The output stage is at the heart of any single-ended amplifier. It is the design of the 845 output stage in the Monaco which makes it so different from other 845-based amplifiers.
A total of five deliberate design decisions, each of which has significant listening consequences, combine in the Monaco to result in an 845 output stage with approximately 10 watts of output power per channel—and a new level of sonic superiority. Why only 10 watts? This is, after all, an 845, an output tube routinely found in other amps rated at 20 watts or more. Here's why the Monaco is different:
* First, the nature of the 845 at high voltage, e.g., above 1,000 VDC, may be just what the doctor ordered for inefficient speakers lacking conviction. But experience has shown that, when this tube is used at such high voltages with relatively efficient, high resolution transducers, the sound is nothing short of sibilant and annoying. Fast, efficient, high resolution speakers are exactly what is preferred for low-power SET amplifiers, therefore the 845s in the Monaco are run at the lower voltage compatible with such speakers.
* Second, operating the 845 at high voltage requires an output transformer with a higher impedance primary. Typically, an output transformer with a 10,000-ohm primary impedance is specified for an amplifier using the 845. In contrast, the Monaco 845, by employing a lower plate-to-cathode voltage, is able to use a 5,000-ohm primary impedance transformer. Everything else being equal, this means the transformer will have a much better bandwidth, particularly with respect to the high-frequency performance.
To this end, and with respect to both output power and bandwidth, the James Audio 6123HS output transformer was selected. Before arriving at this selection, the 6123HS was compared to similar offerings from both ISO (Tango) and Tamura, as well as a number of domestic manufacturers. The James gives up nothing. In fact, we are told the James units are built on the same production line as the much more expensively priced Tamuras.
* Third, the latest listening tests demonstrate that vacuum tubes sound the most natural and last the longest running at 62.5 percent of the recommended maximum dissipation rating. The Shuguang 845M in the Monaco is running at approximately 60 percent.
* Fourth, there are only three decibels of difference in output between 10 watts and 20 watts. For the layperson, this translates to one or two clicks on the typical volume control. Hardly worth worrying about in light of the musical benefits offered by the Monaco 845.
* Fifth, the lower plate-to-cathode voltage and dissipation of the Monaco 845 allow this amplifier to take full advantage of the Shuguang 845M output tube. The 845M is a close copy of the 1940's vintage Amperex brass-base 845, but is built with two notable features which, with one exception, sets this tube apart from all other current production 845s: It has a metal plate as opposed to a graphite plate and it lacks a mica top spacer. The only other 845 with such a build characteristic is the Shuguang 845B.
