Choice of tube amp?

If you are running ESLs it is the nature of the technology that you will have to run tubes for best results. ML has set up the speakers at a low impedance in an attempt to make them work with transistors, but it is only half successful as you still deal with a dry coloration that is inescapable with most transistor amps due to the characteristics of the speaker (including the impedance curve).

The way you drive the speaker with tubes is to use an outboard device called the ZERO. This is an autoformer that is set up to allow a tube amp to be optimally loaded while at the same time being able to drive a very low impedance load. It would be nice if ML offered a higher impedance tap so that both tubes and transistors could be used...

Bombaywalla, the first thing to do is drop the notion that tubes are voltage devices and transistors are current devices. While it is true that tubes run more voltage and less current, and transistors more current and less voltage, to think in black and white terms in this respect leads to confusion. Both devices actually are power devices. The two appraches do behave differently, but to ignore the power aspects of them is to ignore Ohm's Law, the fundamental law of electricity.

The Quad has an impedance curve that is not based on resonance; it is based on capacitance. As a consequence, at 50Hz (where there is a realitively high impedance) it needs the same amount of power to make 92 db as it does at 10KHz (where the impedance is much lower). An amp that puts out 'constant voltage' (i.e. most transistor amplifiers) will be bass shy and bright on a speaker like this. OTOH tubes will *attempt* to put out the same power with respect to these two different impedances, making for more linear response.

It is true that the newer Quads have lower impedances than the older ones (ESL57 and ESL63). This is an attempt to make the speaker more compatible with transistors, but ignores the very nature of the speaker itself. Thus tubes do have more troubles with the newer models. The ZERO is a solution, but Quad ought to rethink the scenario and at least offer a high impedance tap for tubes and a low impedance tap if you insist on transistors. IMO your investment in the speaker and transistors is not good as you will not realize the performance of either; quite simply its a mismatch.