The choice of output transformer should be made based on the desired operating point for the given tube.
Bieworm wrote:
I read that when the primary impedance is lower there is going to be a nicer distortion. Can anyone confirm this and give me some advice? There also is the 1750N, used in the JCM800 / JMP, which has a 3.2k prim imp and 50W rating...
Where did you hear/read this? The answer is not so straightforward because, again, it depends on the operating point. Generally though, increasing the primary impedance will rotate the load line counterclockwise, dropping the 0V gridline intercept below the knee, causing an increase in distortion at the cost of output power. This comes with another price and that is increased screen current, so one must be cautious. The Trainwreck amps (Express and Liverpool) are good examples of this operating choice. The Express ran at 6k6 primary (double the Marshall) but also reduced the plate voltage and utilized the large 1k 25W screen supply resistor to develop a significant voltage drop and screen compression for large voltage swings. Also, individual 1k screen resistors were used to further protect the rubes. The Liverpool also used 6k6 for a quad of EL84s. Both amps are all about output tube distortion and the preamp stays virtually clean.
Another word of caution about classic Plexis and output transformer choice. Some of them ran very high plate voltages (500V or more) but with the 1k7 primary for a quad of EL34s. With the 82k plate load on the inverting side of the PI, the tubes would often redplate.
EL84s don't take much to drive them to clipping so a 4k primary is a good choice at 320-340VDC to give decent headroom. Trainwrecks distort very early but are intended to be controlled dynamically using the guitar volume knob. The magic is that with the screen compression, the clean volume is almost the same as the distorted volume, giving the player the seamless transition from "clean to mean".
