Firefly low-wattage tube amp

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

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

(click for full size)

(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.

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

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.

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.

A lot of people have put the below "FireZog" line-out circuit in their fireflies. I was more interested in a headphone out and someone said swapping the 150 kOhm for a 100 Ohm would work for driving headphones. I also used a 500k pot instead of a 1M since that's what I had lying around. It does work but has a fizzy sound to it. Haven't figured that out yet.

This little amp hummed like a mother once I had it all put together. Which was odd because it sounded pretty quiet when I was building it. I powered it up and removed the lid and - whoa, not so noisy any more. After poking at the wires with a wooden spoon (and one hand in my pocket) I determined when some of the wires in that rats nest get closer to the output transformer there is a ton of hum. Solution...

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One little cable tie and the hum is completely gone. It's so silent you can't even tell if the amp is on or not.