Rectifier Types
So now we know what the rectifier does. In guitar and bass tube amplifiers, there are two different types of rectifiers you will see, solid state and tube. Now we all know what a tube is right? The little glass bottles that plug into our amps that sound so good. There are several commonly used tube rectifiers in guitar amps, 5U4, 5AR4 (GZ34), 5Y3, and 6CA4 (EZ81). Each has different voltage handling and characteristics for different output levels and circuits but each accomplishes the same task in the circuit. Some amplifiers will utilize two or more rectifier tubes as in the Mesa Boogie Dual and Triple Rectifier series. There are many misconceptions as to why they use multiple rectifier tubes but I’ll let you in on a little secret…they need more than one to provide enough current for 100w and 150w output. That’s it, no magic, no mystery, purely a power issue. One 5U4 cannot provide enough current for 100w output so they use two, two on the other hand cannot provide enough current for 150w output so they use three.
A solid state rectifier is made up of two to four diodes in a circuit (generally, there are single diode rectifiers but they are somewhat uncommon in guitar amps) or an IC that is made up of diodes. The solid state rectifier, while being diodes and not tubes, accomplishes the same task as a tube rectifier, it is simply there to convert AC voltage to DC voltage. A lot of modern tube amps use solid state rectifiers such as Soldano and the Fender Hot Rod series.
You’ll notice I am repeating myself about the function of a rectifier, this is for a reason. In my experience it is a common misconception that the rectifier passes audio, it does not. I am merely trying to make this clear.
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Tone
So if a rectifier does not pass audio, how can it affect the tone of an amplifier? Well, the rectifier is the first post-transformer link in the power supply and changing the power supply voltage will cause different tonal characteristics. Tube rectifiers have an internal resistance so the more current that is pulled through a tube rectifier, the more the voltage drops which in turn causes the output power of the amp to drop. This drop affects the rise time of the tube (the time taken to produce the proper voltage output) and as the note decays the voltage builds causing a compression like effect. This is commonly referred to as “sag”. Since low frequencies require more current to reproduce, this sag causes a tube rectified amp to have a spongier low end and distort easier. This natural compression and sustain is sought after by many guitarists of certain genres (blues, classic rock, country) but not for everyone. Many players feel that a tube rectified amp responds better to touch dynamics and sound warmer than the solid state counter-parts. Different types of rectifier tubes will have different tonal characteristics in the same circuit as well but, as with all tube substitutions, BE CAREFUL. Not all rectifier tubes are interchangeable so consult a qualified technician before substituting.
A solid state rectifier has no internal resistance so when differing amounts of current pass through, there is a consistent fixed voltage coming out. The rise time is very fast as the proper voltage is reached almost immediately equating to less compression, more headroom (less distortion at higher levels) and a punchier, more articulate tone with a tighter low end. Diodes also can provide more power, run cooler, and are less expensive than tubes. You will notice little difference in tone between different diode types as they all provide a consistent voltage drop (about .7 volts). There are a number of companies out there that are making drop in solid state replacements that will plug directly into a tube socket for those seeking a tighter tone from a vintage amp.
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Reliability
Solid state rectifiers are generally thought of to be more reliable than tube rectifiers. While this is true in certain ways, it implies unreliability on the part of vacuum tube rectifiers. In general tube rectifiers fail due to either physical damage or bad manufacturing. Unfortunately, a lot of Chinese made tube rectifiers seem to fit the bad manufacturing bill with up to a 50% failure rate at some points. In general for a reliable tube rectifier, check out the European tube manufacturers like JJ Electronics, Electro-Harmonix (their factory is in Russia), or Sovtek. Solid state rectifiers, due to the lower operating temperature and soldered connections tend to be less prone to failure by damage or physical upset. Basically, a good tube rectifier should be every bit as reliable as a solid state rectifier but if you own a tube rectified amp, it’s always a good idea to have an extra or two lying around or in your road case.
While there are many, many more factors to be discussed regarding rectifiers, this should give you a basic idea of what they do and how they affect your tone without having to be an engineer to understand it. While a lot of modern guitarists will swear by a solid state rectifier, there are those of us who love that “sag” of a tube rectified model. No matter what genre you play, as always, let your ears lead you to the tone that is right for you. Thanks for reading folks and we’ll see you next time, in the Corner.