Building a Firefly 1.5 watt Tube Amp
By Paul Marossy
Last updated 2/18/04
Here is a cool
little tube amp project that is a good first time build for those brave enough
to venture into tube territory. It is Doug Hammond's Firefly,
Rev. 3, a push-pull 1.5 watt tube amp utilizing two 12AX7 preamp tubes and a 12AU7 preamp
tube in a self-split power tube configuration. One of the 12AX7 tubes is in
a switchable cascode boost circuit. Not only is this an interesting and unusual
way to use preamp tubes, but it also sounds great! My Firefly is built per the Rev. 3 schematic, except for an added resonance control. The resonance control I used is a 5K pot with a built-in switch. It can be switched in or out of the circuit. The cap value is flexible and can be any value between 10uF and 47uF.
This is actually my first tube amp build. I've wanted to build a tube amp for at least a year before I built this project. Below is some details of how I approached making what I like to call my "Deluxe Firefly".
|When I made my first speaker cabinet, I cannibalized a cheap Crate bass amp that had a hum I just could not rid of. This is the raw chassis from that amp before my modifications to it. It was a real pain getting the sticky stuff off of the front of the enclosure after removing the faceplate. It measures 14"x6.5"x2" and is made out of aluminum.|
|Next, I got an extra fine point Sharpie marker and designed a chassis layout. I marked out where everything was going before drilling any holes. One immediate problem was that large opening where the power transformer was.|
|Here is the chassis with the terminal strips installed and most of the holes drilled. I found a way to cover the large opening...|
|To cover the hole where the power transformer was, I cut some pieces I had leftover from an aluminum enclosure that I had cut down and covered the opening with them. To the left is a hole with a black plastic insert for the new transformer wires. The green perfboard will house the rectifier diodes and wiring from the new power transformer, and wiring to the standby switch. The power transformer I decided to use is a Hammond 269EX, which has a 190-0-190V center-tapped secondary and a 6.3V heater filament all in one transformer.|
|Here is the openings for the preamp tubes. To drill these holes, I used a trick I heard about at a guitar amp forum - a 3/4" or 7/8" size wood drill bit, depending on the socket you use. It works on aluminum because it is a soft metal, and it makes a pretty clean hole. A rat tail file cleaned up the edges where needed.|
|I covered the face of the chassis with four layers of 2" wide aluminum tape, known as "metal repair tape", sold at places like Home Depot. It has lots of uses. This will get covered with some graphics which I designed via AutoCAD.|
|Here the wiring is about 25% complete. Still waiting for parts before I can continue...
I have a new appreciation for amps that are wired point-to-point. It's kind of an art when you are used to PCBs and stripboard. It's rather tedious and labor intensive for complicated circuits. No wonder they charge so much for PTP amps!
|Here the chassis wiring is complete. I used wire from a dead computer power supply, which is rated for 300 volts. It gave me lots of different colored wires and didn't cost me anything. I kept all wiring runs as short as possible and followed good design practice everywhere that I could. One small change I made is powering the indicator lamp with the 6.3V heater supply instead of 120V as shown on the schematic.|
|Here is a close-up of the preamp tube sockets. It was somewhat of a challenge getting all this wiring soldered up in such a small area.|
|Here is a view of the bottom of the chassis. I used tube shields for the preamp tubes for a variety of reasons, but mostly because this a relatively high gain circuit. I wanted to minimize problems with any kind of noise as much as possible.|
|Here is a view from the back. When I stripped down the original chassis, I polished the back and bottom surfaces of the chassis with an SOS pad. It leaves a nice and shiny surface. The power and standby switches were purchased at RadioShack. They seem to be seem to be satisfactory for this application in terms of quality and they meet the ratings needed to be used with this amp.|
|With the chassis done, it was time to build an enclosure. I decided to use the enclosure I had leftover from the Crate bass amp I cannibalized for my speaker cab project. This is what it looked like before I did my surgery.|
|After peeling all of the old Tolex off, I marked up the enclosure for cutting down to size with a jigsaw. Here is the enclosure after I was done cutting and reassembling the pieces. It didn't take very long to do all the cutting and it was pretty easy, all things considered.|
|Here is the enclosure with a new cloth covering. It is applied with some spray adhesive. I used the corners that were on the original cabinet. The grille cloth also comes from the same bass amp cabinet. I cut it down to the size that I needed. The logo is my own design, just to make it look like a commercially manufactured amp. You can't see it this picture, but there is shielding on the top and sides of the enclosure where the chassis is.|
|Here is the finished product. The overall dimensions are 16"x8"x7.5" high. Weight is about 12 lbs. The graphics were done with my usual sticky back method as shown on my Building Stompboxes page. It's not perfect, but it works pretty well for my purposes.||Finally, here is a view of the back of the finished amp. I have since painted the inside of the enclosure black.|
FireFly Chassis Layout
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