Introduction

Power attenuators are very useful tools for tube amps. As you know, tube amps sound best when cranked all way up. The only problem with that is that most of them are PAINFULLY LOUD. So it would be the best if we could crack amp and then somehow reduce output volume to level that will not make our heads explode. L-Pad is the simplest available power attenuator. It consists only two resistors (one in parallel with driver and one in series) which means it’s purely resistive.

Simple resistive attenuator

This could be a problem because at higher attenuation levels (lower output levels) there’s more treble loss. There’s also variable L-Pad available and it looks like huge potentiometer, but you can build fixed L-Pad very easily (and even cheaper). However, you must use resistors that will match your amp power and output impedance. You can further improve this design by using non-polarized treble bleed cap across R1 to compensate treble loss (for variable L-Pad it should go between input and output pads).

Calculator

You can use this calculator to determine resistor values for desired attenuation level.

Variable L-pad

For more flexibility you can get variable L-Pad. As you can see below, it looks like rather large potentiometer. Treble bypass cap is installed between lugs 1 and 2. In my Firefly build I used 2.2uF Solen Fast cap. These L-Pad attenuators are linear which means that if you use it with 10W amp you get 5W output with L-Pad set to 50%. Since humans hear is logarithmic there will be only little volume drop with half power. To get half volume (-3db) power needs to be reduced ten times.

Here’s interesting and easy to do implementation of L-Pad. Using three way two pole switch (either slider or rotary) you can get nice versatile output. One position would give unchanged output level and other two positions can decrease level in two stages, for instance 3db and 6db. Of course, one could use switch with more positions and more options, this is just the idea.

Also, treble bleed caps could be added across R1a and R1b to reduce treble loss caused by attenuation. Below are listed resistor values for 3db and 6db attenuation with 8 ohms impedance. Cap values are usually 1uF-5uF, try different values to see what suits your needs.

Simpler solution would be using only SPDT switch for one attenuation step. Below is perfboard layout that shows both parallel and series resistors and treble bleed cap Cs that can be omitted.