Bias Procedures

The Manley STEREO 75 + 75 watt Amplifiers use a fixed bias system that requires very little attention. We recommend checking the bias every 2-3 months or any time you replace an output tube. To adjust or check the bias, follow the following steps.

For this procedure you will need a voltmeter (a hand held autoranging DMM digital multimeter is the easiest) and a small flat screwdriver.

1.On page 6, which shows the top view of the amplifier, you can see4 black tip jacks towards the front of the amplifier. To measure the current draw of each output tube place the positive meter probe into the first black tip jacks which corresponds to tube number one. Place the negative meter probe into the negative black output binding post to make a good ground connection.

Set the meter to read 'millivolts' DC (direct current).

2.Each of the four output tubes can have its own bias adjusted by adjusting its own bias adjust pot. On the top view picture on page 6, you can see the numbering sequence for the 4 output tubes. The corresponding tip jacks (measurement points) and bias adjust potentiometers are also shown.

3.The first step to setting the bias is to turn on the amplifier and ensure that there is zero signal input, either leave the preamplifier or source signal turned off or unplug the amplifier input. Also, leave your loudspeakers connected as these provide an ideal load on the output. (DO NOT EVER OPERATE YOUR AMPLIFIER WITHOUT SPEAKERS CONNECTED!) It is recommended that you leave the amplifier on long enough to ensure that the tubes have reached their stable current draw, at least 1/2 hour is recommended.

5.With the meter probe in the first black tip jack, adjust the first bias trim pot slowly until you measure 275mV to 300 mVDC (0.275V to 0.300VDC). Since you are measuring across a 10 ohm cathode resistor, this would correspond to a 27 to 30 mA current draw for each tube by Ohm's law.

6.If you are unable to bring the bias voltage down to 300 mV on any tube, you may alter the setting on the master bias adjust pot which will change the bias for all of the tubes at the same time. Rebias all the tubes as described above. If after lowering the master bias adjust to its minimum setting you are still unable to bring the voltage down to 300mV on any tube, then we recommend that that tube be replaced.

7.Follow step five with the remaining tubes, switching to the next test point and adjusting the next trim pot each time. After you have adjusted all the output tubes, recheck and repeat the procedure as they will drift a little bit during adjustment. Once they are all set, your amplifier should be in perfect operation.

STEREO 75+75 WATT AMPLIFIER specifications

The Manley Labs STEREO 75+75 WATT AMPLIFIER is a remarkable piece of audio equipment designed for both audiophiles and professional sound engineers. This amplifier exemplifies the perfect blend of power, quality, and innovative technology, providing an immersive listening experience that brings music to life.

At the heart of the STEREO 75 is its unique design that utilizes a combination of high-quality vacuum tubes and solid-state components. This hybrid approach allows the amplifier to deliver a warm, rich sound characteristic of tube amplifiers while maintaining the precision and reliability of solid-state designs. The amplifier’s output stage features a pair of powerful 75-watt outputs, ensuring that it can drive a wide range of speakers with ease and clarity.

One of the standout features of the STEREO 75 is its use of Manley's proprietary transformers, which are engineered to minimize distortion and enhance frequency response. These transformers play a pivotal role in delivering the amplifier's exceptional audio fidelity, with a frequency response that spans from a deep low end to crystal-clear highs. This means that whether you’re listening to classical music or rock, every detail is reproduced faithfully.

The amplifier is equipped with an advanced power supply that ensures stable performance, even during dynamic playback. This power supply is designed to provide consistent energy delivery, reducing the risk of clipping and distortion at high volumes. Additionally, the STEREO 75 features an innovative feedback circuit that enhances transient response, allowing it to handle complex musical passages without losing detail.

In terms of connectivity and usability, the Manley Labs amplifier offers multiple input options, including balanced and unbalanced connections, making it compatible with a variety of source components. Its elegant front panel design includes easy-to-use controls for volume and a selection switch for input sources.

Another noteworthy characteristic of the STEREO 75 is its build quality. Like all Manley products, this amplifier is hand-crafted with an emphasis on durability and aesthetics. It features a robust chassis, premium components, and a stylish finish that makes it an attractive addition to any audio setup.

In summary, the Manley Labs STEREO 75+75 WATT AMPLIFIER combines traditional tube warmth with modern technological advancements. Its powerful output, exceptional sound quality, and thoughtful design make it an outstanding choice for anyone seeking a high-performance audio amplifier that delivers an unparalleled listening experience.