Firefly low-wattage tube amp - Projects - Audio Artillery

Firefly low-wattage tube amp

  1. Intro
  2. Power section
  3. Internal wiring.
  4. All wired up
  5. Adding an FX loop
  6. Adding a headphone output
  7. Hum reduction

1: Intro

This is a build of the AX84 Firefly tube amp project. It's a 1.5W tube amp that is great for indoor playing.

schematic


 

2: Power section

First power up of the high voltage rail. This is pretty scary the first time.


(click for full size)


 

3: Internal wiring.


(click for full size)

I used a wire-wrap board to do most of the wiring. This is convenient for debugging but you end up with a lot of wiring between the tubes and the board. I'm not sure I'd do this again that way.


 

4: All wired up


(click for full size)

Not much to look at but it sounds great and is just the right volume.


 

5: Adding an FX loop

I wanted to add an effects loop so that I could run a reverb pedal. The below is what I attempted. It's a passive breakout in the middle of the V2 preamp stage. The extra capacitors are to cut any DC going to or coming from the pedal.


(click for full size)

It didn't work very well. The voltages can get quite large at this stage, especially with the overdrive enabled. With the single preamp stage enabled and low volumes it worked very well. But when the voltage gets high enough the pedal couldn't keep up and would reset. I'm lucky I didn't smoke the pedal.

My take away from this is that unbuffered inputs to an amplifier aren't a good idea. The real way to do this is to knock down the signal going into the loop, and then have a dedicated tube to drive the output of the loop back up.


 
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