Linear Tube Audio ZOTL Ultralinear Power Amplifier

Twenty Watts Of Joy

Equipment report
Solid-state power amplifiers
Linear Tube Audio ZOTL Ultralinear Power Amplifier

Say hello to the latest member of the ZOTL amplifier family. The ZOTL designation describes amplifiers that use David Berning’s patented tube output stage that doesn’t require an audio output transformer. The new 20Wpc ZOTL Ultralinear (UL) amplifier reviewed here expands Linear Tube Audio’s (LTA) portfolio of David Berning designs yet again. Kudos are due to LTA’s Mark Schneider for that. It also marks David Berning’s resumption of his “love affair” with TV sweep tubes that was interrupted by the EL34-based ZOTL 40 that I reviewed in Issue 273. Berning’s embrace of Compactron TV beam pentodes goes way back to the early 1980s. Some of you will recall the EA-230 amplifier, a screen-drive design based on the 6JN6, a tube that aside from heater characteristics is identical to the 17JN6 used in the UL amplifier. One of the attractions of the Compactron beam pentode is cost. They’re available today as new old stock (NOS) for as low $3 a pop. The reason for the low cost is that they were mass-produced in the early 1960 for use in horizontal-deflection amplifiers of early color TVs. With the advent of reliable power transistors in the late 1960s demand for Compactrons faded leaving thousands stranded in warehouses. 

As Berning has made clear, high plate voltage, on the order of 1000V is a desirable for screen-drive designs since a high plate voltage facilitates increased power output and efficiency. The first high-power ZOTL amp released circa 1996, the ZH270, was configured for screen drive, but the Ultralinear amp reviewed here is driven conventionally via the control grid. The question that quickly comes to mind is why use a sweep tube in a UL connection? Traditional UL operation is achieved by connecting the power tube’s screen grid to an intermediate tap of the output transformer’s primary winding. When the tap point is at about 40%, the overall effect is to linearize the tube characteristic curves thereby reducing distortion products. The UL amplifier breaks new ground in emulating a traditional UL output connection in a ZOTL setting. It turns out that the decision to use a TV sweep tube in such a setting wasn’t random at all. In fact, standard audio power tubes will not work well in a UL-connected ZOTL circuit. 

The UL amp is quite similar to the ZOTL 40. The power supply is pretty much identical, and with the exception of the ultralinear connection there’s not much difference in the audio circuity either. A 12AX7-based input- voltage gain stage is followed by a 12AU7 phase splitter that is directly coupled to the control grids of the 17JN6 TV sweep tubes operating in push-pull configuration. All of the tubes are auto biased so there should be no worries about matching tubes. I should mention that the new casework, designed by Fern and Roby Audio, is a marked improvement over the old-style boxy looks of the ZOTL 40. The front and rear panels are now milled from slabs of aluminum, and feature Cardas RCA jacks, WBT copper binding posts, a front-panel industrial toggle-type power switch, and a stereo/mono switch. 

Since I’m lucky enough to own a Berning EA-230 amp, recently upgraded to auto bias operation by Berning himself, it seemed interesting to start off with a shootout of sorts between the old and new guard. And to make it even more interesting, I included the ZOTL 40 amp in the mix. The catch was the speaker load: the QUAD ESL-57, not an easy load but a fair one for these low-power tube amps. The musical fare was mainly violin concertos, including the Bach violin concertos (Perlman-Zukerman, EMI Classics). It turned out that the coupling of the QUAD with the EA-230 was a match made in heaven that generated enough tonal sweetness to satisfy even the most jaded audiophiles. It was also the most dynamic with the most solid bass lines in this trio of Berning designs. The UL proved to be most problematic at this early stage. It dialed back the violin’s upper register sheen and struggled to create a convincing soundstage. 

It was time to switch over to my Basszilla DIY high-sensitivity loudspeaker. And slowly, over a period of days, the UL amp continued to improve. It seemed to take forever to fully break-in, so I decided to let it continue to cook for over a week before resuming critical listening. Patience was finally rewarded and I began to appreciate, or should I say enjoy, its virtues. This is an incredibly detailed amp. I mean detailed in a good way, without sounding analytical in the process. Its ability to unravel complex passages resulted in a supreme sense of spatial clarity. This was a function of preserving transient decay into an exceptionally low noise floor. Hall ambience became easy to resolve, which nicely fleshed out depth perspective. 

The UL amp beat out the ZOTL 40 when it came to bass definition, which would be expected from a UL vs. pentode output stage, even though on paper their respective damping factors are similar. The UL amp sounded distinctly less tube-like than its pentode sibling, a finding that can’t simply be explained by total harmonic distortion (THD) measurements. Theoretically, UL should be more linear than the pentode connection, but the ZOTL 40 also uses more global feedback, which acts to reduce THD. My guess is that the UL amp’s distortion spectrum contains less second-order harmonic distortion. In any case, the end result is a leaner and less liquid sonic character that eventually made me miss the sumptuous textures of the ZOTL 40. It occurred to me that the UL had more in common with the First Watt SIT-3, my favorite low-power, push-pull, solid-state amp, than it did with the ZOTL 40. It made sense to undertake another comparative listening test. The results confirmed strong similarities with the SIT-3 in detail resolution and harmonic textures, though the UL was more dimensional, especially with respect to soundstage depth.