Typically tube amps maintain the same power output at all impedences. It doesn't double or halve as you go up and down as can happen with high current ss amps.
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Most tube amps have impedance matching output transformers built into them. You have to select the most appropriate impedance tap for your speakers and it "should" produce rated power into that load. Having said that, there aren't any speakers that i know of that maintain a steady state non-reactive load that is of a consistent impedance. As such, the output transformer is somewhat of a buffer, but power output WILL vary even with it in place.
As a side note, many tube based amps do fall off in power as impedances are dropped, especially at the very low impedances that you mention. If i was going to run a pair of VERY low impedance speakers with a tube amp, i would probably look into the Mesa product line. Since they build / design professional tube based amps, they know how to compensate for very low impedance loads. Whether or not you like the sound that their circuitry produces is a personal thing, but at least you wouldn't really have to worry about straining / blowing up the amp or tubes all the time. Sean
Tube amps drop their output power as the impedance goes down and capable only to drive a high impedance loads opposed to the SS amps that have a large current delivery capabilities certainly depending on output stage configuration and power supply.
An output transformer for the tube amps is an interface to match impedances of speaker with output stage that in real case requires the impedance to be much higher than even 16 Ohms.
Tube output stages have no current delivery possibilities and mainly amplify the voltage. Thus with transformer output stages tube amps deliver same power to the different load values depending on output transformer capabilities to handle the low impedance loads for certain time.
My Martin Logan Prodigys (Nominal 4 ohms) dip down to 1.2 ohms. Keep in mind, that this is at 20 kHz, there simply is not that much information going on in that part of the spectrum.
My amps, the Audio Research VTM200 have no issues whatsovever driving these speakers off the 4 ohm taps. They will recreate staggering volume with no loss of qulaity in my 12 x 26 x 8 room.
I imagine the REF 600 would not have much trouble driving any real world speaker.
No, the power of a tube amp will not increase as speaker impedance decreases. A tube amp will provide maximum rated power only when the speaker load is matched with the output impedance of the tube. Any mismatch will cause a DECREASE of power - regardless of which way the speaker load changes.
The way to match a speaker load is with the output transformer. Using a 2A3 tube, eg, the power rating is 15 watts with a plate resistance of 2,500 ohms, which is also the output impedance. If the load to be driven is 8-ohms, then the output transformer has to reflect 2,500 ohms at the primary when the secondary is 8-ohms. The reason for the 2,500 ohms at the primary is to match it to the tube's output impedance. When these impedances are matched, there is a maximum transfer of power from tube to transformer. All 15 watts are transferred, which is then passed along to the speaker.
If the speaker load changes to 4-ohms and 16-oms, then the primary impedance will change to (reflect) 1,250-ohms and 5,000 ohms respectively, which by ohms law will transfer 13.5 watts to the xfmr (in both cases).
Multiple tap output transformers are used to match different nominal speaker impedances (or else an amp manufacturer will have to wind different transformers and if you change speakers you'll need another amp). It doesn't matter what impedance the speaker is - as long as the appropriate tap is selected, the amp power output will remain the same. If the taps are not properly selected, the amp will put out less power.
A solid state amp can double its power as speaker impedance is halved if and only if the power supply transformer can provide the power necessary for the increased current. Why can't a tube amp do this? Because the power is lost from the tube output to the primary OT transformer winding when the speaker impedance changes. Loss of power equals loss of current at constant voltage, thus the power decrease.
Biamping with a mixture of tube and SS is a thought, but perhaps not practical for sonic performance. I can think of three factors that will affect the sound:
1. You need to match voltage gain (meaning matching volume will be a problem).
2. You need to match input impedence (meaning you can't use use a y-plug to double the input signal and expect good result).
3. You need to match phase coherence (e.g. some tube amps are 180 degree out of phase, which will cause lots of fun at the crossover region, and as well affecting the sonic presentation as a whole).
Tube amp and SS amp are particularly different in # 2 and #3. If you are willing to make the compromise, why not considering a hybrid amp that uses tubes to handle the voltage amplification and transistors the current? The result should be better (IN THEORY) than mixing tube and SS in a bi-amp configuration.