Tube amp power rating question.

It depends on the design , Many tube amps feature multiple taps so that they can deliver the same power to different loads, While others like the VTL ST-150 are designed to deliver the same wattage into different loads. Read the amps specs , look up the amp on the companies website, or call the specific manufacture for details about their product. Its also a good idea to make sure that the impedence of the preamp is properly matched to the poweramp.

This is impossible to understand without a basic understanding of electronic fundamentals. Excuse me if this is long winded.

Power (P) is the product of voltage (V) and current (I). P = V x I
Current is equal to voltage divided by impedance (Z). I = V/Z

In order to deliver more power into 4 ohms than 8 ohms, the amp must be able to maintain it's output voltage even though the impedance is changing. Solid state amps are designed as constant voltage sources, they maintain their output voltage over a wide range of speaker impedances. As you can see from the formulas above, if the voltage is constant and the impedance drops, the current will go up. If the current goes up and the voltage is constant, the power goes up.

Example: Assuming an amp whose output voltage is independent of the speaker impedance.
An amp puts out 16 volts into an 8 ohm speaker. This results in 2 amps of current. 16V/8ohms = 2A. This means we have 32 watts of power, 16V x 2A = 32 watts.
If the impedance drops to 4 ohms we now have 64 watts.... 16V/4ohms = 4A.... 16V x 4A = 64 watts.

This is only possible if the amp has a very low output impedance. As we draw more current, the output impedance of the amp will drop more of the output voltage, and the speaker will get less. Solid state amps can have very low output impedances but tubes amps do not. In fact, the output impedance of tubes is so high that we usually use impedance matching output transformers to get reasonable power to the speaker. Otherwise all of the voltage would be dropped across the output impedance and none would get to the speaker.

Without getting into the physics of transformers, suffice it to say that the most common way to do this is to choose windings ratios that match the impedance of the load to the impedance of the output tubes. The output transformer therefore has taps at different points on the output windings to match the impedance of different speakers. The amount of voltage delivered to different impedances is adjusted via the taps so the power delivered is the same for the different impedances. However, the amount of current drawn from the tubes stays the same for different impedances. This maximizes the amount of power that can be transferred from the tubes to the speaker. We get maximum power transfer, but the power remains the same for different impedances as long as the proper taps are used.

Hopefully this isn’t too much info. It is too complex a topic to give a very simple answer. And this does not even consider other variables such as the speaker impedance varying with frequency and different amplifier designs such as OTL.

BTW, the VTL does not deliver the same power into different loads. It is limited by the same factors that all amps of this design are. See http://www.vtl.com/pages/st150specifications.html
for the specs.

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.