This project is addition to my Firefly amp. I need delay pedal and the thing with delay pedals it that they sound best when used after distortion. Since Firefly amp gets most of it’s distortion from output stage FX-loop is out of question, so only option is to make chain Amp -> Line-Out -> Delay Pedal -> other amp’s power amp or line in. Also, it’s good for home recording directly from amp. Additionally, it features a light bulb attenuator that reduces volume and adds compression.

How it works?
It consists of 3 main sections:

  • dummy load resistor in case speaker is not used
  • bulb attenuator in series with the speaker
  • transformer coupled line out based on Firezog

When speaker is not plugged, dummy load resistor is switched into the circuit. As signal increases, light bulb warms up and increases it’s resistance. That reduces the volume while making compression effect. It may be useful to add a switch that bypasses the bulb when this effect is not desired. It turns out that Firefly has enough compression on its own.

  • 1:1 audio isolation transformer
  • 5K log pot
  • 8.2ohm 5W dummy load resistor (I used 11W resistor, adjust value if using speaker other than 8ohm)
  • 1x 10K 1/2W resistor
  • 3x mono jack
  • 12V 0.2A (2.4W) light bulb with socket
  • Enclosure

Click on an image to see more details.

2 Responses to “Zog-zog”
  1. Maksym says:


    Isn’t isolation transformer a serious pull down load for that? I mean it is in parallel with dummy load resistor and its own resistance is not that big, what about tone loss? Is it dark and compressed?

    • bancika says:

      I haven’t noticed a loss of high gain, although it’s been a while. The isolation transformer is higher impedance (~10K), so it shouldn’t load down the output noticeably.

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    The idea behind this site is to share my experience with Do It Yourself approach to guitars, amplifiers and pedals. Whether you want to save a couple of bucks by performing a mod or upgrade yourself instead of paying a tech, or want to build your own piece of gear from scratch, I'm sure you will find something interesting here. Also, this is the home of DIY Layout Creator, a free piece of software for drawing circuit layouts and schematics, written with DIY enthusiasts in mind.