When shopping for a high quality tube guitar amplifier, one of the potential decision points along the way is whether to purchase an amplifier with a tube or solid state rectifier. What does all this mean, and does it matter?
Vacuum tubes are powered by high voltage DC (250 – 500 volts) while the power coming out of the wall in your house is 120 volts AC. To convert AC voltage to DC voltage the AC alternating current is rectified into DC current. Back in the early days of tube amplifiers the only way to rectify voltage was to use a special type of vacuum tube called a rectifier tube. By the early 60’s electronics had advanced to the point that solid state silicon diodes had become an affordable alternative to tube rectifiers. A diode allows current to flow only one direction, and a simple “bridge” of four diodes is a cheap and reliable way to rectify AC into DC.
From a pure cost standpoint, silicon diodes are very attractive to the manufacturer. They are cheap (pennies really) and they rarely fail. They also produce a very “tight” output, and a power supply with a solid state rectifier is very stable and consistent. And it does not generate any heat to speak of.
In comparison, a tube rectifier requires the tube (dollars not pennies), tube socket, additional wiring, and a more complex power transformer with a special tap (output) to power the tube rectifier. There is also more variation in tube performance than diode performance, and in general a tube rectified power supply is “looser” and it’s actual output will vary more in relation to demand.
The “Sound” of Solid State versus Tube
Besides being a cost advantage, solid state rectifiers provide a “stiffer” power supply to the power tubes which results in more headroom with less distortion. The famous Fender Twin Reverb Amp has always used diodes to provide loud clean sound. An 80 watt amp would overwhelm the power handling capacity of a single tube rectifier, and diodes were pretty much the only way to go. Certain Fender Bassman designs used two tube rectifiers in parallel to share the load and provide better headroom and cleaner sound. If this sounds somewhat like a Mesa Boogie “Dual Rectifier” you are absolutely right. For a 100w or 150w head, two or three rectifiers in parallel are the only way to provide tight punchy sound without going solid state. So in general solid state rectifiers are associated with cleaner, tight, punchy sound with good headroom. And they help keep costs down.
Tube rectifiers has certain characteristics that are deeply ingrained into the Mojo of tube amps. When faced with a strong power demand (striking a big chord or picking very hard) the voltage output of the tube will actually “sag” or drop in voltage. This results in a softer note attack and maybe a little clipping or “hair” around the notes. As the note decays, the voltage comes back up and “pushes” the note giving the effect of a slight volume swell and more sustain. When players refer to an amp as “touch sensitive” it is this effect of the rectifier tube sagging and swelling with the notes, and responding directly to the player’s style. Low wattage amplifiers with tube rectifiers have a nice spongy feel, warm sound and soft clipping that many players love. With all the boutique builders furiously cloning Champs and Deluxe’s it’s clear that this type of tone has some hard core followers.
Is Tube the only “true” path to Sonic Bliss?
If you need a lot of volume, clean headroom, or really like a tight low end to your sound, an amp with a tube rectifier may not be your cup of tea. Folks who like metal or hard rock need not apply, although Boogie has made a real name for themselves with the Dual Rectifier. Of course having two rectifiers sort of defeats the purpose, but it’s great marketing. Even if you really like that softer attack and early clipping, you don’t have to have the extra expense and maintenance of a tube rectifier: Just play a smaller amp! If you are playing a 50 watt amp for jamming or in clubs, chances are it’s too big. Even 30 watts may be too much. Big amps were designed prior to pro audio and commercial sound systems. The big 100 watt amps of today are matters of testosterone and showmanship. Nobody needs that back line of amps — OK maybe Yngwie Malmsteen does — it just looks awesome.
I really like the spongy feel of a fairly low powered tube amplifier. A nice 20 watt amp with a 5AR4 rectifier playing through a 12 or a couple 10’s has great feel and character. And it will handle most club gigs with no problems at all. It may lack a little punch, but this type of setup gives the player great control over the texture of the notes. However, after saying all that, my #1 go-to amp has a solid state rectifier. But it’s still only 20 watts (on half power) and the overall sound quality and touch sensitivity is still there because it’s sized right for the task. There is so much to amplifier design, transformer design, and component selection that picking an amplifier on strictly one aspect — it “must” be Class A for example — just does not make a lot of sense.
Early amplifier builders used tube rectifiers because they had too. If affordable silicon diodes existed in 1950, they would have used them instead, and we would have never known the difference. But tube rectifiers have that tribal folklore attached to them, and for some there can be no other way. They can add a lot of character to the sound of an amp, but like many other “must have’s” a tube rectifier is not instant guarantee of goodness. In the end, buying on sound quality and choosing an amplifier that is sized appropriately for your needs is the best strategy.