The changes in the input and pre-amp circuits are not the only ones, as they also affect the output stages:
“The power amplifier part has also been modified – a triode-ultralinear switch has been added to change the operating mode of the power tubes, and the operating conditions of the amplifier have also been redesigned, mainly through the use of automatic quiescent current adjustment. In a nutshell, the cathode bias (so-called autobias) has been replaced by a fixed (manual) bias, but it is not regulated by the user, but by analogue automation, which monitors the currents flowing through the tubes and continuously adjusts the negative voltages applied to the grids. The elimination of cathode polarisation has many advantages, the most significant of which are increased power, reduced distortion and improved tube operating conditions. Now the tubes can operate at lower currents, so they are less stressed and should last longer. Moreover, as the tubes wear out, in a typical cathode bias circuit we would see a gradual decrease in current and, consequently, in output power. In the new Luna, the automatic system will set the correct current even with more worn-out tubes, which should allow you to squeeze the last drops out of them, with a quality not much worse than that of fully functional tubes. In short, it will allow for longer lifespan and a much later signs of tube wear.”
Please allow me to make an important comment here. The fact that automatic quiescent current regulation is used, and that it operates continuously, is really of utmost importance in this case. This is because fluctuations in these currents in audio amplifiers are a really big and often overlooked problem. They affect not only tube designs, but also transistor circuits, where after leaving the class A region of operation, there is an abrupt transition to class B mode. And that in turn leads a lot of problems, including (but not limited to) a phenomenon known as a GM doubling. The new Luna has an advanced system to protect against such bias changes:
“Luna uses analogue automatic quiescent current adjustment. The system consists of several stages: a current-voltage converter, an amplifier, an integrating circuit with a limiter, and a final amplifier that generates the appropriate negative voltage for the grid. This voltage changes and evolves as the tubes gradually wear out. The automation eagerly takes over from the user, who no longer has to adjust the bias with a multimeter and screwdriver, doing it for them, even more perfectly: more precisely and continuously. Even when the music is playing. This was the whole challenge – how to maintain a constant quiescent current in a class AB amplifier, where by definition the current varies depending on the drive? Suffice to say that between full drive and no input signal, the negative bias voltage does not change even by 1V.”
Another important thing should be mentioned here. That is the use of toroidal output transformers. FEZZ uses them in all of their vaccum tube amps. Worth mentioning is that the Polish company actually makes them on their own – after all, the brand was created by expanding Toroidy.pl to include a division dealing with the production of audio equipment. Mr Jakub Korpacz from FEZZ tells us about the advantages and general characteristics of toroidal transformers:
“The advantage of toroids over popular cores made of waste-free “EI” fittings stems largely from their optimal shape in terms of magnetic field lines – symmetrical, without protruding corners and holes, completely surrounded by windings. Toroidal core has another feature that distinguishes it from a core composed of a stack of laminations – it has no air gaps. A toroid tightly wound from anisotropic sheet metal is a monolith from the point of view of the field lines, which cannot be said about a core composed of laminations, even if assembled with the utmost care and precision. There are microscopic gaps between them, which constitute reluctance inserted into the magnetic circuit. This feature of the toroid is not an unambiguous advantage, or in other words: you have to know how to use it.
A transformer with micro air gaps (moulded), at the expense of lower achievable parameters, is much more forgiving. A few turns difference, carelessly wound winding halves, inaccurately selected power tubes or tubes that wear unevenly, poorly set quiescent current… all these things lead to a partial magnetisation of the core with direct current, which occurs much more gently in the presence of air gaps. And in the case of a toroidal magnetic core – almost instantly. A transformer with a saturated magnetic core has a drastically narrower frequency response and enormous distortion, reaching up to several dozen percent for the lowest registers of the audio spectrum.
Therefore, in order to fully benefit from the advantages of toroidal output transformers, there must be no asymmetry in the anode winding halves, and the currents must be completely identical, down to the milliamperes. Then such a transformer rewards you with parameters unattainable for other types of cores. And while winding asymmetry can be dealt with by simply winding them carefully, uneven tubes, especially those that wear unevenly, are difficult to combat. Only the proprietary analogue automation system used in LUNA, which continuously monitors and sets quiescent currents with an accuracy of tenths of a milliampere, completely solves this problem and allows you to fully enjoy the outstanding quality of toroidal output transformers.”