Why don’t you use a valve rectifier?

Valve rectifiers are an unnecessary expense and complication for a guitar amp. In a few vintage amplifiers they do contribute to the tone, but in many amplifiers they don’t provide any sonic benefit at all. Here’s why…

Valve rectifiers’ most important feature is that they have an internal resistance. Let’s say that it’s 100ohms. When there’s a big change in current drawn through the amplifier – for example, when you go from silence to a big power chord – there’s a corresponding change in the voltage drop across the valve rectifier. This is called sag.

If the change in current is, 100mA, then there’s 100r * 0.1A = 10V of sag. (NOTE: there will also be a little more sag due to the internal resistance of the power transformer.)

In some amps with valve rectifiers, this sag is enough to make a small but audible difference to the attack of your power chord – it sounds a little softer. And as the current demanded by the amplifier drops back, the sag slowly disappears, and the note may appear to ‘bloom’. It’s not just a change in volume, either – the attack and bloom can the way that the distortion is created through the amplifier signal path. For some types of playing, the overall effect is very musical and rewarding. (For other styles – such as ultra-tight heavy metal – it’s an unwanted effect.)

Wot – no sag?
For many amplifiers, however, there’s not very much change in the current drawn through the amplifier. This is especially the case with many small single-ended and Class A amplifiers. And if there’s no significant current change, there’s no significant sag, and no tonal benefit. A valve rectifier in that amp is a complete waste of money, energy and construction effort.

In contrast to the valve rectifier, the solid-state diode rectifier always drops the exact same voltage – typically 0.7V. This does not change with different current demands, and so there’s never any sag due to the diodes.

More or less sag
One aspect of this issue is very useful for the valve amp builder: the whole benefit of the valve rectifier is in its internal resistance. Nothing else matters. This means that you can mimic any valve rectifier with a simple power resistor of the same internal resistance. And all at the cost of a few pennies.

So, for example, you could add a 100r/5W resistor directly after the rectifier diodes to get the exact same sonic effect as described above. Once you decide to use a sag resistor (as they are called) you get a lot more flexibility. You can increase the value to get more sag, or decrease it to get less sag. You can even add a switch to bypass it.