The most critical, least expensive, most overlooked tubes in your amp.

Loni Specter on the left of http://www.ampshow.com/

I picked the photo above of myself and Loni as the 6th annual Los Angeles Ampshow is coming almost here, October 2,3 of 2010 in Van Nuys California.  I will be there as always and will once again be the moderator on the Tone Wizzards panel.

With the ampshow on the horizon I felt that posting something from another side of my life might be a nice change.

Preamp Tubes - The most critical, least expensive, most overlooked tubes in your amp.

The tonal signature of "your sound" and interchangeable without adjustment or the need of an amp tech.....

 Many folks may find this interesting.  It is a good place to get started:
The Phase Inverter.  Most important tube in your amp?
http://www.guitaramplifierblueprinting.com/files/Phaseinverter.pdf


Moving on:  Unlike power/output tubes, which are routinely matched when they are sold (in different ways, some much better than others), preamp tubes are generally tested at best to: (a) make sure they work ? (b) they are not microphonic. In testing, I have found that some suppliers don't seem to test their preamp tubes at all.  At times I have even found one side of the triode dead.

Most folks have some sort of warranty.  Most warranty preamp tubes.  Some warranties are for long periods which on the surface look like a good deal.  Many sellers figure that is cheaper to just replace one tube with another if folks had problems. 

In many cases on less pricy tubes there are many buyers who won't even take the time to pack them up and ship them back.  Groove Tubes had a great warranty and would pay the new tube shipping back to the customer.  Unfortunately the customer had to pay for their own shipping when they sent the bad tube to Groove Tubes.  Their CFO never understood the math here even though Rick Benson tried to explain it to her year after year. 

Many tube folks just send an untested tube 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.

Over the years I have locked on to some great tube vendors that test well, have nice prices and and honest and good people to work with for tube needs.  I have mentioned some of them in various places and some of them are listed on the right side panel in this blog.

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.  This applies in most cases even in multi gain stage amplifier designs that use two, three or even more tubes in the gain stage or amps which are multi channel amps.  It all has to start somewhere unless the imput jack signal path is switched to send the path to more than one initial place.  This is generally not the case.

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.

Another way of looking at this is from this angle. 

Some folks like say they like 6L6 amps.  Are we talking about a Mesa Boogie Dual Rectifier with a quad of 6L6 output tubes or a Fender Twin Reverb with a quad of the same output tube?  Very different amps with nothing in common other than the output tube which is the same tube as used in B52 bomber radios which is nothing like either of these two amps as well.

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, I continually go through batches 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.

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.

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 trying a different 12AX7, even from the same brand, same date code, and same batch.  Just by swapping tubes around which are already in your amplifier. With today's inconsistent offerings, the old tables of gain cannot be used with much accuracy. 

As a side note or tip, a 12AT7 is a terrible tone generator.  These are best for reverb drivers and phase inverters.  If you want something in this gain range in the front end of your amp procure an NOS 5751 or try a 12AY7.

In the tubes I test, keeping in mind our 1.2 mA / 1600 transconductance industry standard spec, samples rangee from 0.5 mA to 2.67+ mA in regard to plate current.  Plate current is generally not measured by most folks that test tubes as it requires pricy equipment such as an RCA WT-100 or something other than the typical transconductance testers. 

Excessive plate current is found on tubes that are of improper design.  The highest plate current I have observed on 12AX7 tubes were on the Groove Tubes 12AX7M tubes made by the Shuguang factory in China.  The excessive plate current was due to plate resistance that was half or less of design spec.  The end result here was a tube that had very low gain, many times less than 1/2 of spec and the tube ran very hot in in amps.  The low plate resistance also allowed the two sides of the tube to crosstalk.  This did not do a lot of good in some amps such as Fender tolex era amps where one side of the tube was gain and the other side was the tone stack element.

Plate current is important in other ways.  We expect 1.2 mA in a spec. 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.  The amp just does not run at it's potential.  Sure it works but there is no magic.  You sit dumbfounded.  how can this be? I just put in new tubes, the same as what I had before? 

When I blueprint amps with a client I have a spreadsheet that I use where all the tubes are documented before work begins.

Doing a bit of blueprinting with Carl Verheyen ( www.carlverheyen.com ) on his
1964 Fender Twin Reverb

Before a single note is even played through the amp I can generally tell the player what we can expect to hear and what is wrong with the amp.  If they tell me they used to love the amp I can generally tell them what they used to have that they lost.  There is a number that I expect to see and the number that I do see on the bottom line which is a compilation of all the tube measurements.

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 900-1200 versus the 1600, the way the tube reacts is differently too. In many cases this also corresponds to rise time being 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. 

This is not an absolute rule as some tubes such as long smooth plate Ei tubes have low transconductance and low plate current but due to very high plate resistance, over 80K, can have very high actual gain in circuit.  These were the highest gain production tubes out there a few years ago.  The very high gain made them more prone to physical microphonics unfortunately but when you had a good one they were great. 

Transconductance in I have measured in testing ranges from less than 750 to 2800+. 1600 is the industry standard.  This figure shows that untested preamp tubes are a total crap shoot but you can use this to your advantage to make adjustments to your tone and response in your own amplifier.  Many of my clients have tube sets with numbers on the tubes that they swap in and out as their needs dictate for a given project or show. 

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.

There are also tubes used as preamp tubes such as the EF86/6267 in a number of amps.  This is a small single pentode. These are not interchangable with a dual triode such as a 12AX7.

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 the right panel of this blog as I mentioned before.

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 Telefunkens. 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. Today's designs look for two or three or more stages of gain, channel switching which we did not have on the older amps of yesterday, and flexibility. The only flexibility we had when I was the age of most of you, was a high gain input and low gain input or tuning the reverb on or off.

My advice is try to find a tube vendor that can supply you with the tubes you need with some degree of classification. This way, if you have a 1.3/1670 tube in there now, and you want to tone it down a bit, then maybe go for a 1.1/1400 range tube. 

If you want tonal changes in color, rather than gain and compression, then you want to go with a little stash of tubes, depending on your use for the day or evening. Most of my clients keep a number of tubes on hand.

If you are in Southern California and want to know and see what some of your own preamp tubes are actually doing feel free to drop me a note.  I would be happy to arrange a meeting to test and trace your tubes at no charge (not hundreds but a handful).  I am located in North Hollywood in the 65 Amps complex.

Myles Rose is the founder of Guitar Amplifier Blueprinting which provides support services and training for touring bands and individuals as well as blueprinting services.   I offer consulting services to various amplifier designers and manufacturers.  The link www.guitaramplifierblueprinting.com has links to music industry forums which I moderate and direct email contact information.  My shop is located in the complex at www.65amps.com.  If you wish a tour of 65 Amps please feel free to contact me directly.  When I am not doing something related to music I do what I can to support the homeless of Los Angeles either directly on the streets at night or through my good friends at Union Rescue Mission - www.urm.org