Building a Champ out of Junk Parts

By John Fisher

Since Christmas was coming I was thinking,"What could I get my son that he would really appreciate but that wouldn't cost me much money." After I built him his Strat guitar I was a little worried that (like many kids do), they play with something for a while and then get bored with it. It seemed a little like this at first but then by a miracle he started really getting into playing the guitar and recently (as of this writing) made great progress. So I was encouraged.

I was looking at all my junk that I have collected throughout the last couple of years and I thought, "Wow I have just enough parts hanging around to make some kind of a small amp for my son." I was going to possibly make a solid state one but I was happy to see that I had the stuff to make a small tube amp. It is almost against my religion to even use solid state amps for guitar. Once I started using tubes for guitar amps I would never go back to solid state again. They just sound so much better in my opinion.

Seeing what I had to work with, I realized that the closest thing I could come up with was a Fender Champ style amplifier. There are several different models of Fender Champ amplifiers that were made a long time ago. There are some that just have one knob for volume and others that have tone controls. I planned to make this one like the old blackface Champ that has a Bass and Treble control incorporated into it. They are usually about 5 or 6 watts. They are just loud enough to get on your nerves and disturb the whole house if played at the wrong time but not loud enough to have the police at your door for disturbing your neighbors.

I didn't have the exact parts. The Output transformer was from an old tube reel to reel tape recorder that someone gave me as junk. The power transformer was from: "I don't know where or what". It seemed pretty close to what I needed except that it didn't have the 5 volt output windings needed to power the heater filament for the 5Y3 tube rectifier. So I had to make some modifications with this and use regular diodes.

 

Above is a drawing of the original schematic from the 60's of a "Blackface Champ" For the motherload of Fender circuits and info about vintage Fender amplifiers you can go to Fender Amp Field Guide.

As you see from the above circuit, this is a fairly simple circuit and take note that this one uses a tube rectifier in the power supply.

Above is the schematic that I came up with based on the parts I had sitting around. It is very similar to the original except for some minor details. The most significant change is that I used 2, IN4007 diodes in the power supply instead of the 5Y3 rectifier tube. What is the difference in performance? First of all using diodes will put out a little higher voltage then the 5Y3. The DC output of the power supply was around 390 volts compared to the 360 volts that the original circuit puts out. This will cause the amp to be a bit louder and clearer at higher volumes. People usually prefer the 5Y3 because it produces a softer more compressed sound then using the solid state diodes. I agree with that, and also would have preferred to use the 5Y3 but the power transformer that I used wouldn't permit it because of the lack of a 5 volt winding.

I made a few other minor changes in the circuit. You will notice that I used 2, 1 watt 100 ohm resistors off each leg of the 6.3 volt heater supply that then were tied to ground. This helped a lot in reducing AC hum from the tube filaments.

Unlike the original circuit I added a 3 prong wall plug that grounded the chassis to the wall socket ground. I also omitted the extra guitar input and only have one input going into the amp. I always do this on my amps because I look at 2 inputs as unnecessary and I always would get annoyed when someone wanted to plug their guitar into my amp when I was playing. Well!, to each his own.

Somebody just happened to give me an old 30 watt 10" Fender speaker that was perfect for this project. I didn't spend hardly anything on this project except some of my time. I made the cabinet out of some junk wood pieces from the yard and I covered it with black thin carpet that is used for covering sound equipment.

Since this is a amp made of junk parts I made this quick chassis by simply screwing on 2 pieces of 2" aluminum U stock onto a piece of sheet metal and presto!, an instant chassis. The aluminum is usually used to make aluminum doors or windows.

Here it is roughly wired up and at this point I am experimenting with it to see how it works. It isn't state of art or a perfect layout by any means but it works good without any problems. One thing that happened the first time I turned it on was it had a horrible feedback or squealing. I then reversed the 2 leads on the primary windings of the output transformer and then it was OK. To make a quick way to make connections I just got some circuit board and burned it so that there was just a bunch of squares on it to be able to solder parts to at random. Since this circuit is so simple I didn't need to take much time in perfectly planning out everything but just soldered parts where is was convenient.

I did follow a few different layout rules. For example I made sure that I "star grounded" everything. That is where each ground point at each stage of the amp has it's own wire that goes back to a common single grounding place. I usually make a common grounding point right near the first Power cap in the power supply section where all the ground wires will meet. This is important to give a noise free, or hum free amp avoiding ground loops. I usually at that point also connect it to the chassis. I used an input jack that is made of plastic where the Jack itself is insulated from the chassis. This enabled me to connect a separate ground wire from the jack to the main central connection of the "star ground". Separately grounding the input jack is probably one of the most vital factors in star grounding the amp to make sure no hum from the other stages that draw more current will modulate the ground connection of the very sensitive input. I also twisted the wires together that go to the heaters of the tubes which helps to reduce hum from the heater wires.

I made an ultra simple faceplate on the front by spray painting it with silver auto lacquer and then using leraset or dry letter transfers for a few very basic markings. I made a black line border around the faceplate by using some pin stripes that you get at an auto supply store. I later sprayed clear automotive lacquer over it to protect the markings. I did all this very simple and fast as not to spend too much time on details. Other times I have done this more elaborately with good success.

Here is the underside of the chassis where some of the bulky parts and tubes hang out. The beat up looking output transformer had some of the paper ripped up and some of the wires were not so secure and there was a risk of some of the wires ripping out so I just got some masking tape and rapped it around it a few times.

Here is a drawing to give you and idea how I made the box. This is nothing like an original Champ cabinet but it worked out fine. Because it is an open back cabinet, the dimensions are not critical and I just made this the way I did for practicality of size and with the materials that I had. I didn't spend any time on making it slanted in the front which would have been nice, but it was just a quickly made square box. I used scrap pieces of pressed board and plywood that were sitting around that I glued and screwed together. I then covered it with thin black carpet that I glued on with contact cement. I put 4 rubber feet on the bottom and a handle on the top.

Here it is put together. The wide angle lens on the photos distorts the shape a bit but it actually tuned out real nice and simple and the looks are socially acceptable. The chassis is screwed to the the box with 4 black drywall screws coming from the top of the amp.

Here's the back. I put a 1/4"jack in the back where the speaker plugs in, in case it was ever desired to plug a different speaker into it. The jack is wired so that if you were to unplug the speaker, it will short itself out. This is to protect the amp because unlike solid state amps, leaving the amp turned on with no speaker load connected to it or an open circuit can damage tube amps.

Here is a closer look at the very simple face plate. I did a minimal amount of work on it to keep it simple to save time but it still came out fine. Fortunately, the knobs themselves that I found in my junk even say "Volume, Bass, Treble" on them.

This is the quickest amplifier that I ever made. I took me about 1 week from starting from scratch to finishing it. I did this in my spare time and in the evenings while the wife and kids were in bed. Ha!

 

Some Modifications

Although the amp worked perfectly fine and sounded good, I couldn't resist the temptation to do some tweaking or modifying. The amp was a bit clean sounding for me even at pretty high volumes. The first thing that I did was disconnect the 2k7 feedback resistor that goes from the speaker output to the #8 pin on the 12AX7 preamp tube. This made the amp sound louder and more gutsy sounding. This can also be a mod that you can put in a switch to give you the option of the feedback resistor or without it, but I just left it without.

The second modification was disconnecting the ground wire from the tone stack. If you look on the schematic you will see a 15K resistor at the bottom of where the tone controls are. If you disconnect the ground wire from the 15K resistor it will disable the tone controls but you get a much louder signal that makes the amp sound a lot much gutsy and not so squeaky clean sounding. I really liked this sound so I put a switch on the amp to connect or disconnect the ground wire from the 15K resistor. Many of the early Champ guitar amps in the late 40's did not have any tone controls at all but just a volume control. The Blackfackface Champ, which has the tone controls was introduced in the 60's. I just happened to have a 250K potentiometer that had a pull out on and off switch on it so I installed it for one of the tone controls and used the pull out switch to either disable the tone controls giving a really nice rocky sound or when the knob is pulled out I get the Blackface tone control option. So now the amp has the best of both worlds.

Here is the first modification illustrated above. You see the switch in red that simply disconnects the feedback resistor from the speaker output

Here is modification #2 which is also a simple switch that disconnects the tone stack or tone controls from the ground.

We'll see how my son likes it.

 

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