How does cathode bias work

If the difference between the two tubes is smaller, which is desirable, leave them in place. If the bias difference is larger swap them back. Don't worry too much about a bias imbalance because it can actually improve the amp's tone by adding even order harmonic distortion. Using matched power tubes is not important in guitar amps. If your amp is cathode biased or has 1 ohm bias setting cathode resistors then you can calculate bias current by measuring voltage drop across the cathode resistor.

To do this set your multimeter for DC Volt measurement, clip the black probe to a chassis ground so you can use just one hand to do the measurement and put the red probe on the tube socket cathode pin 8. You can also put one probe on each leg of the cathode resistor. The voltage shown on the meter is the 'voltage drop' disregard any minus sign. Note the Cathode Resistor at center and the power tube plates and cathodes at bottom center.

Measure the Plate-to-Cathode Voltage by placing one meter probe on the plate pin 3 and the other probe on the cathode pin 8. Once you have the voltage drop across the cathode resistor and Plate-to-Cathode Voltage recorded go to my Bias Calculator and enter your Tube Type and Plate-to-Cathode Voltage at the top of the page , then go down to the Tube Dissipation Using Cathode Resistor Voltage Drop section and enter the Number of Tubes that share a cathode resistor , the measured Voltage Drop across the cathode resistor and the Cathode Resistor's Ohm Value and click Calculate.

The Bias Calculator webpage assumes 5. You can get a more accurate bias measurement if you measure the resistance of your cathode resistor.

It may be marked as a ohm resistor but actually measure To measure the cathode resistor turn the amp off and measure the resistance from the tube cathode pin 8 to ground. This method has you directly measure the power tube plate current but it is much more dangerous than the previous method for reasons you'll see below. Another reason to avoid this method is typically milliamp measurement is less accurate than volts and ohms for most multimeters. The first step is to measure and record the plate DC voltage on both of the power tubes' plate pin 3.

The plate pin usually has a blue or brown wire coming from the output transformer. The voltage should usually be between v to v DC. Then set your multimeter to measure DC milliamps. Normally this involves moving the meter's red probe to another meter socket. If you have two multimeters use the best one to measure DC current and the other to measure DC voltage so you don't have to go back and forth on one meter. If you do use one meter don't forget to switch the meter probes and settings as you go back and forth between measuring DC current and DC voltage.

WARNING : When measuring milliamps current as soon as either meter probe makes contact with a voltage that same voltage passes through the meter to the other probe-- both meter probes will be hot with high voltage so you must control both probes carefully. You must wear safety glasses because if a meter probe accidentally contacts a grounded component an arc will occur and molten metal will spray out from the arc. This procedure is dangerous and if you are not trained for working with high voltage I recommend you take your amp to a professional tech.

Place one probe on either power tube's plate pin 3. The other meter probe now has high voltage DC on it so be careful as you place it on the output transformer center tap.

In many amps the standby switch is connected directly to the output transformer's center tap so you can use either standby switch terminal as the output transformer center tap. Refer to your amp's schematic and layout diagrams to find a place to sample the output transformer's center tap. With your meter contacts on a plate pin and output transformer center tap read the plate current in milliamps. Carefully do the same for the other power tube's plate pin 3.

Be careful not to make contact with anything as you remove both meter probes from the amp chassis. Output transformer center tap points at center and power tube plates at bottom center. Morgan Jones, Valve Amplifiers , 4th Edition.

Robert C. It includes many graphs to help make the material easier to understand. What is Bias? Right Power Tube Red Plating The red plating is caused by too much heat from too many electrons pounding the plate.

Measure Bias With the Output Transformer Resistance Method This is my recommended method of measuring plate current and bias for both fixed bias and cathode bias amps. Measurement Overview 1. Bias Points in the 5E3 Deluxe On many amps the output transformer center tap can be accessed at either terminal of the Standby Switch or where the output transformer center tap lead is connected to the circuit board.

However, we can supply correctly balanced and biased valves for all Mesa amps which mean you will not need to bias the amp. These valves are a direct swap for the Mesa tubes supplied with the amp.

Simply tell us it is for a Mesa amp in the notes field when you order and we will do the rest. Fixed bias on the output tubes means they are capable of being adjusted to raise or lower the bias current. This is usually done with a pot or swapping out a fixed resistor. Neither you nor a tech can adjust the bias on these amps.

Bias in Boogies can be adjusted by changing the bias resistors or by fitting a pot to convert it to adjustable fixed bias. They just make it difficult for you, so they can scam you with their overpriced valves. I have the vox ac4 bl, it is not listed above as a cathode biased amp but it has the EL84 on the output so when replacing it does need to be adjusted?

Read more Accept X. Manage cookies. Cathode Bias — What Exactly is it? Do I need to bias my amp? Bugera: V5 and V Cathode biased amps are less efficient than fixed bias amps. The same amp in fixed bias would be about 40 Watts.

It is important to note that the volume difference is actually quite minimal. Many tube amps are over powered for the average users volume needs. One thing to keep in mind is that fixed bias and cathode biased amps are often biased differently.

Cathode biased amps are often bias very hot and near Class A tube amp rarely actually meet the standard for being true class a. The idea was often to maximize power. These two bias conditions sound. Many cathode bias amp will benefit from reducing the dissipation a bit by increasing the value of the cathode resistor. The tubes will last longer and the amp will sound better. Biasing should really based on what sound best rather than an attempt to maximize power.

Notice the adjustable bias pot. I build these on request! Some Famous Cathode Biased Amps with my versions model numbers there too :.