these two. Generally speaking, you will want to get a tube that matches your amplifier lest you risk changing the overall sound of the amp. If you know exactly what you’re doing, this could be a good thing but in most cases it doesn’t turn out great.
6L6 tubes generally have a lot more headroom than EL34 tubes. So, if you need a clean guitar tone or are replacing a tube in a stereo amp a 6L6 tube is a much better option.
The socket pattern on both of these tubes are the same. This means they can be plugged into amps that cater for the other type. Yet, just because you can do something doesn’t mean you should.
Given that these are two of the most common types of tubes for guitar amps, they are widely available on the open market. They also have a lot of NOS tubes that you may want to get your hands on.
The standard bottle shape for these tubes is also similar, as is their size. You can, of course, get some that break the mold in both types of tube. This will provide a varied look and a slightly different sound.
In most cases, you can! This is because they have the same pin-out configuration. But, that doesn’t mean it’s a good idea. You will compromise the overall tone of your amplifier if you change out the tubes to a completely different tonal profile.
This can result in a unique sound if you pull it off but do so at your own risk. You may find that on rare occasions replacing the tube with a different type can cause issues with your amplifier too. These issues can include overheating, blowing fuses, and terrible sound. Personally, I wouldn’t bother but each to their own. They are reasonably cheap and easy to replace so you could always give them a try.
NOS tubes are popular for a reason. The thought process is that they were built better in the past than the current tubes that are available now. They are a lot more expensive though and there are plenty of decent newly produced tubes available. If you stumble upon a good deal on a NOS tube go for it. I wouldn’t be losing any sleep if you can’t find the exact tube you’re after though.
Still unsure? Check out this detailed look into these tubes by MesaBoogie.