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.

Components

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. AX84.com offers many low-wattage amp designs, but I finally decided to go with FireFly, 1.5W 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 (and one Xicon Poly film in tone stack), Silver Mica for pF range, filter caps are F+T electrolytes and few caps are great Solen Fast poly. One of those Solen caps at 2.2uF is used as treble bleed at L-Pad. Those ECC83 and ECC81 tubes are EI and 12AT7 I experimented with is a NOS General Electric tube. Hookup wire is 1000V solid core in different colors, a gift from AmpMaker.com (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.

Mods
  • 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 beadroom-friendly levels.
Other Mods
  • swapping the output tube with 12AT7 (ECC81) adds sustain and distortion and brings the volume down. I tried both, but preferred the sound of ECC82.
Project Files

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

Update (June 2010)

From this perspective, the only mod I would keep is the L-pad. It’s unnecessary to have both that and the Master volume. This amplifier is really designed to have the tone and master cranked all the way and you lose some signal by introducing those control. In the meantime, some good folks started making PCBs for this great little amp which makes it even easier to build, so I’m considering rebuilding it in a smaller enclosure having this in mind.

Pictorial

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 AmpMaker.com. 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!

Useful links

AX84.com project page
Harmony Central AX84.com Firefly reviews

Comments
5 Responses to “AX84 Firefly”
  1. emanuele says:

    Great! Can you please tell me how can i add master volume and tone control like you ?
    TKs !
    emanuele.borgato@gmail.com

  2. 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 J.kohout2006@seznam.cz Tahnks!

  3. Nikola says:

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

  4. 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.