How do tube Watts compare to SS Watts?

Rascal, the most important factor when using a tube amplifier to drive speakers, is that the speakers must have a benign impedance curve. If the speakers have an impedance curve that varies significantly, even a 1000-watt tube amp will have problems driving them. The nominal impedance numbers i.e. 4 ohm or 8 ohm is not really important as most tube amps have 4ohm and 8ohm output taps.

In short, if your Thiels do not have a benign impedance curve, go solid state.

I drive 88db speakers to very loud levels with a 12wpc amplifier, so in theory you should be able to do the same with and amplifier rated around 20wpc. Again I repeat, this will hold only if your speaker has a benign impedance curve.

There is definitely something different between the rated output of a tube amplifier and a transistor amp. Wattage is measured the same, that much is true, but anybody that has compared tube and transistor based amps of similar ratings will be well aware that the vacuum tube amplifier can produce much higher SP levels before overloading is detected.

This is partially due to the fact that tube amplifier clip in a harmonic (musical) fashion, while high NFB used in transistor amps make them square off the signal when clipping. This squaring off of the overload is very noticeable and objectionable. As such, transistor amplifiers cannot be run near their maximum rated output. Tube amplifier can.

Another theory I have heard is that naturally dispersed harmonics add to the perceived SP of the signals. Vacuum tube amplifiers tend to produce high levels of naturally dispersed near maximum output. Transistor amplifiers tend to produce unnaturally dispersed harmonics which are both very objectionable and add nothing to the perceived SP levels. Tube amplifiers therefore deliver a bit more than what their rating suggests.

Regards
Paul

03-14-07: Eldartford "The sound level that can be produced by a tiny amount of power"

Now that I can agree with ... :-)

Bartokfan. A pentode is a vacuum tube that has three grids, namely a control grid, a screen grid and a suppressor grid. The screen grid was added to isolated the control grid from the plate to reduce Miller effect.

A tetrode is a tube that has a control grid and screen grin (no suppressor grid). The suppressor grid is added to reduce secondary emissions from the anode.

A Triode has no screen or suppressor grid.

Triodes, Tetrodes and Pentodes can all be used in a push-pull or single ended configuration. When used in single ended configuration they all have to run class A, in push-pull they can be run in either class A or B (or combination thereof.)

Tetrodes and Pentodes have their screens grids biased to +. If you bias the screed grid from the anode, it is know as triode mode. If you bias the screen grid from a tap on the primary winding of the output transformer, it is called ultraliniar mode. If you bias the screen grid from the power supply, it is called tetrode/pentode mode.

The method of biasing the screen grid alters the character of the tube slightly. Many amps have a switch to toggle between two of these modes.

A KT90 is actually a kinkless tetrode (the KT stands for Kinkless Tetrode). I believe that is actually a pentode with the suppresser grid tied to the cathode. And EL34 is an example of a pure pentode.

Regards
Paul

Bartokfan, the assumptions you make pertaining to the characteristics of tube are incorrect. There are a great deal of variance is sound characteristics of any given tube from different manufacturers. A Tungsol 6550 sound very different to an Electra Harmonic 6550, which in turn sound different to SED, Svetlana or Phillips 6550s.

Additionally, factors such as the circuit design, quality of components, screen grid bias method etc. etc. have dramatic effect on the sound.

EL34 are generally known to be very melodic when run in triode mode, yet partnered with the right speakers they produces a prodigious bass slam in my V12i. KT88’s are known to have bass slam, but they are wonderfully melodic and detailed in my Audionote.

Observations on what a amplifier/tube sounds like, should be made after auditioning, not before.

Regards
Paul