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.
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.
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