Tube vs. Solid State Rectifier

There's lots of talk among gear enthusiasts about the differences between amps with solid-state and tube rectifiers. Some players prefer one or the other or even claim one is better than the other. So what is the big deal anyway? In this article I will briefly enumerate the differences between the two.

What is a rectifier?

The whole subject of tube vs. solid-state rectification begs the question: what is a rectifier anyway? A rectifier is a device that changes alternating current (AC) into direct current (DC). The power in all wall sockets is AC but all amp s operate on DC so a rectifier is required for operation. It is important to note that the rectifier is not in the signal path of an amplifier but rather the power supply. 

Tube Rectifiers

 All early tube amps used tube rectifiers because solid state rectifiers had not been invented yet. Yet even after solid-state rectifiers became more common place some manufacturers stuck with tubes because early solid state devices were expensive. Today most new designs have solid state rectifiers.  

Tube rectifiers are inherently imperfect in an electronic sense but they work well in a musical instrument context. They have a voltage drop across them and internal impedance. Lastly an amp cannot use much power filtering with a tube rectifier. So what does that mean for sound? The internal impedance of a tube rectifier means that when the amp calls for current (when you strike a note) there is a slight delay in the delivery of the current and a slight drop in voltage. This is known as "Sag". This leads to a singing sustain and touch sensitivity at high volumes and mild compression at lower volumes. Many players find this very useful. The drawback is that is makes the bass response of an amp flabbier and the low filtering can cause hum and even ghost notes at high volumes. The hum and ghost note problem are easily remedied with proper decoupling and grounding like in the amps we make at Carl’s Custom Amps.

Tube rectifiers are far less reliable than their solid-state counterparts but you can swap them for different types for different feels. The new production rectifier are sometimes problematic. There is high rate of infant mortality and some tubes of Chinese and Russian origin are not exactly the same as their vintage counterparts. Often they were designed for less current draw so cannot handle some amps they are installed into. It’s good idea to carry an extra rectifier tube when gigging.

Here at Carl’s Custom Amps we use tube rectifiers on traditional models as nod to the past.

Solid-state Rectifiers

After the 1950's more and more amps were built with a solid-state rectifier. Today the majority of tube amps use solid-state rectifiers. Solid-state rectifiers cost a few cents, do not require a filament winding on a transformer to run the tube, and rarely have service issues. The solid-state rectifier works more perfectly from an engineering stand point. The delivery of the current is quick and you can use whatever amount of power filtering you prefer. On drawback is that solid-state rectifiers do have some diode switching noise a tube does not. The addition of few resistors and capacitor makes them just as quiet but some designers neglect to do this (i.e all vintage Marshalls and Hiwatts). Some solid-state rectified amps sound tighter and punchier that their tube counterparts if the designer chooses not to introduce sag. It is very easy to build sag into an amp with a solid state rectifier and make it feel/sound like a tube. It’s even possible to make the feel switchable or adjustable with a pot. Few manufactures do this but it’s very easy way to give an amp a vintage feel.

At Carl’s Custom Amps we use solid-state rectifier in designs that call for firm feels like in Marshall style amps. We also use them in amps that have the traditional feel associated with tube rectifiers by using sag enhancing circuitry. We often install a SAG control on our amps with London Power Scaling which allows the player to dial in the sag in the amp. This is especially useful.

Solid-state rectifiers are more reliable and allow the designer to dial in the sound/feel. Overall they are a better technology provided they are used correctly.