Tubes & Biasing

Category: Main -> Tubes & Biasing



Q: When should I change my power tubes?

When any of these conditions occur:

  • When the bottom end sounds muddy

  • The amp lacks punch

  • The high end is muffled or the clean sound is dirty

  • If you constantly blow fuses (Do not replace the fuse with a larger value!)

  • If you hear a low hum, the tube is bright red on the outside plates and the volume decreases Urgent

Good quality, well matched tubes can last 5 to 10 or more years. Chinese tubes last from 2 to 5 years. Proper biasing helps enormously. Some people can go through tubes in 3 months!

Music stores generally do not have the equipment to test power tubes properly. They will almost always tell you that you need new tubes regardless of whether this is true. Go see a reliable service tech who will replace parts only when necessary and is not out to make a profit by selling unneeded parts.

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Q: Why do I need to adjust the bias in my amplifier?

The current flowing through your amp's output tubes is controlled by a small negative voltage that is known as the bias voltage. When this bias voltage is applied directly to the input grids this is called fixed biasing. Usually you will have an adjustable control to set the proper voltage. Even though you can vary the negative bias this is still known as fixed bias because once it is set it does not change. If this voltage disappears, so do your tubes.... they will get red hot and incinerate. If there is too much of it your sound will be cold, choked, and with no sustain.

Used to be that you could replace your output tubes with ones from your original amp manufacturer and not have to worry about setting the bias voltage, but today with the wide range of tubes available it is necessary to that the bias be checked when changing output tubes. Even matched pairs of the same rating as your original tubes can be appreciably different from each other.

A more dynamic method known as self biasing sacrifices some of the output power to provide negative feedback voltage. This technique is used in smaller amps and sounds softer when the amp is heavily clipped. The Vox AC30 is a prime example. A self biased amp is more forgiving than a fixed biased one when installing new tubes, but in extreme cases needs adjustment as well.

See AmplifierBiasing page for more info

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Q: Do I need to bias my pre-amp tubes?

Pre-amp tubes are almost always self biased and require no adjustment. In Fender amps a 12at7 is often used as reverb driver. This tube works very hard and should be selected to provide the best output. If necessary, change the cathode resistor to rebias the tube.

See AmplifierBiasing page for more info

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Q: Can I remove 2 tubes form my 100 watt amp to make it 50 watts?

Removing tubes causes operating voltages to rise in the amp and stresses the remaining tubes reducing their life expectancy. This can be very dangerous in a mid 70's Marshall that runs at 540v on the plates. JCM800's on the other hand might take it. Due to it's larger transformers a 100 watt head will always sound bigger than a 50 watt model even when it is running only on 2 tubes.

The output impedance of the amp will change so that you must now set the amp to 4 ohms for an 8 ohm cab, or 8 ohms for a 16 ohm cab. Because the voltages have changed the amp needs to be re-biased as well.

Never remove a pair of tubes from an amp that has a cathode biased output stage unless the remaining pair is separately biased, doing so will cause the plate current to greatly increase and probably fry your amp.

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Q: Can I replace my power tubes with smaller ones so that my 100 watt amp will run at lower power? (George P.)

Smaller power tubes such as 6v6's or 6BQ5's (EL84) operate under lower voltage and current conditions than do higher power types such as 6l6gc 0r El34's. Power supply voltages must be brought down to safe levels before attempting this change. Using a high-sag tube rectifier such as a 5U4gb can help to bring the voltage down as well. Using two 5U4gb's in series will give you a "Dual Rectifier" with lower voltage still.

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Q: So will using smaller tubes make my Fender Twin Reverb sound like a small 20 watt amp? (George P.)

Well, not really. In a small amp the output transformer is much smaller than the one that is in your amp. It saturates more quickly thereby rolling off some highs and bass when the amp is played loud, which removes some of the harshness from the output tube clipping. Saturating the output stage adds compression and fatness as well.

None of this occurs if you run a 100 watt output transformer at 20 watts, the sound will be more hi-fi and more dynamic but harsher. The power transformer is much smaller in a 20 watt amp as well and causes the power supply voltage to drop when the amp is running full out. This makes for suppression of sharp transients that can fry your tubes while providing a mellower tone.

In your amp the larger power supply will deliver a sharp transient spike if you slap a really high note and that can be the end of your hard working tubes. So the voltage of your amp must be further lowered to allow for these conditions. While not sounding the same as a smaller amp it will nevertheless be operating at lower power and will benefit you at lower volumes.

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Q: How much on average does it cost to replace tubes (having a trained tech doing it)? (Morgan H)

The cost of replacing and biasing power tubes depends on many factors. A good tech will generally make sure that your tubes match by measuring the current in each and will check the signal with an oscilloscope to see if there is any crossover distortion. You must leave the amp on for a while to stabilize the current and switch it on and off several times to see if the tubes are reliable.

Amps that do not have an adjustable bias (Boogies, Hiwatts, Vox AC30) take longer to bias because you must change components to obtain the correct current. In some cases the available bias voltage is too low and the bias supply circuit must be redesigned. The Blackface Twin Reverb reissue is quick to bias because it has an easily accessible potentiometer with sufficient range to accommodate most tubes.

Since your amp is new, retubing should be very straightforward. A lot depends on the quality of the tubes that you buy. Lower quality tubes can cause all kinds of problems from crackling noises to low frequency humming and can short more easily. Any problems such as these will, of course, cost more time. Expect to be charged for an hour's labour.

See AmplifierBiasing page for more info

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Q: What is the correct bias setting for my xxx amplifier? (Greg D)

Most high-powered guitar amps employ push-pull class AB circuitry. In this configuration the power tubes do not operate continuously but are alternately switched on and off. This causes a very ugly glitch to occur known as crossover distortion.

Idealy, the bias voltage is set to provide enough current flow in the output tubes to eliminate this crossover distortion. Too little current will cause your sound to be distorted, too much current will cause the plates to overheat.

When setting the bias you must take into account the variations between the tubes. Even matched sets can vary from 30% up to 100%. I have seen unmatched sets vary as much as 1000% (10 times the current in one tube as opposed to the other) or more! Still, some people question the need for matched tubes! Each tube must be carefully measured before biasing.

The plate voltage of the amp is also important. There is no way to properly bias a '74, 100 watt Marshall operating at 560 volts and to eliminate all the crossover distortion when it is driven to clipping. 50 watt Marshalls can vary from 325 volts to 450 volts and this must be taken into account when biasing the amp.

The quality and brand of the output tubes is naturally extremely important. If you have cheaper brands they won't hold out and you must compromise on the bias setting and therefore on the sound quality of your amp.

Amps are often played in conditions which are far from ideal and are subject to poor ventilation, improper and fluctuating mains voltages, sudden surges, etc. All of this must be taken into account or your amp will blow right in the middle of your big solo.

I strongly suggest that all biasing be done by a skilled and experienced technician, preferably one who will test the amp at full power with a guitar and not with a signal generator.

See AmplifierBiasing page for more info

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Q: My pre-amp tubes seem to be loose, is this normal?

Yes, on chassis mounted tube sockets the pins are supposed to float loosely in the sockets. This helps reduce microphonic pickup and protects the tube from shocks.

Newer amps, unfortunately utilize printed circuits and the tube pins cannot move making these designs more susceptible to vibration and microphonic pickup.

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