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After a couple of weeks I received a picture of two solid Walnut bases each with a slightly different finish. I was asked to choose the one better suited to my speaker cabinets. This was followed a few days later by a picture of the base with an unmounted plaque. After agonies of indecision I gave Attilio the go-ahead for original center position. More pictures which showed the completed amp with no tubes and the internal wiring came. So did a request for an extra month and apologies for the delay. 28 days later all details of the completed amp were subjected to my final approval. Following another Paypal payment, the tiny Tektron was duly delivered to my doorstep in three days (a major Italian miracle this).
Now this was all excellent fun but the facts were that I was buying a—very pretty—pig in a poke. Specifically the amp’s architecture assured very high internal resistance (I would think 4 ohms at the very least). This in turn guaranteed significant interaction with the speakers. Power was hardly an issue. My Tannoys come out of the corner at a 95dB stated sensitivity. And the modulometer on my Nagra 300p informs me that the horrors of room lock are reached in my 40m²+ very reflective listening space well before the 2-watt mark. The trouble? The Canterburys’ impedance and phase plots are very far from those of a large double-ported system such as my preceding 12-inch Yorkminsters. This is because they have Onken-type vertical slots which can be closed by sliding Walnut baffles (a very upscale version of the socks Srajan stuffs in his speaker portholes). Impedance and phase consequently vary wildly. They go from a more or less standard bass-reflex load to something approaching acoustic suspension.
This allows excellent room matching but plays all hell with amp/speaker interactions. That particularly so in my case where best results are obtained by leaving one slot wholly open and one closed on each speaker (in my room wide-open slots result in something not unlike a powerful but underregulated subwoofer whilst fully closed produces bouncy and controlled but essentially tuneless bass). Perfectly damped sand amps like the F5 (or the Job) are totally indifferent to this kind of load. High internal impedance tube designs are a shot in the dark particularly when tasked with controlling a 15-inch very short-throw paper woofer with an extra-heavy motor.
On the other hand the similarly load-sensitive Tripath chip on my Trends amp can drive the Canterburys with no bass bump of the kind that, as Srajan sagely observes, is often associated with room nodes but just as often is the result of an underdamped amp. And though I loathe hum, Attilio assured me that double triodes would not make this much of an issue. Still, when the little amp finally emerged from its double box and its unending swathes of bubble wrap, I was in a stoic mood. In the event pushing on the winsome red-lit button after easily mounting the three tubes produced only a minute amount of very well-behaved noise. This was essentially inaudible ten inches from the speaker cone.
So far so good. Since I had budgeted for a 50-hour burn-in before feeding the amp a steady diet of Abba’s greatest directly from an ancient iPod shuffle, I took stock of its design and construction. The layout was as simple as can possibly be: the upper half of the vintage RCA double triode feeds small signals to the lower which amplifies them and sends them to the 14-amp primary 8Ω secondary home-wound output tranny. Rectification is from a 1952 Raytheon 6X5WGT. And that’s all, folks. The 3-way brass speaker terminals and RCA inputs were wholly adequate. Wiring was workmanlike with solid soldering. For a very nice personal touch Attilio decided on polypropylene caps instead of his usual paper in oils because he felt they would give my Tannoys more air on top.
