I became interested in building a tube amp in the spring semester of my sophomore year at UConn. I had been dismayed at the sound of many commercially available amps and I wanted to see if it was possible to craft my own amp to my own specifications. Of course I knew that I could not simply build a complicated high gain channel switching amplifier right off the bat so I decided to start small. I did some searching on the internet and came across one of the best sites to being learning about building an amplifier, AX84. Their prices were low for entire amplifier kits and they also have a forum where all your questions can be answered. AX84 mainly offers a couple different types of Single Ended Class A amplifiers which are based off amalgamations of Fender/Marshal circuits and some up to date ingenuity. They provide detailed layouts, schematics and kits to facilitate the first time builder. Based off of the recommendation on the site, I chose to build a P1-X, a slightly more complicated and louder version of their most basic amp, the P1. The P1-X is a straight forward single channel Class A Single Ended amp. The design incorporates two cascaded gain stages with a gain control, a Marshall/Fender style tone stack and a octal Class A output stage with a master volume control.
At the same time I also meet another builder through facebook who helped to provide answers to all of my million questions, Mr. Donato Bicicletta. It was sort of random because I saw his ad for custom built tube amps through the marketplace feature on facebook and contacted him. He was and still is a constant source of information as well as a great builder to ping ideas off of.
When ordering the AX84 kit, the site gives you the ability to have your chassis CNC'd so that do not have to drill any holes at all. Since I lacked the necessary tools to drill, I added that feature onto the amp at a slight extra cost. I also added the variable bias option so I could use several different types of output tubes. One of the first things to do when a entire amp kit arrives is to inventory all the parts to make sure they all where shipped. Luckily, I did this and found that they shipped my kit with an older Bill of Materials (BOM) which had a swapped a couple potentiometers. I emailed the site and they quickly sent me the correct parts free of charge. It is worth noting that another great aspect about AX84 is their customer service.
Although the kit included nearly everything needed to build an amp, it does not include wire, faceplates or a wooden cabinet to enclose your amp. This should be considered before ordering a kit from AX84. I sourced the wire from Ted Weber, another great source for tube amp kits and parts.The amp process can be broken down into several stages which was made easier due to the pre-drilling. Dry-fitting the eyelet board, propagating the eyelet board, fitting the components to the chassis, and finally wiring everything up. Afterwards, electrically testing and tweaking to finish up. (Another very detailed guide on amp assembly can be found here)
Putting together the amp was actually much easier than I had originally assumed. I tried to follow the build guide laid out by AX84 which can be found here. The guide provides a much more detailed step by step procedure for building a tube amp and can be extrapolated for many types of amplifiers. Once the amp is assembled, there is a great guide on the first power up written by Paul Ruby. I cautiously powered up the amp and everything seemed okay. However, when probing the 6.3V AC heater voltage I found that one side of the heater voltage was approximately 6.3V referenced to ground and the other side was 0V referenced to ground. I had thought the heater voltage would be 3.15V on each side of the winding so I powered the amp down. I checked and rechecked the circuit multiple times and I found no wiring errors. Fearing that I may have shorted a winding inside the transformer, I re-ordered it from AX84. In the time I waited for the transformer to arrive Donato suggested that I mod the stock design and insert another tube for more gain. Taking him up, I ordered the components to add another gain stage and a cathode follower onto the circuit. I sourced these parts from Hoffman Amps. The great thing about moding the amp was that the eyelet board from AX84 already included the space to mod the amp. The modifications would make this amp somewhat resemble AX84's Single Ended Lead. I also designed and ordered a front faceplate from BNP Lasers.
Once the new transformer arrived and the mods were done to the amp I decided to travel to Donato's house to do the new power up. We noticed that the new transformer also had Heater voltage similar to the transformer I had thought was shorted. It is because the way in which the circuit achieves a ground reference for the Heater winding. The winding is grounded on one side through the resistor/capacitor on the cathode of the output tube. This creates a DC bias on the heater windings. (More on this later). Thus, I erroneously re-ordered a new transformer. Donato helped me through the whole power up process and the first initial playing was quite shocking to me. I had not expected this much gain out of the amp. With my Gibson SG loaded with EMGs, I had nearly a growly, metal sound out of this simple amp. It was also a lot louder than I had expected. At about 8 or 9Ws of output power, the amp was perceptively very loud at high enough volume levels.
Since the first time playing it till now, the amp has gone through several changes and modifications to suit my ever-changing and fickle opinion on it's sound. I am still not entirely satisfied with it's sound. I have changed the power supply to have less voltage on the pre-amp tubes and change cathode capacitors on the first and third gain stages to shape frequency response. A final schematic will be shown below which can be compared to the one listed on AX84's site. The amp has a fizzy quality to it when the gain is pushed to a certain level and has some bass response issues which may be inherent in a Class A Single Ended cathode biased output stage. It is possible I am driving too much signal into the third gain stage since it lacks a voltage divider to pull the signal down. However, the amp sounds amazing when you dial the gain down, drop the tone stack out of the circuit and slam on the blues riffs.
Below are a few shots of the amp.
After almost a year of playing through the amp I decided to bite the bullet and gut the heater wires and redo them. I also changed how the heater circuit was grounded. Instead of grounding one side of the winding through the cathode of the output tube, I used 100 ohm 3W resistors to create a fake center tap. Go here if you want a more detail explanation and scroll down to section on artificial center taps. However instead of grounding the center tap to ground I grounded it to the cathode of the output tube to elevate the heater a few volts. DC elevation reduces audible heater hum. I also rewound all the heater wires traveling to the tube sockets so they were much neater and tidier instead of all over the place. In a Single Ended Amp, any heater noise will be quite audible in the out as the hum cannot be canceled by the output section unlike in a push pull amp which can cancel heater hum in the output stage. Below are some shots of the new heater wiring. Excuse the bad quality they are cell phone shots. Heater wiring is the twisted blue/yellow pairs. Second photo shows the artificial center tap.
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