General:

Uber rare set of Sequerra monoblocks utilizing a plethora of EL34 tubes (some of the best sounding). Featuring no less than FOUR discrete chassis connected by high quality custom umbilical cords.

Original MSRP was $125K in the late 1990's. Yes you read that correctly, and note that MSRP works out to $257k+ in 2023.

SkyFi Technical Notes:
These amplifiers appear to have gone through a series of modifications throughout their lifetime. The removable front panel has a display and a bay of 10 bias adjustment trimmers. This circuit was disconnected at one point and the amp was converted to run on a single bias adjustment.

This modification makes the bias adjustment less complicated, but does require a set of 10 tightly matched output tubes! We fitted both amps with fresh sets of very tightly matched Psvane EL34 tubes for this reason. In addition to the bias modification, there are various unused connections jacks and cutouts on the rear panels.

SkyFi Design Notes:
There is a not a lot of information available on these amplifiers. It's also possible these were an early prototype chassis.

Our circuit study indicates these amplifiers in their current configuration are set up as follows:

• Stage 1 ~ Western Electric 5842 differential input pair.

• Stage 2 ~ 5687 dual triode pre-driver - Direct Coupled to Previous Stage.

• Stage 3 ~ +/- Phase EL34 Drivers - Direct Coupled to Previous Stage.

• Output Stage ~ Parallel Push Pull EL34 (5 Tubes + 5 Tubes Per Phase) wired as Triodes, yielding approx. 90W into 8 ohms.

• Global adjustable tube regulated bias supply (12B4A).

• The power supply is very complex and provides a collection of regulated voltages for the various tube heaters, drivers, and output circuits.

• The amplifiers are fitted with balanced XLR and single ended RCA inputs. If using single ended inputs we recommend shorting pins 1 and 2 of the XLR connectors.

• Cheater plugs for lifting the ground on the power cords may be necessary to avoid ground hum in certain applications.

SkyFi Cosmetic Notes:

• These units may have originally had bottom covers, these covers are no longer present as shown in our photos.

• Some of the paint on the chassis has a sticky texture.

• Empty cutouts and unused connectors on power supply and amp chassis.

• Slight component mismatch between L/R channel chassis.

Ownership:
Unknown

Connections:
XLR Inputs, RCA Inputs, and 5 Way Speaker Binding Posts.

General Sound:
Accurate to the recording

Cosmetic Condition:
6 - See our detailed rating description here.

Working Condition:
Working perfectly and tested in our lab and listening room.

Included:
Just the units as pictured and power cords

Shipping
Will ship safely secured and strapped to a shipping pallet anywhere within the lower 48 US States for free. We can ship worldwide for an addition $400 fee. Local pickup from our shop in Glen Rock, NJ also welcome.

Packing:
Will be packed using our highly developed in-house process and custom packing materials.

Weight:
200 lbs.

Recommended Cables:

Kimber Kable - RCA Interconnects - Better

Kimber Kable - RCA Interconnects - Best

Kimber Kable - BALANCED XLR Connectors - Better

Kimber Kable - BALANCED XLR Connectors - Best

Kimber Kable - Speaker Cables - Better

Kimber Summit Series Monocle XL Speaker Cables (PAIR) - Best

Kimber Summit Series BiFocal XL Bi-Wire Speaker Cables (PAIR) - Best If Applicable

Kimber Kable - Power Cords - Better

Kimber Kable - Power Cords - Best

Testing Process:
The SkyFi Testing Process for Tube Amplifiers:
We start with a visual inspection of all internal components to make sure that there are no signs of heat stress or damage. Capacitors are checked for telltale signs of predictive failure including bulging, shrunken wrappers, or physical leakage. We also inspect resistors and other passive components for signs of overheating. If tube arcing has occurred in the past we can usually spot discoloration on the output tube sockets. On vintage units we often spot check select capacitors for value and ESR.

If the amplifier passes visual inspection, we move on to a full test of all of the tubes. We use an Amplitrex AT-1000 Tube Tester which is capable of testing both emission and Gm with a high degree of accuracy. We document the results of each tube and replace any weak or suspect tubes before proceeding. When we power on tube amplifiers for the first time we usually use a variac and current limited AC supply and slowly raise the voltage up to nominal mains level while monitoring plate, screen, filament, and negative bias supply voltages where applicable. If everything is in order we feed a low level test signal into the amplifier’s input and monitor its output on an oscilloscope across an 8 ohm dummy load. At this point we are just looking to verify basic function and confirm that the output transformers are not damaged. Once we have verified that the amplifier is safe to operate, we connected it to full mains power. For fixed bias amps we set the bias to manufacturer spec. For cathode biased amps we monitor the plate to cathode voltage to determine if the output tubes are operating in a safe range. Once the output section is verified we move onto bench evaluation.

We start by feeding the input of the amplifier with a low level 1KHz test signal, slowly increase its amplitude while monitoring the amplifier’s output on an oscilloscope for signs of noise, clipping, distortion, or improper channel balance. We continue increasing the signal level until the amplifier reaches clipping. At this point we take an output power measurement and compare it to the spec sheet of the amplifier to verify proper performance. We finish off the bench evaluation with a 1KHz square wave check and a 20Hz to 20KHz sine sweep to assess the amplifier’s frequency response characteristics. This battery of tests will usually reveal if the amplifier has any issues that need further attention.

Before the device leaves the bench, we perform a listening test with actual music using a variety of preferred test tracks. Our benches are outfitted with familiar monitor speakers which help us identify inconsistencies that will not always show up on our test gear. The main things that we are listening for are hum or noise with no signal present, proper center image, clicks, pops, or any other obvious undesirable audio characteristics.

If the unit passes all of these tests it is moved to our long term testing rig where we simulate real word operating conditions for 6-8 hours. For tube amps we like to run this test at least twice. This allows us to monitor the unit for signs of thermal runaway or intermittent issues that only crop up when it has fully come up to temperature. We find this step to be essential, especially for vintage units.

Sequerra UA-1 Custom EL34 Tube Monoblocks - Uber Rare! - w/Free Shipping