Which high power SET

I hate to pop your bubble but OTOH, I will probably save you a lot of heartache too so:

The Harbeth is a revealing speaker but not particularly efficient. Its efficiency (and efficiency matters more to tube amps than sensitivity FWIW) is low- only about 85 db. For those that think it is higher keep in mind that Harbeth rates in sensitivity and the impedance of the speaker is 6 ohms. When you do the math 85 db is about right.

You will find that you are better off with a push pull amp that can make some power. There are several reasons for this, not the least of which is that the more powerful the SET is, also the less bandwidth it has. In this case any 20-30 watt amp is going to fall on its face right away.

To take advantage of an SET's characteristics (low distortion at lower power levels) you really do need some efficiency, at least 10 db higher than what you have for the amps mentioned. The only caveat here is if you plan to listen in a small room and in the near-field. Otherwise, seek a 100-200-watt amplifier for the average American listening room.

Edoit, I think you will find its in your best interests to do an audition on your speakers in your home if you can.

SETs with the same power as your 30-watt push-pull amp may not come off as just as refined. I alluded to the issue, but here it is in plain black and white:

If you want to really experience what an SET does, the efficiency of your speaker should be such that the amp *never* is asked to make more than about 20-25% of full power. If you follow this rule you will get the most bang for the buck out of your SET investment dollar. The reason has to do with how SETs make distortion- above that power level the higher order (harsher) distortion elements come into play. As the power demands are decreased, the distortion goes to unmeasurable and is the reason for the 'inner detail' claims you see describing a lot of SETs. This has to do with how distortion can mask detail per the ear's masking rule.

Since this speaker also has some bandwidth, you may also notice that an SET of similar power simply does not have the bass delivery, as it is very difficult to make an output transformer that can deliver bass and full treble extension at that power level.

I'm not a fan of pentode amps FWIW, being a manufacturer of triode amps myself :)

I think you have sussed out that SETs have a following for good reasons- I think if you look into it, you will find that the most successful systems have lower powered SETs (which have more bandwidth) and much more efficient loudspeakers. My speakers are 98 db 1 watt/1 meter, and I find that 30 watts is a minimum in my room, which is an average size. I do have a type 45-based amplifier (push-pull, makes about 6 watts) and it sounds better than the 45 in SET mode on the same speaker; but even so its hardly enough power to do the job!

Now for the math: the 6 watts that I have on my speaker translates to the same sound pressure level on your speaker with about 120 watts. Think about that. There is no point to buying any amplifier and then pushing it as hard as you can.

If it's topology, it means harbeths with its low sensitivity are generally incompatible with any SET amp, irregardless of it's power output. Given a hypothetical world where both push pull and SET generate the same power output, does push pull drive harbeths better simply because of topology?

Yes!! What is not getting the air time it should in this discussion is the fact that to take advantage of an SET, **you must not push it past about 20% of full power**!

You can go past that amount with a push pull amplifier and not have ill effects.

This is all about your amplifier investment dollar- with a speaker of efficiency that low, your investment dollar in an SET will not be served due to the fact above.

The impedance curve is another issue! It is pretty evident that the Harbeth is expecting that the amplifier used with has some negative feedback to linearize the output of the loudspeaker. Your EL34 amplifier has that feedback. If you put a zero feedback amp on that speaker its likely that the range of that high impedance in the midrange will take on some prominence.

To understand more about this phenomena, see this link:

http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

Ait, have measured the bandwidth of these behemoths?

That is quite impressive to get that sort of bandwidth out of an SET of that sort of power! Do you know the configuration- by any chance is it a parafeed circuit?

I remember reading that thread. Nice work!