What is the most important tool for making any decisions about musical gear?

Your ears! (Well, technically the answer is your brain...)

Is there right and wrong when it comes to tone?

There is absolutely no right or wrong when it comes to tone. The right tone inspires you to make music, excites you to pick up your instrument. Even for those of us who gig, honestly, the gear we use matters very little to an audience. The differences between pedals and amps that we brood over for untold hours simply don’t change the way most listeners experience the music.

We have all been to a show where the guitarist had an amazing sound of course, but, really, the relentless pursuit of tone is about us, the players ourselves. The right tone for each of us makes us play differently, makes us play better, and feel more connected to the music. And this the audience definitely experiences.  At Soundfelds we believe the quest for tonal bliss leads to better and more passionate music, and we can’t think of a better reason for the pursuit of tone. 

 Is more expensive better?

Not necessarily! (see first question above) Everything matters in a system that reproduces sound. With HiFi gear you want the rig to reproduce the recordings without coloring the sound, but with guitar gear, coloring the sound is the point. So it all comes down to YOUR ears and how you want to hear the sound of your instrument. We have seen vintage Fender amplifiers that sounded terrible and a look inside revealed that many of the stock parts had been replaced with high end modern parts. Returning the amp to its original specs made it sound good again. Its important to understand that the vintage amps we all love were made largely with stock off the shelf parts, and often the cheapest parts that could be found. Component values also drift over time, and the are some very knowledgeable techs out there who believe component drift accounts for why some vintage amps have a particularly magical tone. A guitar amp is a complex system, and the tone comes very much from the way all the individual parts interact with each other much more than any individual part. So more expensive parts are not necessarily better for a guitar circuit, it just depends on what your ears want to hear.

Are NOS tubes better than new tubes?

Again, see the first question (do you see a theme developing herein?) The main thing with NOS tubes is that the largest consumer of them 60+ years ago was the military. We all know that the term "mil-spec" refers to a part that is very solidly made because the military required quality. The military wouldn't accept a part that was made more cheaply to save some money because lives could be at stake. So NOS tubes are generally very well made tubes. But that's not the whole story. For years there were a lot of NOS tubes available and you could be pretty confident buying NOS tubes from a variety of sources. Now, though, as supply has dwindled, much of what shows up on the market are the tubes that would have been rejected 60 years ago, seconds and odd out tubes. So its become difficult to buy with confidence, and that's why Soundfields is very particular about where we source NOS tubes, generally buying from Beacon Tube Amps, KCA Tubes, or McShane Design.

There is, however, good news about new tubes. The 1990s was a particularly dismal time for tubes, but the worldwide demand for guitar and hifi amplifier tubes made the makers step up their game after the dark ages of the 90s. These days new tubes are very strong, and we happily install tubes by JJ, Tung Sol, Mullard, Genelec (the later three re-issues, now made in Russia), and others. Everyone has their favorites, so always go back to your ears.

NOS tubes also make less of a difference in some amps. For example in the XITS X10 we have not found much improvement in the tone with NOS versions over the Tung Sol and Mullard re-issues that are stock. But with some Fender handwired circuits we have indeed heard what we consider to be a noticeable improvement. So we can’t make a blanket statement about NOS tubes, but often favor good NOS tubes from a reliable source (if you can afford them that is). 

How important is it to have matched output tubes?

We believe it is worthwhile to use matched output tubes, but also consider that in the Golden age of Fender they never worried about this. The tube tolerances were much tighter 60+ years ago so tube matching was not as necessary. As the quality of tubes dropped however, and up to today, matching became more important.  BUT, so-called tube matching by tube re-sellers may not be true matching. First of all, tubes drift over time with use in an amplifier, so if the matching was done without a fairly extensive burn-in time (a few hours), the tubes may not remain matched in your amplifier. Plus tubes should be matched for both idle current draw (for DC current balance) and amplification factor (for AC signal balance) and many tube suppliers only match for one of those. If current draw and amplification factor are too far apart with a set of tubes its possible to loose some low frequency content in the output of the amp, and unwanted noise may also be induced in the output transformer. 

That said, some techs believe unmatched tubes sound better because they don't cancel even order harmonics from the output stage (this does not affect the even order harmonics generated by the preamp stages). 

Do I need to re-bias the amp when I install new output tubes?

First of all it depends on whether your amp's power tubes are cathode biased or grid biased (all preamp tubes are cathode biased). A cathode biased amp is designed so the power tubes will self-bias, whereas a grid biased--or fixed biased--amp will need to be re-biased when the output tubes are changed. Setting the bias is an essential part of allowing the amp to perform at its best. If the bias supply is too low your amp will sound lackluster and will distort early (though not a good sounding distortion), whereas a bias setting that is too high can exceed the tube's max plate dissipation which can burn up the tube. 

Of note, there are incorrect bias settings (too high or low) but no correct bias setting. In other words there is a range in the middle, and you can adjust to your ears within that range.

By the way, If you need to replace your output tubes at a gig and don't have the option to check the bias, replace with the same tube model and when you turn on the amp make sure you don't see a vibrant red glow in the middle of the tube indicating the plates are in the process of melting (which we call red plating). Then have a tech check the bias later.

Can I a substitute different preamp tubes?

Most noval (9 pin) preamp tubes have the same pin out, meaning all the internal components of 12A_7 tubes—also known by other names—are connected to the same tube pins, and may be safely substituted. However, we believe there are some general guidelines for achieving good results, because even though the parts of the such tubes are connected to the same pins, the internal geometries of different models can be quite distinct.

Most of what you read about preamp tube substitution focuses on the amplification factor of the different tubes (also called gain factor). The amplification factor of a tube represents how many times the input signal will theoretically be multiplied by the tube. Theoretically because the number reflects what the tube is capable of but which will vary a little in different circuits. A 12AX7, for example, has a stated amplification factor of 100, meaning the output signal from the tube is specified to be 100 times greater than the input signal, but in your amp it may really only amplify the signal 95 times.

However, you can’t just consider amplification factor. Different tube models can draw different amounts of current and have dissimilar bandwidth due to inherent differences in internal geometry. 

Follows is a chart that shows the bandwidth, plate resistance, current draw, and amplification factor relationships between the principle preamp tubes:

You can see that there is more to the picture than simply considering amplification factor. For example, a 12AX7 has a narrow bandwidth of 10,000 hertz, which works well for the guitar’s frequencies, and only draws 1.2 mA of current. Whereas a 12AT7 has a much higher bandwidth and draws more than 80 times the current. This can make an amp sound pretty sterile, even harsh if a 12AT7 is substituted in, say, the first gain stage. But it also is a very hearty tube that can work very well as a reverb driver, which is a tube that takes a pounding in an amp.

At Soundfields we recommend the following preamp tube substitution guidelines:

  • For the first gain stages, if you want to back off the gain generated by a 12AX7 in that stage, try a 5751, which has an amplification factor of 70 (verses the 12AX7’s 100) but is very close to the 12AX7 in other respects
  • For even less gain go to the 12AY7, with an amplification of 44, but not nearly as far from the 12AX7 in other parameters as the 12AT7 or 12AU7
  • a 12AU7 will dramatically change the amplification factor (at 17), but has 9 times less plate resistance and a lot more bandwidth, which may not work for your guitar sound, but can work nicely if you are trying to amplify an instrument other than a guitar (a harmonica for example)
  • a 12AT7 is a great tube for the reverb circuit of an amp 
  • a 12AT7 may work well as a phase inverter if the amp has a 12AX7 and you want to hit the power amp section with a little less signal.
  • Conversely a 12AX7 can be substituted for a 12AT7 phase inverter tube if you want to hit the power amp section with more signal

Of course, you may find that with your particular set up you like a 12AT7 or a 12AU7 in the first gain stage, which is totally okay! Remember always return to your ears and what you want to hear.

Can I substitute octal pin rectifier tubes?

Maybe.... Rectifiers are not as simple as preamp tubes because in some cases you can damage your amp. The main octal rectifiers have the same pin out (5Y3, 5V4, 5U4, 5AR4) but changing them will shift the B+ voltage in the amp (the DC voltage that is supplied to the tubes). If you have a 5Y3 in your amp and you change that to a 5AR4, it may be okay as long as the increased voltage is not a problem. They both draw a similar amount of current, so the main concern is the additional voltage. In some cases, such as the Deluxe Reverb, which uses a 5U4, changing to a 5AR4 can be a big problem because the plate voltages are already very high in that amp. Changing to a 5U4 can also be tricky because a 5U4 draws more current than the other tubes, so your power transformer must be able to handle the additional current. We generally recommend you talk to a tech before changing the rectifier. There are simple ways to determine if a different tube will work in a particular amp, but they involve opening up the amp which can expose you to lethal amounts of electricity. In other words, if you play around inside an amp, whether it is on or off, you can kill yourself if you don't know what you are doing... Seriously....

What is the difference between class A and class AB? 

Another common question, and its partner question: are cathode biased amps class A? First the later… No! The biasing scheme of an amplifier does not determine its class. Tubes have an operational range, with two extreme points, cut-off and saturation. The cut off point is where there is not enough signal to allow current to flow, and saturation is the max current that can flow, so no additional signal can increase the current. The tube’s operating range is between the two. Now, class A means the signal never pushes the tube into cut off or saturation, and it conducts the entire 360 degrees of the AC signal. The classic class A amp is a so-called single ended amp with only one output tube. With only one tube, if the signal ever goes into cut-off or saturation, that part of the signal is completely lost. With class AB, each side of the output section drives 180 degrees of the signal, so you can let one side fall into cut off while the other is conducting. This also allows you to get more power from the output section than with class A because you can use higher plate voltages and a stronger input signal.

Do I need a buffer for my pedal board?

This is a question we get asked often. And its partner question: What's the deal with true bypass pedals?

First, the true bypass question… It simply means that when the pedal is off, the input is hardwired to the output, essentially adding only some wire to the signal path. However, when you have a number of pedals strung together, that can add a not insignificant amount of wire to the distance from your guitar to your amp (think of the signal path through the interconnect cables, the switches and jacks, the wire to and from the switch, etcetera). 

This alters the tone, which may or may not be a bad thing depending on your needs. It is generally accepted that when you have more than 12 - 15 feet of cable between your guitar and your amp (including all the wire and cable in your pedal set up), the tone is audibly affected.

Without getting too technical, your guitar pickups and your guitar cable and the pedal or amp that you plug into first, form what is called a RLC circuit, which is a circuit that has the electrical properties of resistance, inductance and capacitance. Its not so important to understand an RLC circuit per sea, but the principle is that the circuit is essentially a low pass filter. This means that 100% of the signal’s frequencies will pass up to a certain frequency, after which higher frequencies will gradually roll off until none pass. 

The main thing to understand is if you feel that your tone is lacking in high frequency content, if it seems muddy, then the point at which the trebles start to roll off may be too low for your ears. So what can you do about that?

Well, you can't do much about your pickups without changing them, but cables have capacitance, so you could shorten the cable length. The shorter the cable, the less capacitance, the higher the high frequency roll off will be (meaning, all other things being equal, a brighter tone). But if you have a bunch of pedals, the only way to shorten that part of your signal chain is to remove pedals, which you may not want to do. 

Another solution is to use a buffer, which is really just a unity gain amplifier that electrically isolates your guitar signal from anything downstream, and takes the high impedance guitar signal and converts it to a low impedance signal that pedals like to see. So, the effect of a buffer is to make your amp think the cable from your guitar to the buffer is connected directly to the amp input. This extends the treble roll off point and allows more high frequencies to make it to the amplifier, giving you a brighter tone, which can also seem to have better clarity.

So, to answer the question of whether you need a buffer, listen to your tone and decide if you like it. If you feel like your tone lacks some definition on high frequency content, or if you tone seems a little muddy to your ears, then it might be worth exploring a buffer. If you love your tone as it is, then you don’t need a buffer. There is NO right or wrong here!

In fact, you can even use the electrical properties mentioned above to your advantage, to tame a bright instrument for example. Buddy Guy uses something like a 50 foot cable from his strat which definitely rolls off treble frequencies, but he likes the tone that gives him. I have also seen guitarists playing Trainwreck amps with single coil pickups use very long cables to “tame the high end.” 

Caveat one: There are some pedals that don’t like the low impedance signal that the buffer sends out, such as certain fuzz pedals. If you get a buffer and you don’t like the way it changes the tone of your fuzz pedal, simply put the fuzz pedal before the buffer. Depending on your pedal set up, you may have to explore putting the buffer in different places in your signal chain to achieve the tone you want. Remember there is no right or wrong, just experiment until you find what you like!

Caveat two: some pedals have a buffer built in, and some have the option to have that buffer on or off (check manufacture specs). If you have such a pedal, its okay to have a second buffer in your chain. For example, if you have, say, a Strymon Timeline and the buffer is on, but it is last on your board, after a 12 foot cable from your guitar to the first pedal, and a bunch of pedals between that and the Timeline, you may benefit from having a buffer closer to your guitar cable. You then might explore turning the buffer off in the Strymon, or you may prefer it on. Having multiple buffered pedals will effect your tone, but its not going to do anything untoward. Just play around with your rig, try different things until you are happy with the tone. That's the bottom line, getting a tone you are happy with. 

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