Tube amp power rating question.

Herman,
thanks for the info, I have two questions on this subject, the first how would you set the taps on the Tube amp if your speaker nominal impedance is 4 ohms but it can go down to 2 ohms, would you set the amp on 4 or 2? the second is how one would match the tube amp with preamp also in terms of impedance? should the values be the same or one more than the other but within a certain range? your help is much appreciated

The best way to determine the correct tap is to try both and listen. I know that isn't much help.

The situation with preamp output and amp input impedances is similar, but in this case what you are trying to do is transfer a voltage. The standard in stereo equipment is to take the small voltage from the source (phono cartridge, tape head, etc.) and to amplify it without adding much current until the final stage of the power amplifier. The current flow is very low until the final stage so there isn't much power involved. The ideal for voltage transfer is if the output impedance of the preamp is zero and the input impedance of the amp is infinite. However, the higher the input impedance of the amp the more easily it will pick up noise so a lower value is used. Typically 100,000 ohms for a tube amp and somewhat lower for a solid state. The output impedance of the preamp is usually much lower than this. A typical value is less than 1,000 ohms but varies greatly depending on the design philosophy of the builder. A ratio of 10:1 is probably as low as you would want to go with 100:1 considered very good.

There are a few that advocate matched impedances for maximum power transfer but these are in the minority. To be truly matched, the characteristic impedance of the interconnect cable must be matched also, which is rarely the case except with digital and RF cables. There are even a few system designs that utilize a current transfer function with very high preamp output impedance and very low amp input impedances.