Heavily into measurements, the output transistors of the MSA naturally undergo the most stringent matching.
Here is another test station with an MSA that's now been fully assembled for an entirely different diagnostic routine.
Here we see further MSAs belly up waiting to be cleared for play-testing back in the K2 building.
The power transformer is covered beneath its own aluminum lid which, though most owners will never see it, is proudly emblazoned with the company's logo and finely grained.
Once right-side up, what becomes the strongest cosmetic cue for the Mosfet Stereo Amp is the massive top-cover heat sink. It's machined from solid stock rather than a trimmed longitudinal extrusion. The latter would invariably leave rough edges. Those would require deburring and never arrive at the immaculate finish Nagra insisted on for this new €8,500 machine.
Having now twice had opportunity to compare the MSA to Nagra's mighty VPA monos with dual 845s per side (once in Athens, once in Cheseaux), I walked away twice preferring the far smaller and cooler-running transistor amp. Under casual conditions in unknown environs, that's naturally neither here nor there. Still, I confidently predict that this amplifier will find its way into more than one present 'anti transistor' home. Hence I've requisitioned Matthieu Latour for a review sample in the new year already. He promised a complete front-to-back Nagra stack to also assess what going all the way accomplishes.
While on the subject of the VPA, here we see how to properly precondition this beefy tube amp without a regular speaker load.
The formal load resistor bank for burn-in of these amps is mounted on one massive aluminum slab.
One of the tweaks Nagra performs on the VPA during bench-testing is carefully rotating the two transverse toroidal transformers against each other until their electromagnetic interactions operate at the very minimum. Naturally, all Nagra transformers are wound in-house.
Since the VPA places a lethal 1200 volts on its rails, operational safety was of paramount concern to the Swiss. Hence the circuit clamps down with instantaneous relays should anything with the 845 bottles go awry. 11 years after introduction, the VPA remains a strong and consistent seller. That likely was potent inducement to now follow up with Nagra's unconventional homage to the famous 'Queen of Triodes'.
To properly test a pair of VPAs under high power output requires not only a capacious load resistor bank but forced air cooling to prevent those stressed resistors from incinerating.
Tubes undergo stringent tests as well. Here is Nagra's machine for the 845s, once again of their own design and fabrication.
A small army of 845s throws off tremendous heat, thus this movable station was parked in one of the many hallways of the K2 building.
To arrive at just two
premium-matched ultra low-noise valves for its phono stage, Nagra sifts through 500 tubes. The magic color code for those rarities destined for the VPS is incidentally red, not green.
The remaining 500 valves aren't necessarily lemons though depending on type and supplier, reject rates can exceed 50%. Many tubes which pass simply won't be used in this particularly critical juncture of micro-level phono signals.
Here are two of Nagra's own shots of the VPS phono stage. The lower image also shows the vibration attenuation platforms which use embedded viscoelastic pellets for decoupling.