Single-Ended (SE) 6550 Tube Amplifier Schematic

Magnus

SE 6550 Tube Amplifier Schematic with 12DW7 Driver Stage

This single-Ended (SE) 6550 design is based on a minimonitor constructed by Lars Ohlsson of ElektroAkustik & Music.

Single-Ended 6550 Tube Amp Schematic with 12DW7

Single-Ended 6550 Tube Amp Schematic with 12DW7 Driver Stage

The single-ended amplifier circuit uses a 6550 high power tetrode vacuum tube in the output stage. A 12DW7 or 7247 is used to drive the output stage. The 12DW7 / 7247 is a dual section triode, the first section has a gain factor of 20 and the second section a gain of 100. You can think of it as one-half of a 12AX7 and one-half of a 12AU7 in the same envelope. New production JJ ECC832 tubes are a suitable replacement for the 12DW7 and 7247 which are no longer in production. C2 and C3 should be good quality film capacitors. The cathode bypass capacitors C1 and C4 will also have some impact on the sound and good quality low ESR capacitors are suggested.

Output power will be about 7W. With the 390 ohm cathode resistor (R9) the 6550 draws 91mA which is about 82% of the maximum plate dissipation. A 330 ohm cathode resistor (5W) will increase the current to 103mA which is about 92% of the maximum plate dissipation. The audio output transformer should be 3k and rated for at least 10W.

Power Supply Schematic - Single-Ended 6550 Tube Amp

The transformer ratings shown in the schematic are minimums. A linear power supply using bridge rectifiers is used for the Single-Ended 6550 Tube Amplifier. Adjust the value of R11 to get 400V. To improve the power supply a 5 to 10H choke can be used in place of R11. Pick choke with a similar resistance as R11 and rated for at least 500VDC and 125mA. A film capacitor is preferred for C9.

Power Supply Schematic - SE 6550 Tube Amp