Valve Biasing a valve amplifier

Valve biasing is very straight forward on a Separo valve amplifier. One of the selling points for our amplifiers is that it is easy to change valves and re-bias  the amplifier. Our single ended valve amplifiers are designed in such a way that they do not need biasing if any of the valves are replaced.

If any of the power valves blow on one of our ultra linear valve amplifiers you will need to replace the valve and re-bias the amp. 

You do not need to re-bias one of our valve amplifiers if any of the smaller pre-amplifier valves are replaced. 

Procedure to re-bias one of our ultra linear valve amplifiers 

Biasing is done to adjust the current that flows through the valve. Bias is a negative voltage that acts as a brake on the main current carrying the amplified signal through the valve. By altering the bias you can control the less current that flows through the valve. The less bias voltage you apply will allow more current to flow giving more wattage and power. 

However, the valves are designed to work within a range, if the metal plates you can see inside the glass start to develop a red patch there is too much current going through the valve and the valve is being destroyed, turn the amplifier off and get it re-biased). 

Apart from this safety/valve survival issue, bias adjustment becomes a matter of taste. High current gives you power/volume, clarity, good top end. If the idle current is set too high (under bias) then the valves will run too hot and the life of the valves will be reduced significantly. Less current gives you earlier break-up, with less volume, less clarity and top end. If the current is set too low (over bias) the tone of the amp may be thin and lacking volume. 

Locate the bias adjustment screws and testing sockets on each side of the amplifier. 

You will need, high impedance voltmeter DC VOLTS,you will be measuring around 400-600mV so set the voltmeter in the appropriate range

Ensure the valves have been replaced correctly and that the speakers are connected. 

While the amplifier is turned off; 

1. Turn each of the adjusters, labelled “V1 ADJ”, “V2 ADJ”, “V3 ADJ”, “V4 ADJ” to the minimum position. This is the opposite way to the arrow points. 

2. Turn on the amplifier. Adjust each valve, one at a time, - adjusting valve V1 

3. Connect the BLACK NEGATIVE terminal of the voltmeter to the COM socket next to the “V1” socket. 

4. Connect the RED POSITIVE terminal of the voltmeter to the “V1” valve socket, 

5. Turn the bias adjuster “V1 ADJ” to adjust the voltage so it is in the range 350 - 550 mV. (The actual voltage is down to personal choice and listening preferences.) the maximum voltage of the biasing should NOT be greater than 600mV. 

Repeat steps 3, 4, 5 for each valve until you have exactly the same reading on each valve and they are perfectly balanced. Repeat the process for a final check.