Antique Sound LabsAQ-1006-845usedAntique Sound Labs AQ-1006-845 monoblock tube ampsThese amps are the second iteration of ASL’s single-ended 845 design. This version falls between the original chrome chassis monoblocks and the current black tower version. The most interesting fea...1231.00

Antique Sound Labs AQ-1006-845 monoblock tube amps

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Ships fromSan Jose, CA, 95131
Ships toUnited States
Package dimensions22.0" × 16.0" × 13.0" (104.0 lbs.)
Shipping carrierUPS
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These amps are the second iteration of ASL’s single-ended 845 design. This version falls between the original chrome chassis monoblocks and the current black tower version. The most interesting feature of this amp design, besides the class A triode SE output stage, is a tube regulator circuit that feeds both the driver stage and the input voltage amplifier (the entire front-end, in other words). This is not just a gas tube or zener diode regulator—it is a full-blown, two-stage active regulator with pass tube, voltage reference, and control tube.

This pair of monoblocks has been thoroughly updated since my last auction. They are in excellent working condition. Here is a rundown of the work done at my facility in San Jose, CA:

1. Filter caps were replaced and increased to 15,000uF in both DC filament supplies (front-end & 845 supplies). This is the value used in newer versions of this amp. Older versions used smaller values. Small caps (or worn, leaky caps) in the 845 filament supply are the main source of hum in this amplifier.

2. The stock Nippon Chemi-Con 500V filter caps were checked for leakage and found to be in excellent condition, with negligible ESR and nearly perfect sharing between the series pairs in the high-voltage supply. A leaky series pair will soon fail due to one cap taking on the voltage burden while the other cap leaks current.

3. The stock 450V TRI filter caps were replaced with 500V JJ/Tesla caps. This was done as a precaution in case the TRI caps decide to fail. There were two of these in each amp.

In addition, I made three changes to the tube regulator circuit to improve reliability and performance:

1. The regulator input voltage was increased (but not to an unsafe level). The voltage drop across the pass tube is too low in the stock circuit, and is barely adequate to operate the tube. Increasing the voltage drop improves regulation and reduces ripple & noise.

2. A filter cap was placed across the zener reference diode to reduce zener noise. Low noise in the voltage reference is critical because reference noise is amplified by the control tube.

3. A protection diode was installed from grid to cathode on the regulator pass tube to limit the voltage differential during turn-on. This maintains the grid and cathode at the same level until the tube warms up, which extends the life of the pass tube. It is cheap insurance against cathode stripping. The diode is biased off during normal operation and does not affect the sound quality.

The last three items are optimization techniques I used in the 1980’s during development work on MFA’s Luminescence preamp, which used a tube regulator with nearly the same topology as this one. The difference in sound quality is not subtle. The bass now has real authority due to a stable voltage rail. Top end detail, definition, and quickness are all on a higher level due to reduced regulator output impedance and noise. Midrange image focus is sharper for the same reasons.

Cosmetically the amps would rate 8/10 if not for a small scuff on the front panel of amp #1. The scuff is to the right of the standby switch, about ½ inch long, and is barely visible in the photo. The scuff has been touched up so you may have to look for it.

All tubes test very strong, both in and out of the amp. Tube descriptions are the same for each amp:

The 845 is a factory Valve Art.

The EL34 pass tube is a JJ. KT66’s will also work here, but the output impedance will increase due to the lower mu of the triode-connected pass tube. When the impedance rises, audio signals are more likely to interact among stages. This tends to homogenize details and blur imaging.

The regulator control tube is a 5-strut RCA Red Base 5691. This was the tube of choice in the Luminescence phono stage. Among the octal triodes, it is ideal as a regulator control tube due to its high gain and low noise.

The driver tube is a vintage Raytheon 6SN7GT/VT231.

The input tube is a single-strut Sylvania brown base 6SN7WGTA. This tube has become scarce & expensive. It is to my ear one of the best sounding 6SN7’s ever made.

Amps are sold with stock chrome wire cages. Amps will be shipped in custom cut double-wall boxes with 2” thick ethafoam all around. Ethafoam has more give than Styrofoam, but is denser than other types of foam. All tubes will be packed in individual boxes.

Just insert tubes, set bias to 850, null hum, and play music. And don’t forget to use the standby feature. This allows the 850 to warm up before hitting the plate with nearly 1000 volts via the standby switch.

Any questions as to usage gladly answered. Thanks for looking.

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