40+ watts SET, cost is not a problem

@lynn_olson

but SET output transformers have serious design challenges once the power gets much beyond 20 watts.

In the link provided above, Andy Grove (Audio Note) acknowledged that in his article and explains his remedy. By many listener accounts he was successful with the 845 tube amplifier output transformers dilemma.

I wish the OP could sample your Black bird designed 300b push-pull with the TAD speakers he wants to drive with tube amplifier. 17x13 ft room, pretty much near field environment.

Charles

Hmm, well the Blackbird puts out an honest 22 watts, with half-second transient peaks probably around 60 watts. They power highly reactive electrostats with no issues, because of deep Class A biasing and zero feedback ... the load-lines for the PP 300B’s are nearly perfectly straight, which is ideal for reactive loads. With zero feedback and power supply isolation between drivers and outputs, the amplifier behaves like a limiter/compressor as it is pushed beyond 22 watts. (The power supplies are good for 200 watts per channel, plenty of reserves there. The voltage and current limiting is in the 300B’s.)

But for inefficient speakers with high-order (18 or 24 dB/octave) crossovers ... probably not. Speakers like that need an amp with a high damping factor and at least 200 watts of clean power. High damping factor typically requires high feedback, and high feedback also means hard clipping. Hard clipping sounds terrible and can destroy tweeters, so the amp should never be operated in the clipping region. So lots and lots of power, enough to never approach clipping.

Modern Class D amplifiers, particularly with GANfet switching transistors and well-designed PWM modulators, are probably optimal for these kinds of speakers. This class of amplifiers avoid those annoying Class AB colorations, and don’t have problems with bias drift as they warm up. And power reserves are large enough to avoid the clipping region.

Note there are two approaches with not that much in-between. Either moderate to low power (2 to 20 watts) with very gentle clipping and modest damping factor (between 3 and 4), accepting that clipping will happen but will have low audibility, OR 200 to 500 watts of power and clipping never happening. There’s also a handful of low-feedback low-power Class A transistor amps, which I guess are sort of a halfway house between the two.

@lynn_olson 

But for inefficient speakers with high-order (18 or 24 dB/octave) crossovers ... probably not. Speakers like that need an amp with a high damping factor and at least 200 watts of clean power. High damping factor typically requires high feedback, and high feedback also means hard clipping.

Okay got it..I'll defer to your assessment. No one knows more the capabilities and suitability of this amplifier than you (And Don Sachs).

Charles

Low-order passive crossovers (6 or 12 dB/octave) are less sensitive to amplifier output impedance (damping factor) than high-order crossovers (18 or 24 dB/octave). High feedback is the most direct method of reducing output impedance for both tube and transistor amplifiers.

Which in turn affects how an amplifier clips. Unless a "soft-clip" diode array is used in the feedback loop, but those have other, unwanted side effects, such as increased distortion in the top 3 to 6 dB of the amplifier’s power curve.

It’s considered good practice to let the output devices define clipping. At that point, they are giving all they have, forward gain collapses, and feedback loses effectiveness. The transition zone from linearity to clipping might involve charge storage if solid-state devices are used, or the power supply for the driver stage could sag, and let the drivers clip at the same time. This extends the recovery time from clipping, with power supply rails fluctuating up and down as the circuit recovers.

The SET amplifiers without feedback do not hard-clip, but if the driver and output section have a common B+ supply, that supply will sag when the amp clips, and if the driver and output section are RC-coupled, then the coupling cap must re-charge before normal bias appears on the grids of the power tubes.

If regulators are used, they must tolerate overload conditions and recover quickly, preferably within milliseconds. As you can see, a lot can go wrong if the speaker demands more current or voltage than the amplifier can deliver.

I should mention speakers not only consume energy, but they reflect it back to the amplifier as well. This is what a reactive load does. The most severe condition is a pure reactance with a phase angle of 90 degrees. That reflects ALL the power back to the amplifier, which can create an overcurrent condition in the power transistors. Tubes tolerate this better than transistors because it takes a gross overload lasting several minutes to overheat the plate of the tube, while a transistor can fail in milliseconds.

It’s kind of sad how low speaker efficiency really is. We think of a 92 dB/meter/watt speaker as "efficient", but it’s only 1% conversion efficiency. (A more typical 86 dB/meter/watt speaker is 0.25% efficient.) The 1% efficient speaker needs 100 electrical watts to create 1 acoustic watt (which is very loud).

What makes it worse is the other 99 watts (which are quite expensive) do nothing but heat the voice coil, which is undesirable because it is in a very small space and is not easy to cool. A more efficient speaker has the advantage of cooler voice coils for a given playback SPL, which gives it greater headroom.

Kondo or AN UK are some (or the) of the finest SETs you can find, but I can't comment on the silver used in the Kondo, although silver is the best conductor. However, as other people stated, the TAD speakers are low sensitivity so even if you have 40W, they are probably not the right amplifiers. I would suggests to look at push pull like the EAR 509, which is 100W per channel or the EAR 890 which is also bridgeable to 140W per channel. Otherwise you better change speakers if you want to use a SET.