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Tube Bias Calculator

Enter info in blue and click any 'Calculate' button

Select Tube Type: Enter Plate-to-Cathode Voltage: DC volts (plate voltage minus cathode voltage)

 

Tube Max Design Dissipation = watts  100% Pa

Class A Fixed Bias   70% Cool = DC milliamps    80% Average =    90% Max Safe Dissipation =
Class AB Fixed Bias   50% Cool = DC milliamps    60% Average =   70% Max Safe Dissipation =
Cathode Bias   85% Cool = DC milliamps    95% Average =    100 % Max Safe Dissipation =

Tube Dissipation Using Cathode Resistor Voltage Drop

You must enter Tube Type and Plate-to-Cathode Voltage above before calculating Tube Dissipation.

Enter Number of Tubes that share a cathode resistor:  

Enter Voltage Across Cathode Resistor: DC volts Voltage drop across the cathode resistor.

Enter Cathode Resistor: ohms  Cathode resistor ohm value.

  

Total Cathode Current = DC milliamps   = Voltage Across Cathode Resistor / Cathode Resistor.

Total Plate Current = DC milliamps  = Cathode Current minus approximate screen current of 5.5%.

Plate Current per Tube = DC milliamps  = Total Plate Current / Number of Tubes.

Plate Dissipation per Tube = watts   = Plate Current per Tube x Plate Voltage.

Plate Dissipation per Tube % = % Plate Dissipation per Tube % = Plate Dissipation per Tube / Tube Max Rated Dissipation.

If your tube is a triode (or dual triode with both triodes used) your plate current is milliamps, your plate dissipation is watts and %.


Tube Dissipation Using Plate Current

Use this calculator when you measure the actual plate current. You must enter Tube Type and Plate-to-Cathode Voltage above before calculating Tube Dissipation.

Enter Plate Current: DC milliamps

 
    
 

Plate Dissipation = watts  = Plate Voltage * Plate Current.

Plate Dissipation % = %  = Plate Dissipation / Tube Maximum Rated Dissipation.

 
Calculator updated 11/2016 by Rob Robinette

Note: You must re-calculate after changing values by clicking any Calculate button. Tube Type and Plate-to-Cathode Voltage must be entered for all calculations on this webpage.

WARNING

Amplifiers have high voltage that can KILL. Do not attempt to set an amplifier's bias unless you are competent with test gear and very familiar with the dangers involved.


Tube Bias Calculator Instructions & Help

Enter values in blue and click any Calculate button.

Select the Tube Type from the scrolling list. If your tube isn't listed then pick one with the same Max Dissipation Rate which is shown to the right in the tube list.

Enter the Plate-to-Cathode Voltage which is measured between the plate and cathode tube socket pins, not between the plate and ground. You measure it by placing the red + multimeter probe on the plate pin and the black - probe on the cathode pin. You can also calculate the Plate-to-Cathode Voltage by subtracting the voltage measured at the cathode pin from the voltage measured at the plate pin. See this link for more info on measuring and adjusting bias.

For octal (8-pin) tubes the plate is pin 3 and the cathode is pin 8. For 9-Pin power tubes like the EL84 the plate is pin 7 and the cathode is pin 3. For 9-Pin preamp tubes like the 12AX7 the plates are pins 1 and 6 and the cathodes are pins 3 and 8 (measure between pins 1 and 3 for A; 6 and 8 for B).

Octal Tube Socket Numbers

Plate is pin 3 and Cathode is pin 8. Arrow points to the insertion index notch.

Click any Calculate button to see the recommended tube bias currents in milliamps. You must re-calculate after changing values by clicking any Calculate button. Tube Type and Plate-to-Cathode Voltage must be entered for all calculations on this webpage.

Usually single power tube amps are Class A and amps with two or more power tubes are Class AB. Amps with power tube cathode resistors greater than 10 ohms are usually cathode biased. The Fender 5F1 Champ is a Class A cathode biased amp. The Fender 5E3 Deluxe is a cathode biased Class AB amp. The Fender 5F6A Bassman and most Marshall amps are fixed bias Class AB amps. Fixed bias amps have a bias circuit that puts a negative voltage on the power tube grid. The grid of cathode biased amps will be at 0 volts.

Tubes have a maximum dissipation rating in watts for how much power (and therefore heat) they can deal with so we adjust the tube's bias voltage (voltage between the grid and cathode) to keep the bias current below the tube's max dissipation (power & heat) rating. If you bias your amp too hot you may shorten the life of your tubes but bias it too cool and the amp may sound sterile.

Fixed bias amplifiers apply a steady (fixed) negative bias voltage to the power tube grid and the cathode is connected directly to ground (no cathode resistor). It is called 'fixed' bias because the bias voltage does not fluctuate the way it does with cathode bias.

Cathode bias amplifiers use a cathode resistor to generate a positive bias voltage on the cathode while the grid is grounded through a 'grid leak' resistor. As the guitar signal causes the current flow through the cathode resistor to fluctuate so does the voltage drop across the cathode resistor (the bias voltage). Since the bias voltage fluctuates with the input signal it is not 'fixed.'

Be aware the term plate voltage can have two meanings. It can mean the actual voltage measured from the tube plate pin to ground but when measuring tube bias it means the voltage difference between the tube's plate and cathode. To avoid confusion I use the term plate-to-cathode voltage. You can measure it directly by placing the red + multimeter probe on the plate pin and the black - probe on the cathode pin. You can also measure the plate voltage and the cathode voltage separately then subtract the cathode voltage from the plate voltage to get plate-to-cathode voltage.

Triode Tubes

Triode tubes do not have a screen so they do not have any screen current to consider. The dissipation ratings for dual triodes such as the 12A*7 family of tubes is listed as total dissipation using both triodes. For a dual triode tube with both triodes used enter "1" in the Enter Number of Tubes that share a cathode resistor box. If you only use one triode of a dual triode tube you will need to double the displayed dissipation %.

For example my Deluxe Micro amp uses both triodes of a 12AU7 rated at 2.75 watts per triode so the total max dissipation is 5.5 watts. If the amp used only one triode then the max dissipation would be 2.75 watts and I would multiply the calculator's displayed dissipation % by 2.


How to Measure Bias

This webpage offers two ways to calculate tube bias and plate dissipation. For amps with cathode bias resistors you can simply measure their voltage drop and use the Tube Dissipation Using Cathode Resistor Voltage Drop calculator. My recommended bias measurement method is the "Output Transformer Resistance Method."

You measure the output transformer's voltage drop and resistance to calculate the bias current.

You can also measure the actual plate current by shunting the output transformer with a multimeter in the DC milliamps mode and use the Tube Dissipation Using Plate Current calculator. This method is more dangerous because when your meter probe touches high voltage your other meter probe will also carry that high voltage so you must be very careful what those probes touch.

To use the Tube Dissipation Using Cathode Resistor Voltage Drop calculator enter your Tube Type and Plate-to-Cathode Voltage at the top of the page, then 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. If your amp has 1 ohm bias setting resistors then enter '1' into the Cathode Resistor's Ohm Value field. You can get an accurate resistance measurement of cathode resistors by turning the amp off and simply measuring resistance across the cathode resistor. It's more accurate to actually measure the resistor's ohm value rather than relying on its marked rating. The calculator subtracts 5.5% of the cathode current as screen current.

To measure voltage drop across the cathode resistor set your multimeter for DC Volt measurement, clip the black probe to chassis ground (so you can use just one hand to do the measurement) and put the red probe on the tube socket cathode pin . 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 that voltage measured at the cathode pin is the same as the voltage drop across the cathode resistor so you don't have to actually put your meter pins on the legs of the cathode resistor to measure its voltage drop.

You can get a more accurate bias measurement if you actually measure the resistance of your cathode resistor. It may be marked as a 250 ohm resistor but actually measure 238. To measure the cathode resistor turn the amp off and measure the resistance from the tube cathode pin to ground.

For fixed bias amps with no cathode resistors you calculate the bias current by measuring the output transformer's resistance and voltage or you can measure the actual plate current then use the Tube Dissipation Using Plate Current calculator. Measuring the actual plate current by using the "output transformer shunt method" is more difficult and dangerous than the "output transformer resistance method or using the cathode resistor voltage drop method. See my Bias How-To for more info. After selecting your Tube Type and entering your Plate-to-Cathode Voltage at the top of the page simply enter your Plate Current and click any Calculate button.

I recommend you play your guitar through the amp every time you make a bias adjustment. You may find you prefer a 'cool' or 'hot' bias with your particular playing style, guitar and amp. The amp's tone is what really matters so don't just set a standard bias target like 70% for Class AB amps and forget it. Your amp's "best" bias setting is pretty subjective so play it while you adjust it.

See my How to Bias a Tube Amp for more info.


If you prefer spreadsheets you can download the Excel and OpenOffice Tube Bias spreadsheets.

I also have a free Android Tube Bias Calculator app available here in the Google Play Store or just search the Google Play Store for "Rob Robinette."

Have comments, corrections or suggestions? Send them to robinette at comcast dot net.

Updated June 2016, added the KT120 and KT150 tubes to the database.

By Rob Robinette


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