Project Progress

This project is a four-watt-per-channel stereo amplifier using two 6SN7s in each channel as output tubes. The 6SN7 is known for its full-bodied, rich sound.
Two 6SN7s paired in parallel can deliver up to four watts of Class-A sound comparable to more expensive triode output tubes.

Finding transformers for a project like this can be a challenge. The power and output transformers were purchased from Musical Power Supplies in Columbia, Tennessee. The PT275.2 power transformer has two 6.3-volt windings, one with a 5-volt at 2-amp tap. The high-voltage secondary has a 55-volt bias tap.

The output transformers have dual primary impedance taps of 5,000 ohms and 7,000 ohms. The secondary taps are 0, 4, 8, and 16 ohms. Using different output taps, the primary impedance can be set to different values. Amplifier output would be considered 8 ohms regardless of which output taps are used.

The circuit above is what will be used for the power supply. The filament load is divided between the two transformer filament windings. The filament winding rated at 3.5 amps has V1, V2, V3, and V4, the four output tubes with a total load of 2.4 amps, 4-6SN7 at .6 amps each. With the 3.5 amp winding only loaded at 2.4 amps, to set the filament voltage closer to 6.3 volts, it will be necessary to add a little resistance in series with the filament line. The filament winding rated at 2 amps has V5, V6, V7, and V8; 2-6SN7 at .6 amps each, with a total load of 1.2 amps. If necessary, some resistance will be added later depending on the actual voltage with a 1.2 amp load.

I turned 80 last May, and time has taken its toll. Some issues have developed affecting my hand dexterity, creating difficulty when wiring a chassis. The smaller terminals of a miniature tube socket are difficult for me to wire. After pondering my situation, I did a little design change and will eliminate the 12AY7s. It is an experimental chassis and could see more changes.
EJ Jurich