When I first developed an interest in building a small vacuum tube audio amplifier based on the Mullard 3-3 circuit, at the beginning of 2016, my intention was to construct it using point-to-point wiring, in the same manner as in vintage radio receivers, as can be seen in the various amplifier models presented on the pages of this website. A good friend of mine, Emilio Roldán, assisted me in designing the layout of the components within the Hammond chassis that I had previously selected based on the size of the transformers. Emilio’s signature, dated August 26, 2016, can be seen on the component layout sketch of the first 1955 model, which I now also refer to as the 1955-1.
I built the prototype of this first 1955 amplifier model at the beginning of 2019, connecting the components point-to-point according to the guidelines indicated in the drawing. The considerable time that elapsed between the conception of the idea and its actual construction can be explained by the fact that, as this was purely a hobby, I only devoted time to it whenever I felt inclined to resume the project. In Alicante (Spain), we are fortunate to enjoy such a privileged, mild, and pleasant climate for most of the year that one is often more inclined to walk along the seashore, go hiking in the nearby hills, stroll through the city, or simply have a coffee on a terrace, rather than remain indoors at the small workbench I have at home.
Years later, as a form of recreation, I decided to design a printed circuit board for the amplifier with the aim of simplifying component assembly. I had previously used free software applications to create printed circuit boards for other leisure projects, which also helped me clear my mind and relax from my daily work. This printed circuit board would also make it easier for others to assemble the amplifier on their own, particularly for those who might find point-to-point wiring challenging.
With the intention of making the printed circuit board as versatile as possible—so that, in addition to being designed for installation in the chassis of all versions of the 1955 amplifier, it could also be used in other electronic equipment such as radio receivers, record players, or other constructions—I decided to design it for a single channel and to use two boards for the amplifier. In order to maintain the symmetrical design of the amplifier, I later realized that it was necessary to create two mirrored printed circuit boards. To be entirely candid, I must add that my use at that time of the free version of Eagle, with its limitation of 80 cm² for the board area that could be created, significantly influenced the decision to design two monaural boards instead of a single stereophonic one.
Furthermore, the objective of versatility—and the possibility that the printed circuit board could be used either beneath a perforated chassis, as in the 1955 amplifier, or above a chassis or cabinet without perforations for the tubes—led me to draw the passive components, such as resistors and capacitors, on both sides of the board. The names of the wiring connection points are likewise printed on both sides.
These printed circuit boards include the names of the vacuum tube electrodes for the EL84 and EF86 tubes—cathode, plate, and grids—so that the electrodes can be easily identified and the operating voltages measured while the chassis is open.
Download wiring schematic in PDF
