As mentioned, the impetus for the HP model was an exercise in pushing the existing power limits of the 350M. This current HP has as its heart a new dual-core custom power transformer providing higher voltages and currents. "The larger core is for supplying the power stage; the secondary transformer is used for generating the voltage for the driver stage. The two transformers are completely separated. Just the casing is common. Internally the two transformers are separated by a metal sheet that is isolated magnetically so they cannot interact. If there is a big demand on the larger transformer, it will not influence the smaller transformer by way of magnetic coupling."

Logically, this new transformer is not a plug'n'play replacement. From the transformer on out, the design requirements for the high-power upgrade spread like tentacles throughout the amp. Jim explains that "to honour what we accomplished in the original 350M both in the midrange and treble, it was necessary to change the tubes. If you change the characteristics of the output stage by increasing the power and reducing internal impedance, it changes residual distortion characteristics. If you examine the signal with a spectral analyzer, you will see differences between the 500-watt power stage and the 350-watt power stage. The harmonics are not placed in the same way. To get the same sonic results, we had to compensate by adjusting the driver stages in a different way."

Whilst the amplification stages are essentially the same, Michel changed bias points to get a higher output voltage for additional power from the circuit, allowing them to output 65Vrms to drive the Mosfets. Tenor continues to use the 12AX7 in the first gain stage. However, for the 350M HP the brand changed from Tung-Sol to Mullard. Jim says that while some believe all 12AX7 are essentially the same, they aren't. "For the 350M we chose the TungSol for its harmonic characteristics. For the HP we went with Mullard. All have different characteristics and we had comfort with both brands. When we changed the output stage, we found that we had changed the amp’s harmonic structure. Our objective was not to do that. Our objective was to stay with the same characteristics of the 350M. Therefore we tried other tubes of which the Mullard was one of our favorites. It compensated for what we needed. The compensation was also made by adjusting the bias of the other tubes. This required changes in resistor values and voltages. Simulations, measurements and testing confirmed the changes in each stage. Finally, measurements and testing of the entire system ensured that we had the same harmonic structure as before."

The 7044, a second-stage tube used in the 350M, has been replaced with an E182CC. This has more transconductance, meaning a lower output impedance to drive the transistors. Although the 7044 have a very low failure rate of 5% to 7%, the E182CC are even more reliable and in Tenor’s experience, virtually perfect. Finally, as with the 350M there are two valve-based gain stages and a buffer stage to drive the outputs. The Mosfets provide no voltage gain. Their sole job is to convert voltage to current.

Another change from the original 350M is tighter tube QC. Tenor’s goal is to select, burn in and test the tubes so that in the final selection, those tubes that make the cut have potential lifetimes in excess of 10 years of normal use.  A high-performance car that spends an inordinate amount of time in the shop will have diminished ownership value. The same applies to ultra high-end audio. With their global reach, retrieving an amp with a blown tube from China, Russia or Bangladesh provides significant logistical problems. With thousands of tubes in stock and a decade of data, Jim believes that Tenor have the most robust tube testing and QC in the world. Each tube type passes a unique test regime and in-house burn-in of up to 700 hours to eliminate infant mortality. The tube is then subjected to bombardment in an acoustic chamber with white, pink and brown noise whilst being attached to a measuring device. Output, distortion and microphony are measured and categorized. As many as 60% of tubes fail Tenor’s testing. This combined with Michel’s conservative circuits suggests a lifetime of more than ten years of daily use.

Their fanaticism extends from the tubes to the output Mosfets. Those too are burnt in, tested and matched. Tenor claim that due to their testing procedure, they never had a Mosfet fail in the field. The specific brand and type of transistor are the same in the 350M and HP. The latter’s circuitry was simply modified to handle the additional power and lower output impedance. With up to 1'500W, protecting amps and speakers from every conceivable electrical and human failure was critical. It is not unreasonable to think that a Tenor/speaker combo might exceed $250'000. Protection must be absolute but individual component testing will only go so far. "Therefore we had to examine whether the 350M protection circuitry that we spent the better part of a year designing would also protect the HP. We had to go through all of the original testing and make some adjustments to components. We had to verify that the safety factors were there so that the amp would protect itself."

What could possibly go wrong? Let's see. How about a dead short? Excessive current? Overheating? Polluted ground? Clipping? Catastrophic tube failure? Voltage between neutral and ground? Output transistor failure? A party guest throwing a coat on the amps to suffocate ventilation? Tenor's protection system covers all these contingencies, producing nothing more than a flashing red front logo to indicate a problem. Upon detecting a fault, the amp will either go into protective mute in less than 1ms; or perform a hard shut-down depending on cause. If an amplification stage were to fail completely or the buffer stage experience catastrophic failure, another mechanism cuts the power supply to the filaments to protect the other tubes in the circuit. If the high-voltage supply fails, an LED will light inside the amplifier indicating the problem. Again, a bit more insight into Tenor’s design obsessiveness.