Biasing Power Tubes

By Paul Marossy
Last updated 05/31/05





It is generally a good idea to check the power tube bias each time the power tubes are replaced, especially in a fixed bias amp. If the power tube bias is incorrect for the power tubes being used, they can sound bad and/or can literally burn themselves out by dissipating too many watts, sometimes with disastorous effects. Setting the power tube bias within specs will make the tubes last longer and will also help the amp to sound its best. Of course, like most everything else, this is a subjective topic, and everyone seems to have a different opinion about what is best. The purpose of this page is to explain how to bias the power tubes in your own amp. I will not cover all the theory about power tube biasing as there is numerous articles on the subject on the net. If you want to know more about the theory behind power tube biasing, check out GEOFEX.com

Warning: This page also assumes that you have data sheets for your power tubes in hand, that you know how to proficiently use a digital multi-meter, are familiar with Ohm's Law, that you know about the dangers of tube amps and that they can KILL you if you are not careful. If you do not know any or all of the above, please read everything you can find on the topic and learn it well before attempting to do this yourself, or take your amp to a competent tech to have it worked on!




Cathode Bias:

I will start out with the easy one first. The most common cathode biased amp is one that uses EL84 power tubes. It really is a fairly simple operation to check the plate dissipation using the following calculations.

Using a digital multi-meter, determine the cathode current using this formula:
Cathode Current = Cathode Voltage/Cathode Resistor Value (Ohms)
This will give you the cathode current in milliamps.

Again, using your digital multi-meter, determine the screen current by using this formula:
Screen Current = Voltage Drop across Screen Resistor/Screen Resistor Value (Ohms)
This will give you the screen current in milliamps.
(If no screen resistors are present, you will have to determine the screen current directly by using your digital multi-meter)

After these two steps have been completed, you can determine the plate current by using this formula:
Plate current = Cathode Voltage/Cathode Resistor Value (Ohms) - Screen Current
This will give you the plate current in milliamps.


Finally, you can now calculate plate dissipation in watts by using this formula:
Plate Dissipation in Watts = (Plate Voltage - Cathode Voltage) x Plate Current


Assuming that you have a cathode biased amp using EL84s, you do not want to dissipate more than 12 watts, which is the quoted "design center" value on every EL84 data sheet that I have seen. Exceeding this value can dramtically shorten the life of the power tubes and possibly cause major problems if one should fail while in operation. It's not a bad idea to at least check the plate dissipation when replacing the power tubes in a cathode biases amp - no two power tubes are exactly the same. Some may tend to move more current than others. If you find that the plate dissipation is over 12 watts, increase the size of the cathode resistor until the plate dissipation is at 12 watts or under - 10.5 to 11.5 watts is a pretty good operating range and will give longer tube life. Cathode bias is generally very forgiving due to its very nature. However, don't assume that just because your amp is cathode biased that it is bulletproof - you just never know what you are going to get with new manufacture tubes.

I have made this handy
spreadsheet to help with calculating plate dissipation for cathode biased power tubes in pentode operation. And this page has some power tube bias charts for EL84 power tubes. One last word of caution: If you have an amp that has a tremolo which modulates the power tube bias by using a phase shift oscillator, make sure you have the bias speed and intensity controls at zero, or it will drive you crazy trying to measure anything!



Fixed Bias:

The safest, easiest way for the do-it-yourselfer to measure bias current in a fixed bias amp is by using the cathode current method. The first step would be to add some 1 watt, 1 ohm, 1% tolerance resistors between the cathode and ground. This will allow you to measure the cathode current and easily convert your reading to milliamps.

Before I go on, I must give another warning: most power tubes used in guitar amps (except EL84) share a common pinout and the pin numbers referenced below assume that you have "standard" power tubes. If you have an oddball power tube, compare data sheets to find out what each pin is before doing anything else! OK then, next, with no power tubes in the amp, turn the amp on and leave it on standby. Set your digital multi-meter to the highest DC voltage rating. Measure the voltage with respect to ground from Pin 5 of any power tube socket. If your amp uses EL34 or 6V6 power tubes, you should measure something in the -35 to -50 volt range. If your amp uses 5881, 6L6 or KT66 power tubes, you should measure something in the -45 to -60 volt range. KT88 and 6550 power tubes can range as high as -100 volts.

Now you want to find the bias adjustment pot and adjust that until you have the highest negative voltage possible at Pin 5 of the power tube socket. If you see no negative voltage on Pin 5, your amp may be an older design where the bias voltage is controlled by the standby switch, in which case you may want to have that modified (or your amp has a serious problem!). At this point, you can install the power tubes, keeping the amp on standby. Wait for the tubes to warm up. After the tubes have warmed up, switch it off of standby and measure the voltage with respect to ground at Pin 3 of any power tube socket. Write this number down for future reference.

Set your digital multi-meter to the lowest DC voltage rating (200mV or higher) and measure the voltage across the 1 ohm cathode resistor. This measurement will give a direct reading in milliamps. For all power tubes listed above, except for the 6V6, adjust the bias adjustment pot until the reading across the 1 ohm resistor is around 30-40mV. For a 6V6, start at around 20mV and work up from that. Multiply the voltage measured previously at Pin 3 (with no power tubes in the amp) by the cathode current. This will represent the static plate dissipation. (Example: 450V x 0.035 = 15.75 watts) At this point, you can take another reading of the voltage at Pin 3 now that the power tubes are installed. The voltage measured should now be lower than the earlier measurement because the power tubes should now be drawing more current and the plate voltage will drop some amount of volts. The static plate dissipation ratings should be within the specs quoted on these
bias charts. If not, then the bias will need to be adjusted until the plate dissipation is at an acceptable level in terms of tube operation and personal taste. Yes, power tube biasing is also somewhat subject to personal taste - some like it biased hotter and some like it biased colder. Biasing them colder will extend tube life, but may not sound quite as good. This is a source of controversy in many circles, but ultimately you have to decide what you like and know the possible risks involved with how you like the power tube bias to be.

A difference of a few milliamps in the cathode current between tubes is quite normal, due to variables in the amp and the power tube characteristics. You should try to to balance both sides of an amp that uses four power tubes, if possible, as having a balance between the two sides is more important than having exactly the same readings at each tube. For example, if the right pair reads 32 & 33mA (total 65mA) and the left pair reads 32 & 32mA (total 64mA), then the output stage should be fairly well balanced. During the first few hours of operation, keep an eye on the power tubes to make sure nothing funny is going on inside of the tubes and take one last reading to make sure that the bias supply is stable. If things have changed considerably from your earlier readings, then readjustment may be necessary. If the plates (grey structures inside tube) are glowing cherry red at any time in this process, turn the amp off immediately! This indicates that the tubes are moving way too much current and the condition must be corrected before going any further since this is a dangerous condition for amp operation.



Fixed Bias Circuit Types:

Many fixed bias amps will have an adjustable bias supply with provision to make the bias more or less negative. Making the grids less negative will cause more current to flow through the tube. Making the grids more negative will cause less current to flow through the tube. Some amps do not have an adjustable bias supply, and will use a fixed resistor instead. In these types of amps, the fixed resistor will either have to be changed or the amp modified to make it an adjustable resistor, which is preferable to a fixed resistor. And there are some amps (like a silverface Fender) that employ a balance type bias adjustment, which only allows for adjustment of the negative grid voltage between the two halves of the output stage. In this case, I would modify the amp to use both the balance adjustment and an adjustable bias supply. More on how to perform this mod can be found
here.




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