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Critical OX Operation Notes

Caution: Connecting and/or operating OX incorrectly can cause permanent damage to your amplifier, OX, and/or other equipment connected to OX. Universal Audio is not responsible for equipment damage resulting from improper OX connections and/or operations. Review the important information in this article before connecting or using OX!

Critical Notes — Avoid Equipment Damage

Amp Output Load
DON’T operate any tube amp without an output load such as a guitar speaker or OX. Using a tube amp without an output load can permanently damage the tube amp.

Caution: Always POWER OFF the tube amp before connecting or disconnecting its speaker output to guitar speakers or OX.

Amp Power Rating
DON’T use OX with amplifiers exceeding 150 Watts RMS output power. Any tube amp with an output power rating of 150 Watts RMS or less can be safely used with OX.

DON’T connect the tube amp’s high-power speaker output to ANYTHING other than OX’s FROM AMPLIFIER input jack. The nut on this jack is RED to indicate this input is ONLY for the tube amp’s speaker output.

OX’s TO SPEAKER Output Jack
DON’T connect OX’s high-power TO SPEAKER output jack to ANYTHING other than a guitar speaker cabinet to avoid equipment damage to other gear.

Speaker Cable Insertions
DON’T partially insert 1⁄4” speaker cable plugs halfway into the jack or only to the first click. The cable from the amp’s speaker output must be fully inserted into OX’s red FROM AMPLIFIER jack to connect the ground return for proper amp loading.

Speaker Cables
Use only high-quality 12 to 16 gauge unshielded 1⁄4” mono TS (tip-sleeve) speaker cables for OX’s high-power FROM AMPLIFIER and TO SPEAKER connections. “Speaker” is usually printed on these cables.

Tip: As a general guideline, the longer a speaker cable is, the smaller its cable gauge needs to be (smaller gauge = bigger wires). 

Rated Speaker Power
DON’T connect OX’s TO SPEAKER output to any speaker that is rated for a power level that is significantly less than the connected amplifier’s output power to avoid speaker damage.

Moisture and Liquids
DON’T expose OX to moisture or liquids. Make a habit of never putting beverages on your OX. If liquid is spilled into the unit, immediately unplug the power cord from the AC socket.

Cooling Airflow
DON’T cover OX’s air vents or otherwise restrict air ow around OX. The amp’s output power is converted to heat by OX’s fanless attenuation circuit, and the air vents must be unrestricted to dissipate this heat.

Solid State Amplifiers
Using OX with solid state amplifiers (amps without vacuum tubes) is not recommended. OX is designed and optimized for use with tube amplifiers only. 

Important Notes — Reduce Equipment Wear

Circuit Wear with Power Attenuation

Circuitry within an attenuated amp is subject to the same wear and tear that would occur if its output power was not attenuated. If an amp is constantly run at higher power levels, the lifespan of an amp’s components is reduced accordingly.

Increased circuit wear applies especially to the amp’s vacuum tubes, capacitors, and transformers due to the increased heat and current generated at higher power levels.

Tip: A tube amp’s sweet spot might not be at full output power — a “dimed” amp doesn’t always sound better. Less power and soak might deliver better tone AND better tube life.


Set OX’s IMPEDANCE knob to match the impedance of the tube amp’s speaker output. Similarly, the total impedance of speaker(s) connected to OX’s TO SPEAKER jack should match OX’s IMPEDANCE knob setting.

Many tube amps have multiple speaker output jacks with different impedance labels (4Ω, 8Ω, etc.) for each output jack. If the amp’s speaker output impedance cannot be determined, set OX’s IMPEDANCE knob to 8 ohms.

Note that when multiple speakers are combined (such as in a multi-speaker cabinet), the total impedance depends on how the speakers are wired together. For example, two 8 ohm speakers wired in series has a total combined impedance of 16 ohms, but if they are wired in parallel, the total combined impedance is 4 ohms.

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