Within a month of completing the prototype circuit described below I was building an aluminum chassis for making a full dual mono single ended triode amplifier. I folded up the chassis from 2 mm aluminum, used 3 mm for the front and turned transformer pots from 4 mm x 80mm diameter tube. A mechanic at work helped out by CNC machining covers for the tubes. After drilling and cutting all the holes and polishing I have now mounted the hardware. Originally my plan was to have all the transformers potted, but two of the covers came out badly, so for now I am content just to leave the output transformers as they are and go on with the building.
In this picture all but two transformers and a socket are in place. I have rubber grommets for every hole. From left to right in the picture;
Power switch, fuse and AC socket are at the back. 235Vac to 12.6Vac transformers are at the back (left). Next come 12.6 to 210V plate voltage.
the high tension power supply is planned to go on a PCB under these transformers in the middle of the amp. Star ground will be on the bolt in the middle of the white reflection, between the output transformers. Then come the octal sockets. There is a ALPS 100K logarithmic potensiometer in the center of the front along with two red LED´s that will monitor the charge of the smoothing capacitors.
I am buying my components from parts-express.com, also a bit from Partsconnexion.
I use ruby gold capacitors for the power supply and 1N4007 diodes. From a local hardware store I got two cheap battery chargers rated at 2,4 amp 240v for the power supply, the first makes 13v for filament heating, the second is reversed and brings voltage back up to 230 or so. Since I am in a 240V county I don´t have to build the voltage doubler Nachbaur specifies, but in stead have to bring down the B+ voltage (for the plates), by tweaking the Capacitor, Resistor Capacitor (pi) filter. For understanding power supplies, I really recommend Duncan amps PSU DesignerII Its free and really cool software.
I went for a dual mono design, figuring it was best to follow the plans precisely. That was before PSU Designer. (PSU=Power Supply Unit) In retrospect I would rather build it with a common power supply to reduce parts count. The PSU was soldered on stripboard, experimenters printed circuit board (PCB).
The rest of the amp, is built by point to point wiring around the sockets. Which are octal by the way, I dont think Nachbaur mentions this.
I have a habit of raiding the local electronics shops dumpster and pulling out amplifiers and TVs. I used a couple of components from them as well as wiring from them. Mostly 18gauge 900v rated wire. A bit thicker for the filament. Remember to twist the filament wires tightly and keep them away from other the components. I kept the filament wires in the corners of the amp, and crossed other wires as close to 90 degress as possible to reduce hum, this is good practice. Another important thing is to orient your output transformers at 90 degrees. (Hammond 125CSE) Collect all the ground points at one spot for a star ground. This keeps ground potential even.
For input signal I used Solen SM series foil caps and a logarithmic Alps potensiometer, and then more Solen caps for the coupling capacitors.
This stoy continues with Final notes on the 13EM7