AX84 Firefly

This is the first amp I ever built. I opened it up recently to fix a joint that went bad over time and realized how much progress I made since then regarding soldering, wiring and component placement. Regardless of that, it’s a great little amp and below is the original build report I wrote back then.


There aren’t many good audio-grade components here in Serbia so most parts are ordered from Germany, UK or USA. I purchased good metal film resistors in local store and power transformer is also built here. offers many low-wattage amp designs, but I finally decided to go with FireFly, a one or so watt hi-gain amp which offers good cranked tone at bedroom level and is not too complicated to build for a newbie. Power transformer was custom made by Trafomatic and output transformer is Hammond 125A. For caps I used mostly Orange Drops for nF range, silver mica and ceramics for pF range, filter caps are F+T and Nichicon electrolytics and few caps are nice Solen Fast polyester capacitors. I experimented with several different brands of 12AX7/ECC83 and 12AU7/ECC81 tubes and tried 12AT7/ECC82 as output tube. Hookup wire is 1000V solid core in different colors, a gift from (HUGE thanks). As for the speaker, one of the few guitar speakers you could find in Serbia back in 2005 was Celestion Greenback G12M, so I got one of these to make a 1×12″ cabinet to go with the Firefly. I later replaced it with a much more compact 1×10″ cab with Celestion Vintage G10 speaker.

Mods (2005)

These are mods I decided to include in the initial build of the amp without even trying how it sounds stock. Many years later I revisited this amp and re-did the mods. Keeping this section for reference only, but the current version of the amp with new mods is shown below.

  • added single knob tone control similar to that on the Marshall 18W for more tonal options and versatility. On the schematic below, components C9, VR3 and R13 in the left green “blob” of components are added to the original circuit. I chose this control because it doesn’t cause too much signal loss.
  • added Master volume control. On the schematic below it’s marked as VR4 and it replaces R12 from the original schematic.
  • added L-Pad power attenuator between the output transformer and the output jack to be able to crank the output stage to the max, get that creamy distortion and bring the volume down to bedroom-friendly levels.
Mods (2017)

Looking back, the mods I originally included did improve on versatility of the amp, but they are implemented in a way that makes it impossible to never hear the stock circuit as it was intended. I noticed that my Tone knob was maxed all the time, so it only robs the amp from some of the gain. And although Master volume and L-Pad controls did help bring the volume down, they also made tone suffer noticeably at lower settings. Having all that in mind, I decided to change mods to better suit the amp and my needs.

  • Take the L-Pad out and replace it with a more modern VVR (Variable Voltage Regulator) approach. Instead of messing with the output signal, VVR adjusts the high voltage used to power the tubes to control volume. Lower voltage = lower volume. In theory, it should have much less impact on the tone of the amp than resistive attenuation. I re-purposed the standby switch (who needs a standby switch anyways?) to control whether VVR should apply to the whole amp or just the power amp stage. The first option should keep the character of the amp about the same as the voltage is reduced. The second option leaves the preamp at high voltages, but with the reduced headroom of the output stage we are able to drive the output harder and produce more output stage distortion. Sounds like an interesting choice to have.
  • Add treble bleed circuit to the Master control and move Tone control inside the treble bleed circuit to be able to dial in the exact amount of treble to be recovered. I used a Fender-branded CTS “No-load” pot which breaks the connection when the control is maxed. That way it can break the treble bleed circuit and disengage it from the rest of the circuit. One big advantages of this arrangement is that I can bring the treble even higher than neutral when Master control is not maxed. So it behaves almost like an active control. Disadvantage is that it has no effect on the tone when Master is maxed. But I’m OK with that.
  • Keep the Master Volume control as it doesn’t change the stock amp too much when it’s maxed. The only difference in that case is the 500K resistance to ground instead of the stock value of 330K (R12). In conjunction with the treble bleed and newly redesigned Tone control, Master control is much more useful.

Below is the schematic originally drawn by Doug H which I updated with my mods. Please note that this is not the official revision 4 of the project, just my take on it. Highlighted blue is the mod around the Master and Tone controls, orange designates VVR mod and green is the heater elevation circuit.

Project Files

Original Firefly Rev 3 schematic drawn by Doug H
Turret board layout drawn by Paul Marossy


Chassis started its life as an old piece of metal I found in my basement. It was cut to size and then bent into shape at a local metal shop.

It’s far from a Hammond, but after spray coating it into black, it looks semi-decent 🙂

For the board I used insulator sheet I found in a local electric store for cheap and turrets came from After I placed it inside chassis there were few slight modifications: I installed 22K resistors missing in power supply part and replaced that thin ground buss with a thicker one.

Here’s the inside view with new ground buss installed. Note dummy load resistor on output jack, I used switching jacks so if I forget to plug speaker cable in output jack OT won’t blow up. Also note that the treble bleed Solen 2.2uF cap for L-Pad is switchable. When switch is on it’s not used in circuit and when it’s on it bypasses some treble to compensate loss caused by attenuation; on photo below that switch is marked as “bright”.

Here you can see front panel made of plexiglas. I designed it in CorelDraw and had a local laser engraving company cut it and engrave custom graphics on it. Being 4mm thick, it made it a bit problematic to install power and standby switches which are designed for thinner panels. It looks very cool, especially with tube glow behind, but be aware that it “catches” fingerprints like crazy.

The back panel is also custom made plexiglas.

Head cabinet is made of particle board, glued and screwed together. Those two vertical pieces of wood are there to screw the plexiglas front panel into. All edges have been rounded. After that it was painted in black. I wanted to tolex it but I don’t have time/nerves/knowledge to do it. I know that tolexing rounded edges can be real pain in the a**, so cabinet got matte paint :). It’s much less durable and much less good looking than tolex, but I was exciting to get the amp running, so I didn’t want to spend too much time and money on visual.

And finally the finished head.

Meanwhile, we were making a huge cab for the little amplifier 🙂 . It’s also made of particle board, since I couldn’t find decent wood at the moment. Everything is cut at wood shop, so only speaker hole is left for use to do. You can also see wooden reinforcement bars on the box.

All done!

New photo from 2017 after performing VVR, Master/Tone and heater elevation mods.

Useful links project page

7 Responses to “AX84 Firefly”
  1. Andy says:

    I really like this project. Could you please tell me why you’re using the 0.0022 / 1kv across the primary of the transformer?

  2. emanuele says:

    Great! Can you please tell me how can i add master volume and tone control like you ?
    TKs !

  3. Kohout says:

    Hi! Looks really good, nice idea with plexi! I really like it… Can you please tell me how can i add master volume like you? Im going to build with tone controll and master vol like you. Can you please send me your schematic? My email adress is Tahnks!

  4. Nikola says:

    Hi! Great project. Very nice build. Can you just tell me where did you get Hammond 125A?

  5. Anwar says:

    Love your project! GREAT idea with the plexi face!! You know what, I used fabric to upholster my Fender Frontman 25R like an old Fender Tweed and it looks awesome.

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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.