This single-Ended (SE) 6550 design is based on a minimonitor constructed by Lars Ohlsson of ElektroAkustik & Music.
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.
Single-Ended (SE) 6550 Tube Amplifier Schematic - [Link]