Output Tube Bias FAQ

Setting the bias on one's output tubes is one of the most asked questions presented to us. By way of this short discussion, it is hoped that some light will be shed on exactly what "bias" is, how to adjust it, and what effect it has on your amp's tone.

1. -What is bias?

"Bias" is an "offset" or correction. In our case it is a voltage correction applied to the circuit to alter the the relationship of the output tube's grid and cathode.

2. - Why do we need bias?

In a tube, voltage on the grid controls the current flowing from the cathode to the plate. As we apply an AC (audio) signal to the grid of the tuve, the current flowing through the tube will vary proportionally with the signal. However, if the signal on the grid were positive and negative in relation to the cathode, we'd have way too much current flowing thru the tube, and the excess current would burn up the tube.  The grid signal must be a little more negative  - that is - offset, or "BIASED"  in relation to the cathode in order to keep the flow of current under control.

3. - How does bias effect the tubes operation?

The bias effects the output tubes operation primarily in two related ways.  One is the way the tube will react to it's input signal, giving more or less compression or breakup, the other is the Class of operation.  First lets discuss the Class of operation.

Amplifiers are catagorized into several "classes" of operation. These classes are named Class-A, Class-A/B and Class-B. There are also a few sub classes of the ones already mentioned, and a Class-C, but we'll dismiss those for this discussion to keep things clearer. Simply stated, the distinction in Class catagories is dependant upon that amount of a sine-waveform the output tube(s) conduct. Keep in mind that a sine wave has both positive and negative halves.

You might exerience a sine wave, or part of a sine wave being specified in either percentage, or in degrees, 360 degrees being equal to 100 percent of a full waveform.  We'll use the percentage method, since most folks are familiar with percentages.

A very Simplified Explaination of Amplifier Operational Classes

In a Class-A amplifier, the output tube(s) will conduct both the positive and negative halves of the waveform. Single output tube amplifiers are Class-A by definition, since the single output tube must conduct both wave halves, or put another way, 100 percent of the full waveform. In that case, the bias voltage needs to be set at the mid point of what ever voltage it takes to provide no current flow, and maximum designed current flow through the tube. That "mid point" voltage is the correct bias voltage for Class-A amplifiers.

In amplifiers with more than one output tube, the circuit is often designed such that half of the output tube(s) conduct the positive half of the waveform, while the other output half conducts the negative half of the waveform. These are called push-pull output amplifiers, and may operate in Class-A, Class-A/B or in Class-B, but are popularly found as Class-A/B designs.  There are some Class-A push-pull amplifiers, notably the Vox AC30 and some others, the vast majority of push-pull guitar amplifiers operate in Class-AB.

In a push/pull Class-A/B amplifier each output half conducts greater than 50 percent and less than 100 percent of the waveform.  As soon as a push/pull type amplifier conducts 100 percent, it's no longer Class A/B, but Class A instead.  Some books describe Class A/B as being significantly greater than 50 percent and signficantly less than 100 percent waveform conduction.  In practical terms, that's absolutely correct.

In a Class-B amplifier each output half conducts half of, or exactly 50 percent of the total waveform. Class-B designs are not used in any popular guitar amplifiers.

Musicians also find that there is a tonal change as the conduction increases higher toward the Class-A theshold (100 percent conduction) from that of being just over the threshold from Class-B into Class-A/B (50 percent conduction).  In other words, the tone changes as the amount of conduction increases toward Class-A operation.  When the tube is set at a higher bias point, or one that conducts more current, it's closer to it's saturation point, and the incoming signal will be able to more easily push it to the point of saturation (and distortion).

4. - How does the bias effect the operation Class of the amplifier?

As the bias voltage is made increasingly negative in relation to the cathode, the tube will conduct an increasingly smaller amount of the total waveform. In a push-pull amplifier, if each output half conducts less than 50 percent of the total waveform, there will be a distortion known as crossover distortion. This is particularly un-musical sounding. In a Class-A/B guitar amplifier we'd like to get each output half conducting more than 50 percent, and less than 100 percent of the total waveform.

5. - Why not just set it so each output half conducts the whole waveform?

Well, doing so would make the amp a Class-A amp, and the Class-A push-pull amplifiers do just this. But, Class-A operation has it's price. Since each output half is working 100 percent of the time, conducting both halves of the waveform, the demand on the associated components, such as the power and output transformer, are much greater. There is a lot more heat developed, and the current used (and heat generated) is more or less constant from zero volume to full volume. Setting the bias of a Class-A/B design amplifer such that it actually is running in Class-A operation is a sure way to damage it, since it's just not designed to deal with the current levels being used.

In the more popular Class-A/B push-pull guitar amplifiers, the lower percentage of conduction means that there is less current being conducted at low volumes than at full volume. This also means that the total current being used and total heat generated can be considerably less than in a true Class-A push-pull design. However, many designs allow the maximum current being conducted through each output half to be greater, thus allowing for higher volumes to be generated at the speaker, by way of a greater peak current flowing through the output transformer.

6. - How do we know how much of the waveform is being conducted?

We really don't unless, we make a pretty sophisticated test. The test includes the use of a dual trace oscilloscope and a signal generator and some other test fixtures. The test involves measuring the amount of simultanious conduction of each output half of the amplifier under real world actual load conditions. Although the traces on the scope can be seen visually, many scopes today allow for the scope's cursors to perform math fuctions which allow for precise measurement. Using a scope of this sort, or just using the oscilloscopes display grid, the technician can find the amount of overlapping conduction. This sort of measurement is beyond the capability of most musicians and many of the amplifier repair shops.

7. - So how can the bias be adjusted if the amount of conduction is not known?

Fortunately for us musical types, there are much simpler ways to set the bias voltage than doing complicated testing with an oscilloscope. One way is to observe the effect the voltage has on the current flowing through the tube. Over time, the general range of acceptable no-signal current flow through each output tube has been established for popular amplifier designs. The acceptable range of no-signal current flow is actually quite wide which leaves some leeway for the musician to adjust for the tone he/she desires.

8. - How does the bias effect tone?

As the bias is set lower the amplifier will operate closer to Class-B.  The tone will be a little "thinner" or less full since the amplifier will not be hitting it's output tubes saturation point quite as easily.  If the amplifier is biased very low, higher volumes will encounter crossover distortion.  Properly biased in a lower range, the overall clean headroom of the amplifier will be greater.

As the bias is adjusted higher, toward Class-A operation, the tone will be "thicker", "fatter" or more full. The overall headroom of the amp will be less, since the signal level at the grid, added to the bias voltage will be closer to the tubes saturation point.  The amplifer will distort easier (musical distortion, not that nasty crossover distortion stuff).

9. - If I set the bias too high or too low, will it hurt the amplifier?

If you set it too low, then no harm can be done. If the bias is set too high, then there is the possibility that the output tubes will not be able to dissipate the extra heat, the plates will start to glow. The tubes will do a major meltdown. There is a possibility of power transformer and or output transformer failure if the failing tubes short.

10. - Sounds dangerous, how can I be sure it's not set too high?

For one thing, you can start low and work up gradually to get the tone you desire. Another good idea is to observe the tubes under actual playing conditions at fairly high volumes. If the plates start to glow red, turn off the amp immediately and allow it to cool completely. After that, set the bias lower immediately upon restarting the amp.

11. - How can I measure the no-signal current though each tube?

There are a few methods. One is called the transformer shuntmethod. This involves measuring across one side of the primary winding of the output transformer with an ammeter. This can be a dangerous method since the voltages involved are quite high.

Another method is the bias resistor method. In this method, a small value resistor is inserted between the tubes cathode and ground. In operation, a small voltage drop will occur at the resistor. This may be converted to the amount of amperage by math and ohms law. If a 1ohm resistor is used, then each 1 millivolt dropped will equal 1 milliamp of current flow. This method is also not our favorite, since it involves being close to higher voltages in the amplifier and also has the tolorance of the resistor to contend with (and your meter's ohm scale tolorance, if you choose the resistor value yourself).

There is a method called the crossover notch method, which involves an oscilloscope and signal generator. This is not the same simultanious conduction test mentioned before. It is a test using only a single scope trace and uses observation of the shape of the waveform to determine the Class-B -/- Class-A/B threshold. This method will in fact determine that the amplifier is or is not operating in Class-B. For some, it may be a good starting point for experimentation, allowing for small incremental increases in the bias to make the amplifier operate more toward Class-A operation. Observe the cautions outlined above to avoide exceeding the tubes heat dissipating ability. This method will always set the amp very close to the Class-A/B - Class-B threshold and result in a thin tone.

We happen to feel that the direct measurement method, measuring the current through the cathode by means of a tube socket mounted device is the quickest, easiest, most repeatable, and safest way for the musician or unsophisticated technician to set the bias to the proper range. The SwAMProbe is such a device, and is inserted between the tube and socket. The SwAMProbe's leads are attached to the ammeter input of a standard VOM, and the amplifier is switched on. The reading is directly measured in milliamps with no math conversion required.

12. - How do I make a measurement with a SwAMProbe?

(the following is reprinted from the SwAMProbe instructions)

1. Turn off the amp, and allow the output tubes to cool.

2. Prepare the SwAMProbe by connecting it to you volt meter. Be sure to use the
milliamp range inputs if they are different from the normal volt/ohm inputs. Set
the meter to read a range with a maximum scale of 100 milliamps or higher.

3. Pull one output tube. Be sure the tube you pull is an output tube, not a rectifier
tube. Tube numbers 6V6, 6L6, 5881, EL34, KT88, 6550, and KT90 are some
common designations for output tubes. Tube types 5AR, 5U4, 5Y3, GZ34 are some
typical rectifier tubes. The rectifier should be left in place.

4. Place the SwAMProbe into the socket, making sure the connection to the meter
has not been disturbed.

5. Place the pulled output tube into the SwAMProbe.

6. Power up the amp normally. It is suggested that the amp be placed on stand-by
for at least 30 seconds if it is equipped with a stand-by switch. If you care to
prove to yourself that tubes need full warm-up, and don’t mind abusing your
tubes, just turn on the amp with the stand-by switch in the on or play position. The
current will slowly rise showing the tube is still warming up. This test is not good
for the tubes, so don’t do it on tubes that you care about. Applying full voltage to
a tube whose cathode has not been brought up to temperature, will cause “cathode
stripping”, a condition where the rare-earths are blown off the cathode,
damaging the tube.

7. Observe the milliamp reading on the meter.

8. Refer to the chart for the suggested acceptable current range, according to your

9. Remove the SwAMProbe by first powering down the amp, and allowing the
tubes to cool. Pull the tube from the SwAMProbe, then remove the SwAMProbe
from the chassis socket.

On amplifiers with more than one output tube, it is suggested that each tube be
tested by keeping the SwAMProbe stationary in one socket. An output tube must be
installed in each chassis socket to perform this test. Rotate one tube at a time into
the chassis socket position containing the SwAMProbe. In this manner, each tube
can be tested for “match” or balance, and slight differences in the amplifiers
circuitry will not alter the results. It is suggested that you number each tube with
a magic marker, and write down the each tube’s test results.

After testing each tube, choose one tube to test in all of the chassis sockets. Again,
all output tubes must be installed to perform the test. By using the same tube to
test each socket, differences in the output circuitry, especially the transformer,
can be noted. Number each tube socket, and record each socket’s test results.

If the tube’s milliamp indication varies by more than 5 milliamps, replacement of
the tubes with matched tubes is suggested. If the amplifiers sockets vary by more
than 2 milliamps, then you might want to consider having a repairman match the
bias feed to the output transformer for the most pleasing tone and most efficient

Many times it's easier to use a pair of SwMAProbes instead of switching only one
unit into all the different sockets. This is entirely easy and acceptable!
When using a pair of SwAMProbes one of the two methods must be followed. Either:

A. Use one meter, and connect the leads of the un-metered SwAMProbe together
with a female- by-female adapter (Radio Shack item) or,

B. Use two meters, one on each SwAMProbe.

Some knowledge of the operation of the amplifier is need to actually adjust the
bias level of the output tubes. If in doubt, refer to the manufacturers instructions,
a reputable text, or seek advise from a qualified tube amp repairman.

Most blackface Fenders, and most Marshall’s have a bias level control on the
bottom of the chassis. This control alters the overall level of the bias for the entire
output section.

Silverface Fenders usually have a bias balance control. Do not confuse this with
the hum balance control found on the back panel of some silverface Fender amps,
which is for the elimination of heater hum. The bias balance control makes it
possible to use unmatched tubes by varying the balance of the bias level between
the output halves, but does not permit an overall level adjustment. Fender started
using this balance control to offset some quality problems.

If you have a silverface Fender, it is suggested that you have a qualified
repairman perform two modifications:

1. Return the bias control to the older blackface specification (level control
instead of balance control).

2. Substitute the 220k bias feed resistor on one of the output halves with a resistor
of a slightly different value, if needed, to correct a potential output balance

By performing those modifications, the bias will be truly balanced and the level
will be easy to set. For the absolute best results the output section halves should
be balanced to within 1 milliamp variation between the halves. A balanced output
will create the most pleasing tone when overdriven.

Some older Fenders, and some other newer amps may be cathode bias designs. In
this design, there is no bias control. The bias is set by the relationship of the grid
to the cathode, which in turn is set by the cathode resistor. Cathode bias
amplifiers are self biasing. The SwAMProbe will verify whether the amplifier is
working correctly by showing the cathode current. If the amplifier is out of spec,
the correct solution is to replace the cathode resistors. When replacing resistors,
the value of one resistor may be slightly altered to correct any balance problems
experienced. Usually this is not needed unless the output is considerably

Sometimes the bias level pot is located within the chassis of the amp, and requires
that the chassis be removed to adjust the bias. This is a job for an experienced
repairman, since very high (very dangerous) voltages are present inside the
amplifier chassis. However, it is usually a very easy job to modify the amplifier to
permit the mounting of a control potentiometer with an external adjustment. This
modification should be performed by a qualified technician.

Some amplifiers have a non-adjustable grid bias (as opposed to self adjusting
cathode bias). These can usually be converted, as described above, to a
potentiometer accessed without pulling the chassis from the amplifiers case.

The modifications mentioned in the preceding sections should not be too terribly
expensive, probably the minimum bench charge, plus parts. The SwAMP shop
usually does these in less than one hour, including those which need conversion to
blackface spec, and those with unbalanced output sections.

13. - Why is there no bias adjustment on the preamp tubes?

There is! It's done through the selection of the cathode resistor for that tube. However, pramp tubes are all designed for Class-A operation, and the currents being conducted are very small (about 1.5ma to 2ma), so there's no need to make adjustments as in the case of the power tubes. You can make sure the value of the cathode resistor hasn't drifted, and correct the situation as required.

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