Preamp Tubes – most overlooked tubes in your amp

Preamp Tubes – most overlooked tubes in your amp

Unlike power/output tubes, which are routinely matched when they are sold (in different ways, some much better than others), preamp tubes are tested at best to:  (a) make sure they work,  (b) they are not microphonic.  In testing, we have found that some suppliers don’t seem to test their preamp tubes at all, as we have found one side of the triode that is dead at times. Since most warranty preamp tubes for up to 6 months and longer, they possibly 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.  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.

Amp Design

Today’s modern amps get just about all of their 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.

Amps such as Mesa Boogie, Fender, Marshall, Bogner, Peavey, and others, all use the same Sovtek, Svetlana, JJ/Tesla, Electro Harmonix, and other power 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.

Inconsistencies

Today’s newly made preamp tubes are very inconsistent compared to the tubes of the 1940s to 1960s.  There is little need in the medical sector or the military sector for tubes.  They are primarily used today in audio applications. For the high end audiophiles, their needs are more easily met, as their tubes are not subjected to the same stresses as those on a guitar amplifier, they use less of them, and they last much longer.  There are high end audio suppliers that will match tubes and hand select them, at much higher costs (check out a Western Electric 300B matched pair for example).  They pop their tubes in, and ten years later, all is still just fine.

Tubes for the guitar and bass player for use in the preamp section, are a different story.  The tubes today are very inconsistent.  You contact your local tube supplier, plunk down your money, and the roulette wheel is now set into motion.

To show the inconsistency, we went through a batch of over 100 tubes that were from the Electro Harmonix 12AX7EH, ECC83, 7025, Sovtek 12AX7WA, LP, LPS, Chinese 12AX7C (old tooling and new tooling), and a few others.

Some people like to use those little references that say 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 capacity 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 today’s inconsistent offerings, the old tables of gain cannot be used with much accuracy.

In the tubes we went through, keeping in mind our 1.2 mA / 1600 transconductance industry-standard spec, we found our samples ranged from 0.7 mA to 1.6 mA.  When you take into account, that the amplification factor of a 12AX7 is 100, there is a dramatic difference in these tubes.

Looking at a 1.6 mA tube, we see a factor of increase over the standard of 33%.  This is a LARGE number.  A 1.0 tube versus a 1.2 tube will turn the gain you loved in your 5150 into something less than what you used to know what you liked there.  You sit dumbfounded –  how can this be?  I just put in new tubes, the same as what I had before?

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 1.1 in there and put in a 1.3, 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.

When we see a transconductance of 1200 versus the 1600, the way the tube reacts is different too, in this case, its rise time is 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.

There is one other aspect of preamp tubes.  Unlike power tubes, where one tube is one tube? a preamp tube is two tubes in one bottle.  There is an A side and a B side.  The are independent units sharing only the heater.  In a Marshall amp as an example, the NORMAL channel input 1 uses one side of the tube, and input 2 (lower gain input) the other side.  The BRIGHT channel uses the other side of the preamp tube.  BUT, anytime we use that tube in the phase inverter position or driver position of the amp (which is the driver for the power tubes), then having the two sides matched is important.  This matching subject has been covered before, so I won’t elaborate on this again here.

New versus Vintage amp needs

NOS tubes are sought after by folks that have original amps like Fender Tweeds and the like.  If you want the original sound, feel, and character of these amps, then NOS is about the only way to go.  Getting NOS tubes for your amp to be correct is much easier in some ways than getting decent new tubes for an amp.  There are folks that deal with NOS tubes.  Some of them are on my website, but I will say here, that I can recommend KCA and Tube World very highly.  Either of these folks have the tubes and the equipment that will let you know that when you purchase a Mullard or RCA 12AX7 or whatever, it?s characteristics will be noted for your information. If you are in Europe, check with Watford Valves, as they also have NOS offerings.  Eurotubes in the USA, a JJ importer, also may have some NOS offerings.

When it comes to new tubes for you modern or class amp, or new boutique amp, there is the problem.  Preamp tube suppliers guarantee the tubes to work, and not be microphonic.  That is about all they can do.  Going though tubes that retail for less than $20 in most cases, one at a time to measure them, is beyond reason economically.  Watford Valves will do this for folks. Other folks offer these services, and in most cases, they are an additional cost.  In my mind, the money is well spent.  When you want a nice high gain
tube for your Rivera or Demeter, putting in a tube that is 30% down from spec., is not the ticket!  At that point, what can you do with that tube?  Take it back?  Why?  It works.  The store or vendor never stated it would do anything more than “work”.  Perhaps they will exchange it, and now you start the process over?  And over.  And over.

Recently I was working with a fellow named Tom Dunn.  He plays a 5150 II. We performed a blueprinting session of his amp, and it was found after going through maybe 2 dozen of his own tube stock, that he had picked out his tubes by ear, and placed them in the most advantageous position in his amp.  You can do this by ear, if you have the ears of this guy, and also the time (he did this over many months), and the tube stock.

Conclusion

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 Telefunken?s.  If you have an older amp with a more moderate gain structure, and want it to sound closer to magic, than this is the way to go.  In a modern amp, a lot of the articulation from the output section is not the target of these designs.

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