Bias FAQ
(Answers to common Bias Questions)

"This is the same current probe used here at the shop." That's what we USED to say at least!  NOW THEY'RE BETTER THAN EVER! The current SwAMProbes have GOLD PLATED sockets, GOLD PLATED PULL TABS, Heavier Silicon Rubber Wiring than before, and Improved Epoxy Potting Compund.  These are quite literally THE BEST UNITS WE'VE EVER MADE, and the price has remained the same.  They're made from Russian sockets and bases. The wiring is heavy 18gauge solid core inside and durable HEAVY DUTY silicon rubber test lead wire outside. Comes with a bananna plug on each 18 inch test lead.  Idle current measurements are taken from the cathode (low voltage side) of the tube using your digital VOM's milliamp range to ensure user safety. By using the SwAMProbe, you can test the cathode current without removing the chassis from the cabinet. If you'd like to really get a grip on the cathode current of each tube (balance) then get a pair, as pictured, and use  'em with a set of meters!

SwAMProbes are STILL ONLY  $29.00 each. Shipping in the United States adds another $4.00 ($33.00 total). If you need COD service, we charge only the actual COD costs ($5.00). If you live elsewhere, we can probably ship to your address, but Email us first before sending payment. International Priority Mail can be as low as $2.00 extra!

Double female adapters for use with a pair of SwAMProbes and only one meter, only $4.50 and no extra shipping charges if ordered at the same time as the SwAMProbe set. A nice little tool to have....


The SwAMProbe output tube current monitor/bias sensor 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 SwAMProbe will work with many common output tube varieties using the standard 8 pin configuration. These include the 6V6, 6L6, 5881, EL34, KT88, KT90, 6550 and others using pin number 8 as the cathode.

CAUTION:  The SwAMProbe 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 SwAMProbe 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 SwAMProbes 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 SwAMProbe, allow it to cool without disturbing the relationship of the parts.  Once cool, it's performance will be renewed.


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 SwAMProbe into the output tube socket.  Replace the tube by installing it into the SwAMProbe socket.

Insert the leads of the SwAMProbe 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, leaving the standby switch in the "standby" mode for at least 30 seconds.  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 SwAMProbe, and restoring the tube into its original position.


Two SwAMProbes may be used simultaneously to observe the current indication on a set of tubes.  A DOUBLE FEMALE ADAPTER can be used to join the leads of the unmetered SwAMProbe if only one multimeter is being used.

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 SwAMProbes being used at the same time.


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 SwAMProbe can not directly measure transconductance, if two SwAMProbes 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.

Your indications may vary considerably from this example.


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 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.

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.

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 a little punchier but 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, and fuller, and overdrive more easily.  The tubes will run hotter.  Extremely high current levels will cause the tubes to overheat (start to glow then short out).


All SwAMProbes made on or after 9/15/1999 are using a new improved epoxy, which we feel is an improvement in strength as well as heat resistance. All SwAMProbes using the improved epoxy compound will be identified with a plastic stick on label as opposed to the older paper label. During the manufacturing process, the epoxy is trimmed from the SwAMProbe 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 SwAMProbe 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 SwAMPRobe are high quality, industry standard plugs.  We have found that some meters may have undersized jacks. This normally presents no problem when using the SwAMProbe. Should the spring metal contact of the banana plug be deformed through use in a very tight socket, it may be restored by gently prying the spring back to shape using a suitable tool (jewelers screwdriver works well).


Tube Series Cathode Current Notes
6L6 32ma - 37ma In Fender's, usually best results do not exceed 35ma
5881 35ma - 40ma The Russian large base (WXT) tube is usually good at around 37ma
6V6 25ma - 35ma Fender Deluxe and Princeton amps at around 30ma, maybe a little less
EL34 30ma - 40ma Wide ranges of acceptable currents. Maybe even higher than 40ma in some amps.
6550 35ma - 45ma Some amps will work best at higher than 45ma
KT88 35ma - 45ma Some amps will work best at higher than 45ma
KT66 35ma - 45ma This will vary depending on whose KT66 you use
KT100 35ma - 45ma Some amps will work best at higher than 45ma


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).


On an amplifier of unknown specification, or on one that's been significantly altered, one way to get the bias set "close" (as a starting point) is to observe the output of the amplifier on an oscilloscope while injecting a sine wave signal at the amps input. The bias may be adjusted to the point where the crossover distortion disappears on the scope. At this point, note the cathode current of the output tubes with the SwAMProbe(s). The current measured is at the low end of the bias range for A/B operation. Careful experimentation should be used to raise the current slightly, in small increments, until the desired tone is attained. If the output tube plates start to glow, you've gone too far. Likewise, if the amp seems like it's running too hot, the transformers are hot, or things start to smoke, back off immediately! Few musicians have oscilloscopes and signal generators. It may be useful to have the bias set initially by a professional, using the above technique. Note the cathode current with the SwAMProbe for later use with other tubes.


At SwAMP the Wavetek DM27XT is the preferred VOM. It measures AC and DC volts, ohms, PLUS capacatance and inductance.  Those ranges are not normally found on most meters. They sell for about $120 through most electronics supply houses.

Of course any meter with a milliamp range of about 100milliamps will do for the SwAMProbe (digital preferred)


If you don't already have a voltmeter, you may want to consider purchasing one - especially if you're a working musician who regularly gigs at different venues.

Stage electric service can be some of the worst around. Normal electrical service is 240volts - that is - two 120volt circuits out of phase, sharing a common "neutral". It is possible to have one phase of power for some of the equipment and the other phase for the rest. This is, potentially, a lethal situation. This situation, by the way, is in violation of the NEC (National Electric Code). However, many establishments added stage electric service as an uninspected afterthought, or equally likely, the musician plugs a heavy duty extension cord into an outlet that is out of phase with the rest of the electrical service. It is possible for the musicians to become live "fuses" (well, sort of) in cases like that.

Check first. Play later. Live to play another day!

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