Preamp tubes are the tonal signature of "your sound" and interchangeable without adjustment or the need of an amp tech. Unlike power/output tubes, which are routinely matched when they are sold (in different ways, some much better than others), preamp tubes are usually only tested at best to: (a) make sure they work and sometimes (b) test to assure they are not microphonic. In testing, we have found that some suppliers don't seem to test their preamp tubes at all. They just figure folks will send them back under warranty. There are good vendors out there that I continually mention. Most vendors figure that is cheaper to just send them out as they get them in, and if there is a problem, it is cheaper to just give the customer another tube when they complain. This is of little comfort to somebody that either has to make another trip to their music store, or worse, box up the bad tube and ship it back to the supplier, and then wait for its replacement. This is one reason to consider a proven supplier when you buy preamp tubes.
Today's amplifiers, whether modern high gain types or boutique amplifiers, have one thing in common; the preamp tube in the first gain stage (usually V1 and / or V2) sets the tone and initial gain structure of the amplifier.
Amp design -
Most guitar amps whether vintage or modern get just about all of their basic characteristics in the preamp section. How the gain stages are set up, how the EQ is set up, gain structure, and tone stacks, all are the main aspect of the sound character of the amplifier.
A bit about output sections and their tubes. Amps such as Mesa Boogie, Fender, Marshall, Bogner, Peavey, and others, all use the same Sovtek, Svetlana, JJ, Electro Harmonix, and other tubes from the same factories. In spite of the same output sections, and in many cases the same range of B+ voltages on the plates of the output tubes, these amps sound different. This is all because of different designs, primarily in the front end, or initial gain section of the amplifier.
Todays newly made preamp tubes are very inconsistent compared to the tubes from of the 1940 through the late 1960s. This was the golden era of vacuum tubes. If you are an end user (not an amp manufacturer) you may want to consider NOS (new old stock) tubes for your personal amp. They sound better to my own ears and will last decades. In the end they are a better value than a tube that may last a few years or even fail within a few months that may look less pricy up front. People that build amps need a continual reliable tube supply so this problem of poor tubes made today hits these folks very hard when they try to produce a quality product. As a side note, I pull tubes to test all the time from amps as old as I am and they generally test better than the vast majority of new production tubes that are fresh out of their box.
I continually test current tube production. I did this on a daily basis for six years for Groove Tubes and continue to do this at http://www.65amps.com/ where I select tubes for their amplifiers before they are shipped out the door to their customers. The deluxe revised edition of The Tube Amp Book with the hard cover has my findings at one time on batches of over 100 tubes from the Electro Harmonix 12AX7EH, JJ ECC83, Ei 7025, Sovtek 12AX7WA, LP, LPS, Chinese 12AX7C (old tooling and new tooling), and a few others.
Basically, the standard 12AX7 spec that applies to 12AX7 / ECC83 / 7025 tubes, has a reference of 1.2 mA at 250 volts with a 2 volt bias. This is standard design spec used by RCA, GE, Sylvania, Mullard, Telefunken etc.
Some people like to use those little reference point which say thinks such as, if you want less gain than a 12AX7, use a 12AT7, as it has only 70% of the gain of a 12AX7 etc. These little tips are cute, but with the wide range of inconsistency out there, they are not all that useful, as it is still a matter of chance. The 12AT7 has a different current output than a 12AX7, so if you are just looking for less gain, then you may, or may not get it with just a different 12AX7, even from the same brand, same date code, and same batch ? just by swapping tubes around already in your amplifier. With todays inconsistent offerings, the old tables of gain cannot be used with much accuracy. As a side note, a 12AT7 is a terrible tone generator. It is better used for phase inverters or reverb drivers. If you want the gain of a 12AT7 in a front end stage use an NOS 5751 which has the same gain structure but is a much smoother tube. SRV used these by the way in some of his amps front ends. Another great lower gain tube is the 12AY7. This is the front end tube of the Fender tweed deluxe and tweed bassman.
Back to testing: In the tubes we went through, keeping in mind our 1.2 mA (plate current) / 1600 transconductance industry standard spec, we found our samples ranged from 0.5 mA to 2.6 mA. The plate current is all over the place as is transconductance (TC) / mutual conductance that ranged from 800 to 2500. Most often the TC was 30% below design spec on average. This makes an amp sound a bit tired or just lackluster. Sustain suffers as does note definition and articulation. Bottom line here ... preamp tubes are a crap shoot, a spin of the roulette wheel and the odds are NOT in your favor. What can you do? Know and trust your vendor.
Some of you have taken an old favorite amp that has all the original tubes it in and thought it would be a good idea for a complete retube. You find that when you are done retubing the amp sounds worse, feels worse and is less dynamic than it was with the old original tubes. You sit dumbfounded, how can this be? Chances are the "new" tubes you put in your amp were down 25% - 40% compared to the tubes that were in the front end already. This is very common in regard to preamp tubes. Power tubes are a different matter as they are generally matched and have known specs by good suppliers. Be sure to keep in mind that the phase inverter is the tube that "pushes" or drives your output tubes and this preamp tube is one of the most critical tubes in an amplifier. There are a lot of folks that believe you can toss anything that works into the phase inverter position and it does not matter. I strongly disagree and have many clients that know why I feel this way and have collections of long and short plate tubes that are numbered for phase inverter use.
You want even MORE GAIN from your Triple Rectifier or Bogner? Look at those first gain stage preamp tubes, and get some tube vendor to measure them for you. If you have a 1.1mA in there, and put in a 1.3mA, you will hear the difference in gain IMMEDIATELY. This is not a subtle change that only the "experts" can hear. Leave the settings on the guitar and amp the same, swap the tube, and listen again. In the above example there is a 20% or so change and this is pretty dramatic. You can also measure other parameters for your testing such as conductance as long as all factors are known; transconductance, plate current, plate resistance and you can calculate true gain based on transconductance and plate resistance. It is not all that difficult to go through the front end of an amp and find the amp being "off" more than 30% and resolving this issue.
When we see a transconductance of 1100-1400 versus the 1600 design target, the way the tube reacts is different too, in this case, its rise time is can also be about 25% slower. This might be just the ticket for a blues player, looking for some nice initial compression on the pick attack, but it may not be the sound for a metal or speed player. Looking at rise time requires special equipment which is not all that common. Over at 65 Amps they often select tubes with a specific rise time requirement when developing a new amp and want to experiment. As a side note in regard to the 65 Amps folks they know what the specs are of each tube placed in one of their amps when the amp ships. It is not a crap shoot in their shop.
Transconductance measure in one good batch of tubes with "tight specs" ranged from 1060 to 1790. 1600 is the industry standard. This was considered to be a good batch of tubes by the expectations of today. I think if you check with most amp makers they will tell you they feel things are getting worse in most cases. There are some nice tubes out there but vary from one run to another. Just when your hope is lifted by a few great runs it is taken as a batch comes in that is not even usable.
Your first gain stage in your amp is its soul, sound, and character. We talked here about gain, and a little about rise time, which is a subject in itself. We did not get much into "sound", such as the articulation and definition that comes from NOS tubes like the Mullards and Telefunkens. There is no right or wrong in preamp tube tone. It is all personal taste and preference. A good rule of thumb is old tubes (NOS) will probably sound better to most folks and will always last longer than the majority of current production tubes.