What are valve rectifiers? | Too Afraid To Ask

rectifying some inequities…

Rectification is a word you will likely have come across if you are interested in the technology of amplifiers. All amplifiers (and many other devices) require rectifiers to work correctly, but what are these mysterious devices?



What is a Rectifier?
A rectifier is a device which lives within the power supply of your amplifier (not in the signal chain!) and its job is to change AC voltage into DC voltage.

In guitar amps a rectifier can either be in the form of a valve or solid state diodes.

AC voltage, supplied from your mains electric socket, periodically flips between positive and negative voltage values. This is really useful for transmitting electricity over long distances with few losses, but it’s not so good for powering elements within your amplifier.

Power valves require high DC voltage for their anodes, without this they would not be able to amplify. DC voltage is a constant positive voltage value.

In order to obtain this DC voltage, the mains AC, stepped up by the power transformer, is rectified. The rectifier takes all the parts of the voltage that are negative and makes them positive.
However even after rectifying we are still left with an undulating voltage. To solve this smoothing capacitors are implemented to even out the transitions in the rectified power, bringing what was an AC voltage much closer to a DC voltage.

Valve or Solid State?
Back in the 50s valves were the only way to rectify. Rectifier valves, containing two diodes per bulb, featured on many now classic amplifiers such as the Vox AC30 and Fender Tweed Deluxe.
Rectifier valves are imperfect, however, and experience ‘sag’, a momentary drop in voltage when an increased load is put on the system.
Digging in with your playing puts extra demand on the power valves, which rely on the rectifier supplying them with constant DC voltage. This extra demand for power is passed to the rectifier valve, which can’t quite handle the increased load and its voltage ‘sags’.

This has a knock on effect for the power valves as they no longer have enough plate voltage to amplify the signal effectively. This leads to spongy, less defined attack on loud notes with valve rectified amplifiers unable to react fast enough to aggressive picking to keep the sound consistent.

Most modern amplifiers you will come across will feature solid state rectifiers. These diode packages are smaller, cheaper, more efficient, more robust and faster in their response compared to valves.
They don’t experience the same debilitating effects of sag making them perfect for fast, aggressive picking, retaining a tight, consistent response throughout.
Also thanks to their superior power handling, solid state rectifiers allowed valve amps to grow in power. You’ll find very few 50W and 100W amps that aren’t solid state rectified.

Which is better?

While it may seem that, objectively at least, solid state rectification is the clear winner, in the world of guitar tone things are never so clean cut.

The change in sound caused be a rectifier valve sagging under duress is still popular among those guitarists with a nostalgia for vintage guitar tone. It’s a unique failure state that can’t be recreated any other way.

However for more modern players, particularly those into hard rock and heavy metal, rectifier sag is undesirable and guitarists opt for high gain, high power, solid state rectified amplifiers for their tight, consistent sound.

It’s a point of stylistic preference and both have their merits.