[Amps] 8122,4662,4cx400a,4cx250b,ncl2000

[George Badger]

The 4662 will replace the 8122 exactly. When I was with Svetlana we retrofitted an ncl2000 with 4cx400a's. The retrofit was straitforwad and it worked well. We chose the 4cx400a for marketing reasons but the 4cx250b would have been a much better choice from a technical point of view beacause the 4cx400a requires jockeying parts around and cutting hard steel  to move the sockets. OTOH the 4cx250b and  socket drops in place easily and few parts need to be moved. Don't worry about the ratings because the cathodes and grids in all these tubes are about the same size. The 8122 and 4662 grid material is soft copper and the others have molly grids which are therefore stronger. The 4cx250b anode size is the same as the 8122 so it is capable of tha same dissipation. and fits in the amplifier with ease. The formal ratings for the 4cx250b are less than the 8122 but it is more rugged because of the grid material.