What is Dual and Triple Recitier?  Why does Mesa Boogie and others use them and how do they affect Tone?

Dual Rectifier...Triple Rectifier...the names roll off the tongue nicely and sound cool.  They could be names for action movies, assault riffles or motorcycles. What are they and why do companies use them?  In this article I'll define what a rectifier does, the history of dual/triple rectifier uses and how they affect tonality in guitar amps. I think most players will be surprised.

What is a rectifier?  

A rectifier is simply an electronic device that converts AC voltage to DC voltage.  All amps run on DC voltage so all amps have a rectifier to convert the AC voltages coming from the power transformer to DC voltages your amp circuit can use.

Historical Context

Tube Rectifiers

At one time all rectifiers were tube rectifiers.  Tube rectifiers were found in just about any vacuum tube-based product in fact in almost all electronics. TVs, radios, amps, transmitters etc. all had them.  Tube rectifiers were far from perfect and were limited in two important ways:

1.  They have an internal impedance which causes sag under high current draw (like when an amp is cranked up and you hit a note). This is important to guitar amps because some players desire the sag of a tube rectifier.  The bottom end is looser and the tone more singing when cranked up. They add pleasant compression to the sound.

2. Tube rectifiers are limited in how much current they can deliver.  This is why you rarely see amps with tube rectifiers over 40 watts. This also limits how much filtering can be done in the power supply. Many older amps had very light filtering which can lead to problems in some cases.

3. Tube rectifiers have voltage drop across them also affecting the sound. 

Solid-State Rectifiers

Later on, solid state rectifiers and diodes (the parts that make up a rectifier) came along.  At first solid state rectifiers were more expensive but quickly became much less expensive than tubes.  They have little or no sag and can provide lots of current and have no voltage drop. Additionally, they are far more reliable.   The result is a firmer sound and better reliability.  Since they don’t have any inherent sag amps sound less compressed with a solid state rectifier.

What are Dual and Triple Tube Rectifiers?

Before solid state rectifiers came along there were efforts to reduce sag and increase current capacity in tube amps.  The aim was to firm up the bottom end or increase power or both.  Some companies would use multiple rectifier tubes to achieve this aim. This was more common in Hi-Fi and organs than in guitar amps but there are few examples. 
 

Why does Mesa Boogies and other companies use Dual and Triple Rectifiers?

Marketing is the only reason.  Pretty much no one plays one of those amps turned up loud enough to get much power section sag. Even if they do the hi gain nature of these amp means the volume levels will be so high you would not be able to hear it anyways. The compression from the distortion and compression generated in the pre-amp swamps out SAG effect.   To top it off  the power supply is made to have very little sag other wise further decreasing the effect.   Having multiple rectifiers is expensive since you have the cost of the tubes and of custom power transformers.  It does sell amps though. Lots of them. Both the Mesa Dual and Triple Rectifiers are simply copies of the Soldano SLO with small tweaks, added features and the rectifier tubes. They are cool heavy sounding amps but do not be fooled by the tube rectifiers; they are there for show. 

You must also question the wisdom of using the dual rectifier in a more vintage style amp as well.  From both a sonic a reliability standpoint it's simply not a good practice. 

One more point:
Designing the amount of sag into an amp is pretty straight forward.  You really do not need a tube to get tube-like sag.  Purists will refuse to believe that sag cannot be had without tubes.  The technology is simple, effective and indistinguishable from tube sag.  Tubes are cool but solid state rectifiers with sag circuits can be more versatile, more reliable, and cost less. Only a resistors or two is needed to create a fixed amount of SAG. Active SAG and compression controls are also possible and are more even more useful. 

  Of course an aesthetic or nostalgic desire for tube rectifiers is another matter and legitimate reason to use tubes.  The ability to switch types of rectifiers can be useful too.  This is why Carl's Custom Amps uses tube rectifiers in their more traditional Tweed Amps but use out SAG control in our more advanced amps.