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The preamplifier is unusually hefty for the breed and looks like this with its sliding bottom cover removed:
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Some might wish for the stepper motor of the attenuator to be concealed inside a housing rather than sitting out in the open. The remote is plastic but controls inputs, volume, mute and power. Now a snafu occurred. After a few sessions with the monos which arrived first, one 300B filament wouldn't come on and the tube's bias meter sat at zero. Replacing its blown 0.6A/250V fuse revived things. Kang Su commented that "sometimes, 300Bs short between filament and control grid to cause wild in-rush currents which can trigger the fuse. Other makers use the self-bias method which prevents that, albeit at a sonic cost. If this fuse blows again, it is not in-rush current but tube failure."


Now the same tube's bias meter showed full current immediately after power-up, not the proper delayed response of its brethren. "We think the coupling capacitor of the troubled tube has short-circuited so with the preamp, we included eight new 1uF 630VDC capacitors. Please change the coupling capacitors connected from the driver choke to the tube grids in both amps. Defective capacitors are very rare." For immaculate professional surgery, I made an appointment with Serge Schmidlin of Audio Consulting and Silver Rock fame who lives 50 minutes west towards Geneva.

In all his career, Schmidlin never saw such a capacitor die but as his meter showed, my culprit indeed had given up the ghost. Despite such rarity, Kang Su implemented an immediate production switch to the same brand of black-wrapped tin-foil capacitor with the higher 1uF value which Schmidlin installed in my loaners.


Meanwhile David Beetles had experimented with the Emission Labs valves. "I put one in wrong. Bias meter immediately went to go during warm-up and when voltage hit the 300Bs, the amp shut down. I checked the mains fuse under the plug, pulled the fuse tray, powered the amp up again with new 5A/250V fuse and tube properly reseated and no bias showed for the errant tube. Checked the fuse by the tube which was blown. Found another fuse in my tickle trunk, popped it in and we were up and running. Great protection. Even with my operator error, the tube survived. The XLS tubes right out of the box are so much better than the stock ElectroHarmonix, it was frightening. My weekend audio guests were staggered and so was I. PRaT seemed to improve along with the detail and soundstage size. And that's new out of the box. I am not going to bother trying the Sophia Carbon plates. These EML XLSs are the real deal." I concur and listened to the monos with a complete octet of these output tubes. Evaluating the L-4000 preamp would be first, however.