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Made from Czech Simax glass which appears to have the qualities and strength of Pyrex, tapping the side of the tube gives the impression of an exceptionally thick and rugged bottle far different than a standard output tube. They are claimed to be exceptionally rugged and dependable, with an internal vacuum equal to outer space. Undeniably exquisite, unique and expensive, it's probably the only new tube designed and built in the last 50 years exclusively for audio [far from it – Emission Labs for one have introduced any number of new audio-only tubes over the last few years – Ed]. Whilst they do throw off heat, one interesting characteristic of the tubes is their light output - or rather lack thereof. One of the first things I did after powering up was to turn off the room lights, sit back and wait for yellow, red or orange glow to illuminate the space. After the lights went down and my eyes adjusted to the dark, there was... well, nothing. If you look at just the right angle, there is a measly sliver of light. Subsequently I learnt that this is intentional. Regular tubes often use a tungsten filament similar to a light bulb, often with rare earth elements mixed in. Conversely the T1610 uses a ribbon filament with a proprietary oxide coating. The result is more focused emission, less heat, longer life and greater reliability but alas no light show.

All KR tubes are handmade in-house, distinctive in both appearance and engineering and in the case of the massive T-1610, incomparable. There are no other tubes that match the physical, electrical and design characteristics of the T-1610. The filament design, high vacuum, propriety coatings and quality build should ensure a long and stable tube life but again, there are no comparable options. Without options, the Kronzillas and T-1610 are married for life. Potential purchasers must clearly understand this. In reality it’s no different than with transistors. No one buying a $44.000 solid-state amp is going to substitute different output transistors. [Rather than any absence of rolling options, the main concern could be ongoing availability. The chance of transistor makers going out of business (which tend to be far larger firms than the usual hifi houses) is somewhat less likely than a specialty tube maker. Also, a full retube here equates to $10.000 which should be far in excess of any transistor replacement costs – Ed.] The T-1610 are spec’d for 10.000 hours. Ten hours a week should get you 20 years of service. With the built-in auto bias, replacement is a snap. Just insert the tubes, press the soft-start button on the front and go.

KR audio is well known for their single-ended designs. When I first saw the DXL, I assumed it to be the DX as from the photos they appear identical. Whilst the monster triode is at the heart of both designs, the DX takes the SET route, the DXL the more traditional push-pull road. Marek states that while "our domain is SE design, some customers prefer more power and bass response. For such needs push-pull is the better solution especially for low-sensitivity speakers or smaller ESL." Whilst I did not have a DX for comparison, Eunice says that under controlled conditions she cannot hear any difference between the DX SET and DXL push-pull. She attributes this to the extreme manufacturing tolerances and the "matching of the tubes". If you're already in love with the KR house sound but want more real-world power, the DXL could be your dream amplifier.

Reverse hybrid.
Visually the 1610 tubes are almost overwhelming. Their grandeur is such that one might not notice the lack of any small signal input tubes, drivers or rectifiers. During the initial design of the amplifier, Ricardo experimented with an all-tube circuit. He found that the traditional small input tubes did not meet his requirements for noise, sound quality or microphonics and subsequently developed a transistor front end like Nagra did for their push/pull 300B integrated.

Marek states that "the DXL is a hybrid push-pull amplifier with our direct-heated T1610 triodes working in class A. The driver and input stages are solid state. We use this hybrid design to obtain the best from both - the tube sound from valves, superior S/N ratio from transistor. Hence we exploit discrete Jfet and Mosfet devices in the front-end circuit and electron tubes with an output transformer in the final stage. Together this creates real valve sound with significantly lower noise and hum compared to tube circuits or hybrids where mass-produced preamplifier tubes operate the input stage."

I refer to the design as a reverse hybrid. Several manufacturers-including my current reference, the Tenor 350M monos-use valves for the input stages and in many cases add drivers and rectifiers. In fact with Tenors all voltage gain is handled exclusively with tubes. Their input stage is virtually a freestanding amplifier. The transistor output adds no voltage but mere current gain – the standard hybrid approach .Having spent considerable time with Tenor Audio’s designer, he believes that such a hybrid approach exploits the strengths of each technology whilst minimizing their weaknesses. KR Audio believes the same but reverses the strengths and weaknesses. Both companies thus put their own spin on the subject to blur the line between these technologies. Though there is nothing unusual about a transistor input stage, using transistors to drive the 1610 tubes is a somewhat unusual choice considering the voltage requirements.

In a departure from the typical KR amplifier, the DXL embraces push-pull whilst looking exactly like the single-ended DX. Both run zero feedback class A to throw off copious heat, about 300 watts per mono. This was not really an issue in my room but smaller spaces might get a little toasty during extended listening sessions. From one who has owned a variety of high-power tube amps, the Kronzilla’s heat was very manageable and nowhere near the space heater category of my previous Tenor OTLs which had the rather unique ability to simultaneously play music and cook a roast. The heat sinks on the side were a little deceptive. One assumes all of the heat is generated by the giant bottles but the sinks did warm considerably, indicating active dissipation from the internal transistor stages.