Elite Vacuum Tube 34 Bias Tube Socket

The Bias Tube Socket (EVT34BTS) enables the technician or musician to directly monitor the DC current flowing through an output tube, allowing the correct adjustment of the output tube bias without disassembly of the amplifier.

The EVT34BTS will work with many common output tube varieties using the standard 8-pin configuration. These include the 6V6, 6L6, 5881, EL34, KT77, KT88, KT66, KT90, 6550 and others using pin number 8 as the cathode.

CAUTION:  The EVT34BTS contains an epoxy insulation that may soften with extended exposure to high temperatures found in tube amplifiers.  While the epoxy insulation is rated to an excess of 250 degrees, some softening might occur if the EVT34BTS is inserted for extended times reading idle current, or shorter times reading current at actual playing volumes.  It is therefore suggested to keep the heat exposure to EVT34BTS to less than 15 minutes of continuous idle current measurement. High volume monitoring of current should be kept to the minimum time needed to get the current indication.  Should the epoxy soften in the EVT34BTS, allow it to cool without disturbing the relationship of the parts.  Once cool, it's performance will be renewed.

OPERATION

 

With the power off, allow the amp to cool.  Remove one output tube.  Output tubes are labeled 6L6, 6V6, KT66, 5881, EL34, 6550, KT88, KT90, etc.  Do not confuse the output tubes with a rectifier tube.  Rectifier tubes are labeled 5U4, GZ34, 5AR, 5Y3, 5V3 etc.  Insert a EVT34BTS into the output tube socket.  Replace the tube by installing it into the EVT34BTS.

Insert the leads of the EVT34BTS into the "milliamps" input range of your mulitmeter.  Set your multimeter to read milliamps, with the smallest maximum scale greater than 50 milliamps.  Usually this is 200 milliamps on modern meters.

Power up the amp; leave the standby switch on "standby” for at least 3 minutes.  Switch the standby switch to "play" and observe the milliamp indication on your multimeter.  Allow the indication to settle by letting the amp warm up and additional five to ten minutes.

Adjust the bias such that the current (milliamps) indication on your multimeter is the desired level. Power down the amp and allow the tubes to cool before removing the Bias Probe, and restoring the tube into its original position.

                                                      USING A PAIR OF EVT34BTS

Two EVT34BTS with two multimeters may be used simultaneously to observe the current indication on a set of tubes.

Remember: The indication shown on the multimeter is the correct "per tube" current, with no doubling or halving of the value required.  The same goes for one, two or four Bias Tube Sockets being used at the same time. 

Note, when using "two" EVT34BTS on amplifiers with four or six output tubes, you have to double or triple the values.   For example when biasing let say a Marshall DSL 50, it haves two output tubes, so when adjusting the bias, your multimeter should read around 40 milliamps, that reading is per tube. (Assuming you are using two Bias Tube Sockets, witch I recommend)  Now lets say you are biasing its big brother the DSL 100, it haves four output tubes. Place the EVT34BTS on the outer output tube sockets and check the multimeter reading. If you adjusted the bias to 40 milliamps then you double that reading, (do not adjust the bias to 80 that equals 160) to equal 80 milliamps, 40 milliamps per tube and so on. Marshall recommends 45 milliamps for the DSL50 and 90 milliamps for the DSL and TSL 100. Practice with your old tubes first because you can fry your tubes in seconds.

    TUBE MATCHING

 

Tubes should be matched for idle current and transconductance.  Idle current is the current that the tube conducts with no signal input.  Transconductance is the amount of change in current that is exhibited with a given change in grid voltage (either bias or signal voltage). While the EVT34BTS cannot directly measure transconductance, if two EVT34BTS are used, the effect of transconductance can be observed in the following manner.

1. Set the bias on the amplifier to the point that gives an idle current somewhat lower than what would be normally used for the tubes being tested.  For instance, setting the idle at 20ma instead of 35ma would be fine.

2. Observe and record the current reading of each tube.  Note the difference in current, such as 3 milliamps.

3. Set the bias of the amplifier to the point where the idle current is somewhat higher than what would be normally used for the tubes being tested.  For example, setting the idle current to 50ma instead of 35ma would be fine.  CAUTION - do not run the amp for extended periods of time with higher than normal current settings.

4. Observe and record the current reading of each tube.  Note the difference in current, such as 4 milliamps.

5. Reset the idle to the normal indication.

If the difference in current conduction stayed about the same as the current went from extra low to extra high, then your tubes have a good transconductance match.  Normally there will be some variation.  Consider the tubes matched when the difference on conduction varies by no more than a few milliamps in the low, medium and high idle current ranges.  For example, tubes may show 20.5 and 23.5 at low idle current, 34.0 and 36.5 at medium idle current, and 49.5 and 50.5 at high idle current.  The difference has changed only two milliamps, and that would be acceptably matched.

Don’t worry all output tubes are matched with the Maxi-Matcher and powered with a variac transformer for precise matching.

BIAS ADJUSTMENT

Some knowledge of the operation of the amplifier is needed to actually adjust the bias level of the output tubes.  If in doubt, refer to the manufacturer's 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. If you have a silverface Fender, it is suggested that you have a qualified repairman rewire the control for level operation.

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 EVT34BTS 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.

Some amplifiers, such as tweed and brownface Fenders 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.

MAINTENANCE

 

All EVT34BTS made today are using a new improved epoxy, which we feel is an improvement in strength as well as heat resistance.  During the manufacturing process, the epoxy is trimmed from the Bias Probe and some small flakes may occasionally be seen inside the units.  These are just a few stray bits still remaining for some reason after being dusted with compressed air, and represent no failure, nor do they detract from the Bias Probe performance.  The contacts of the base and socket may be cleaned periodically with denatured alcohol to eliminate any oxidation or contamination that may have been picked up during operation.  The socket pins seem to be cleaned well with a Q-Tip whose end has been broken off.  The paper shaft of a Q-Tip fits well within the socket contact. The banana plugs on the Bias Probe are high quality gold plated, industry standard plugs.  We have found that some meters may have undersized jacks. This normally presents no problem when using the EVT34BTS.

                                       SUGGESTED CATHODE CURRENT SETTINGS

 

IMPORTANT NOTICE

 

These cathode current settings are intended as starting points. There is no one absolutely correct bias level or current level. The range of acceptable cathode current levels is fairly wide. Lower current levels will sound weaker, thinner and the amp will run a little cooler. Extremely low current levels will sound distorted (terrible sounding crossover distortion). High current levels will sound louder, punchier and fuller. The tubes will run hotter. Extremely high current levels will cause the tubes to overheat (start to glow then short out).

What is Biasing?

General

When people talk about "biasing" an amplifier, they are referring to setting the "idle", or quiescent, current in the power output tubes.  All tubes must be biased, both preamp and output tubes, but it is not always clear whether or not the bias needs to be adjusted when changing tubes.

Why do you need to bias a tube?

Tubes have to be properly biased in order to function as amplification stages.  A tube is biased by setting the amount of DC current that flows in the tube when there is no signal present at the tube's grid with respect to it's cathode.  This DC bias current can be set in a number of ways.  The bias point determines several things about a tube amplification stage. It determines the power output, amount of distortion,  headroom (the size of input signal that can be applied before the output signal clips),  efficiency of the stage (the amount of output signal power vs. DC input power),  gain of the stage (the magnitude of the output signal for a given input signal), noise of the stage, and class of operation (class A, AB, etc.).  The proper bias point is a tradeoff between all of these factors, and selecting the optimum bias point can sometimes be difficult, and it will vary depending on the amplification stage requirements.

Biasing methods

There are two main types of biasing: fixed biasing and cathode biasing.   Fixed biasing does not mean the bias is not adjustable, in fact, it usually means the opposite. Cathode biasing is usually fixed, and not adjustable, and fixed biasing is usually adjustable with a small trimmer potentiometer, or "trimpot".  It is no wonder the subject is confusing to people!

Fixed biasing means the tube is biased by means of a DC voltage, which is usually a negative voltage applied to the grid of the tube with respect to the cathode.  As the negative grid voltage is adjusted, the bias current will increase or decrease, depending upon the direction the bias voltage is going.  In general, as the bias voltage becomes more negative, the bias current becomes smaller, and the tube is biased "colder".  As the bias voltage is adjusted less negative, towards zero volts DC, the bias current becomes larger, and the tube is biased "hotter".  This is because a tube is a "normally on" device; that is, it allows current to flow from the cathode to the plate when the grid is at zero volts with respect to the cathode.  The tube can be turned off, and the current flow stopped, by making the grid voltage negative with respect to the cathode.   The tube can also be biased by referencing the grid to ground, or zero volts DC, and applying a positive DC voltage to the cathode.  This is the same as keeping the cathode at ground and applying a negative DC voltage to the grid, because it is the grid voltage with respect to the cathode that determines the amount of bias current in the tube.

Since vacuum tubes are "normally on" devices, a trick can be used to bias them without having to supply a negative DC voltage source to the grid.   If a resistor is placed between the cathode and ground, and the grid of the tube is referenced to ground (usually by connecting a large value resistor, such as a 1Meg, from grid to ground),  the tube will try to conduct a large current from cathode to plate, since the grid and cathode are initially at ground potential.  However, this cathode current flow will cause a voltage drop across the cathode resistor, making the cathode voltage positive with respect to the grid.  Since the cathode voltage is now positive with respect to the grid, the current flow will decrease, and the tube will head back towards cutoff.  A point of equilibrium will quickly be reached where the increase in current is offset exactly by the increase in cathode voltage, and the bias current will stabilize at some particular value.  It will remain at this value unless the resistor value is changed, or a different tube with different characteristics is plugged in.  This allows the desired bias point to be set by varying the value of the cathode resistor.

When do you use fixed biasing instead of cathode biasing?

Since cathode biasing eliminates the need for a special negative DC bias supply, why don't all amplifiers use cathode biasing?  Well, cathode biasing is not without its faults.  It turns out that in order to keep the DC bias supply voltage at the cathode constant while the input signal is changing, the cathode resistor must be bypassed with a large capacitor.  This capacitor effectively "shorts" the AC signal component to ground, while allowing the DC voltage to remain relatively constant.  If the capacitor is removed, the cathode DC voltage will have a signal voltage superimposed on it, which will subtract from the grid-to-cathode signal voltage, and reduce the gain of the stage.

The problem comes in when there are large signal level changes, and the average DC level of the cathode voltage changes.  This causes a bias shift, usually in the direction of a colder bias point. This bias shift can be audible, but is sometimes desirable for guitar amp use, as it adds varying harmonic overtones to the sound.   If the bias shift is severe, the tube will go into cutoff, and large amounts of "crossover" distortion will occur.  In addition, the current flow through the cathode resistor generates a necessarily large bias voltage on the cathode for proper tube operation (typically 30-50 volts for most higher power output tubes).  This voltage subtracts from the total plate voltage, which decreases the available output power.  Between this voltage decrease and the bias shift, the output power in cathode biased operation is reduced when compared to fixed bias operation.  Therefore, fixed bias is usually used for higher power amplifiers (50W and higher), and cathode bias is usually used for lower power amplifiers.

Preamp tubes are almost universally cathode biased, because they are used for signal amplification, not power amplification, and the side effects of cathode biasing are not as important.  Also, cathode biasing makes the circuit less dependent on tube parameters, and more forgiving with respect to bias point.  This allows replacement of tubes without having to rebias the amplifier stage.   Output tubes that are cathode-biased should always be checked when replacing tubes, however, because they vary widely in terms of idle current for a given cathode resistor value, and it may be necessary to change the cathode resistor value to return the output stage to it's proper bias current.

                                                                                                                                        Aiken Amplification
Bias Probe
           

You may decide to purchase a "bias-probe" type device; this is a gizmo which consists of an "interruptor" socket/plug assembly that goes between the tube(s) and the amp's socket. This test adaptor will have a couple of testleads hanging out through a hole in the side, for connection to your meter(s). If you do get one of these, there is no need to install the 1-ohm shunt resistors on the tube sockets as outlined above. You can use the readings obtained from the adaptor sockets in place of the readings normally taken across the one-ohm resistors. BE AWARE THAT THERE ARE TWO TYPES OF THESE ADAPTORS COMMONLY AVAILABLE. One type *breaks* the cathode connection, and instructs you to connect the testleads to the CURRENT jacks on your meter. The other type contains a one-ohm resistor in series with the cathode pins, with the testleads connected to either side of the resistor; this type instructs you to connect the testleads to the VOLTAGE jacks on your meter. It has been my experience that some amps (especially old Marshalls) do not react well to having several feet of wire inserted in series with the power tube cathodes, and will oscillate like crazy. Therefore, if you decide to get a set of these test adaptors, get the ones which use an internal 1-ohm resistor.

REMEMBER...THERE ARE VOLTAGES PRESENT INSIDE EVEN THE SMALLEST TUBE AMPLIFIER WHICH WILL KILL YOU JUST AS DEAD AS A HAND GRENADE WILL!! If you're not familiar with high-voltage safety, seek guidance from someone who is. BTW, an oven mitt or a pot-holder will come in handy for handling hot power tubes if you need to switch sockets; you don't want to let the tubes cool off too much while you swap them before taking new readings.                                                                   
                                                                                                                                    Duncan Amps.