What is bias in my Cary Audio vacuum tube amplifier and why do I need it?
The term bias refers to the amount of voltage held on the grids of the output power tubes. Bias is associated with the “Operating Class” the power amplifier is working in, by design. There are three classes used in hi-fi: Class A, Class A/B1 and A/B2. Class A means that the output tubes are biased so that both tubes are always conducting. In class A/B, the bias is set so that on a signal peak, one of the tubes is off for some part of a signal wave form cycle. In class A/B1, no grid current flows into the grid of the tube, and in class A/B2 some grid current is driven into the grid of the tubes. There are also class B designs where both tubes never conduct current at the same time, only alternately. We at Cary Audio do not make Class B designs. They are frequently used in radio transmission designs, such as amplifiers used for amateur or ‘ham’ radio.
The operating Class of the amplifier is determined by how much bias current is present. If there is a lot of bias voltage, only the tube which is driven by the positive going half wave of the signal at any moment is conducting. Class B sounds distorted because the point where the signal ‘crosses over’ from positive to negative and begins to drive the other tube is not reproduced cleanly, and this creates crossover distortion in the circuit. As the bias voltage is made less negative, crossover distortion diminishes swiftly, and you are in class A/B2; a little less negative, and they both conduct more, and you have class A/B1. If you go further, you get to the point where both tubes always conduct, making the amplifier work in class A, which has the least crossover distortion and best sound of any of these operating conditions. (Cary Audio Class A single ended output tube designs have ’0′ crossover distortion due to circuit design and their operating parameters).
All power tubes have slightly different DC gains, so the same bias voltage on two different tubes produces two different current levels. ‘Matched pairs’ are two tubes selected to be close together for DC gain. The idea of matching output tubes usually refers to ‘push-pull’ stereo or mono circuit designs.
When should I bias my vacuum tube amplifier?
You should re-bias the amplifier whenever you change the output power tubes. Each output power tube needs a certain bias current to keep it operating at the point where the amount and type of distortion under normal conditions is well controlled. Individual tubes vary widely in the grid bias that sets the correct idle bias current. If you change tubes, you need to make sure the new tubes are operating in a way that sounds good and does not over bias them. Always rebias new tubes!
Warning! Vacuum tube circuit designs use high, potentially lethal, voltages inside their chassis and they can kill you if you are careless! Don’t remove the bottom plate to expose the circuitry inside your Cary Audio vacuum tube components!
How do you correctly bias a Cary Audio vacuum tube amplifier?
- Disconnect the speaker cables and the input RCA or XLR cables from the input jacks.
- Connect the provided Bias Cord into the bias connection jack. (If you have the Cary Audio MA-300 bias meter then skip to step 4.) It has a 1/4 inch mono ‘phone plug’ on one end and a pair of alligator clips on the other end. Plug the phone plug into the jack on the chassis. The red alligator clip is for positive electrical connections, the black one is for negative electrical connections on the milliampere (mA) meter.
- Set your meter to the milliamperes (mA) reading range. NOT to the MilliVolts (mV) range!
- Turn the bias adjustment screw on the chassis all the way counterclockwise so that the meter will read zero (or close to it) when you turn the power on. Some amplifiers will not go to complete zero.
- Turn on the amplifier power and wait 1 minute for the voltages to come up and warm up the tubes.
- We are not going to adjust the bias to its full recommended setting yet. Set it to about 10ma lower than needed as the bias will climb as the amplifier warms up and we don’t want it to go over the recommended setting. This also helps to put a gradual load on new tubes and allows them to warm up. Now SLOWLY turn the bias adjustment screw clockwise while you watch the meter for the correct mA reading. Now unplug the bias meter plug from the bias jack and move the meter’s plug to the other channels bias jack if this is a stereo amplifier. You will need to move the meter back and forth adjusting the bias a little at a time until set correctly. Using two meters is very helpful here.
- Let the amplifier run without an input signal for another 5-10 minutes and check it again. Readjust now to the recommended bias setting. You can find the recommended bias setting in your product’s owner’s manual. Adjust both channels until they each have the same reading.
- After another 5 to 10 minutes check and readjust the bias setting again, if necessary. New output tubes should be checked a couple of times in the first two weeks as they ‘burn in’ to their normal operating mode as they may keep changing over time. After that, check them within a month and then every 2-3 months just to be sure all is well.
Turn off the amplifier, reconnect the speaker cables and the RCA or XLR input cable, turn on the amplifier and enjoy the music!
NOTE: Some other tube amplifier designs will oscillate or go into overload if they are used without a speaker or resistive load attached to the speaker output terminals. Cary Audio tube amplifiers are inherently STABLE DESIGNS and may be operated without a load on the output terminals for adjusting bias or while in burn in.
Trouble shooting a vacuum tube amplifier while adjusting the bias.
Plug the bias cable into the bias connection jack on the chassis and connect the mA meter. Start by setting the bias potentiometer screw all the way counter-clockwise on your Cary Audio vacuum tube amplifier. This setting should yield a zero mA meter reading if the all of the output tubes are functioning properly.
If your amplifier should go to complete zero and you find that a zero mA reading on the meter is not possible when you turn the amplifier on it is possible that one of the output tubes is trying to ‘short out’ or has already failed.
With any version V-12 power amplifier (and a few others) it is easy to check the ‘good health LED’ next to each output tube to see if it is lit when the amplifier is set to the zero, fully counter clockwise setting, of the bias adjustment potentiometer screw. When set for zero bias, if any single LED is lit the output tube next to it is shorted out or conducting when it should not and it should be replaced. Put on a glove to protect your hand from high heat and pull out the suspect output tube. Plug in a new replacement tube of the same type. With the power on, as the tube warms up the reading on the mA meter should remain at zero if you have found all malfunctioning tubes. The spare set of output tubes shipped with a V-12 can be used to substitute for any suspicious tube.
After replacement of a suspect tube, check again for a zero reading on the mA meter. If you still have a mA reading of more than zero when you should see a zero reading, check the other output tubes one at a time. Sometimes a gentle thump with a pencil eraser on an output tube will cause it to suddenly draw current if it is about to fail (intermittent short). Never do this when you are playing music and have your speakers attached to the amplifier. When all the output tubes are functioning properly in the circuit and the mA reading is showing zero, you are ready to set the bias in the amplifier.
With any version Rocket 88 amplifier the LED will light if the bias fuse is OK and will not give an individual tube reading. If you cannot get a zero reading when you should, replace each output tube, one at a time, until you are able to get a zero reading. A spare tube may be substituted singly for each output tube to check for a potentially defective tube.
Other Cary Audio designs do not have an LED next to an output tube to show bias levels. The same overall trouble shooting method will work for all of them though. (As well as most brands of tube amplifiers.) Replace each output tube, one at a time, until you find the defective tube. When you have a zero reading with a mA meter and the bias potentiometer is set full counter clockwise you are ready to begin to set bias in the amplifier.
Use the 0-500 mA scale on your meter. SLOWLY turn the bias setting screw while watching the meter to reach the mA level specified for your amplifier. When it is correctly set, turn off the power, disconnect the bias cable, and reconnect the speaker cables and the input cables. Turn on the amplifier and play some music to confirm proper operation.
Congratulations! You have successfully biased your Cary Audio vacuum tube power amplifier.
What speakers will be compatible with my low powered amplifier?
This is the primary question for audiophiles that buy or want to buy low powered single-ended, or push-pull amplifiers. We will start off by giving you the rules that we use to determine if a speaker is compatible with a low powered amplifier.
Rules of Thumb:
- The impedance of the speaker is probably the most important specification to look for. You want to find out the nominal, or average, and the minimum impedance. If the speaker is rated at 8 ohms it should not drop to lower than 6 ohms. If the speaker goes higher than 8 ohms that is acceptable. 4 ohms is all right, but you must make sure that the amplifier is wired for 4 ohms. Our amplifiers are wired at standard 8 ohms unless specifically ordered to be 4 ohms. Each amplifier has 4 and 8 ohm capability, and some even have 16 ohm taps.
- The SPL rating is important but it does not have to be 95db+ as commonly thought. We have used low powered amplifiers on speakers with SPL ratings as low as 86db. The reason an amplifier will drive speakers with relatively low SPL ratings are that the impedance of the speaker is close to the nominal impedance over the whole frequency range. If, at the lower frequency ranges the speaker drops to 4 ohms from 8 ohms, the amplifier would drive the speaker but not to its full potential.
Room size and speaker size play very important roles in your selections. In a larger room it is important to remember that you have to move more air to fill the room. So, a larger speaker may be more desirable. A large speaker in a small room will overpower the room, and the bass will be boomy. Large speakers and a 5 to 10 watt amplifier?….yes! Today more and more manufacturers are making larger and more efficient speakers for use with single-ended amplifiers. This has already greatly increased the speaker selections for low powered amplifiers.
Though we are not here to recommend speakers as they are ever changing and we have no way to test these out we have listed below some speaker manufacturers who have speakers in their respective product lines that will work well with low powered amplifiers. We believe that taste, system components, your room size and acoustics all have to do with the speakers you will like and choose. One size does not fit all! Work with a good dealer, friends and the internet for good speaker information. You may have to do a little homework, but to get a midrange to die for, highs that are open and extended, and a very natural bass, it may be a learning experience and an assignment that you will thoroughly enjoy.
When is it best to turn my equipment off?
This brings us to an ongoing debate. Which is better-leaving the product on 24 hours a day or turning it on and off? Both will shorten the life of your tubes. So what should you do? The answer lies somewhere between the two. If you listen faithfully for several hours a day then leave the unit on. You do not want to turn it on and off several times a day. This is worse than leaving it on 24 hours a day. If you listen two or three times a week or just on weekends, turn the unit off when not in use. In this case, allow one hour for warm up time. For the weekend listener, turn the unit on Friday and turn it off Sunday night. This will optimize tube life for your amplifier.
Preamplifiers and CD players should stay on all the time. The tube replacement cost for these units is considerably less than amplifiers. Most of our amplifiers have a Standby feature. The Standby is there to pre-warm the tubes before operating. Tubes generally will last longer if they have a few minutes of warm up time before turning on the high voltage.
Most tubes will last for many years. Some will fail after a short period of time. As more tubes are being manufactured, the quality is excellent and the life is longer.
How will I know when to replace my tubes?
There are several different reasons why you would install new tubes. Some are more obvious than others. As tubes are used over several years, they will get weaker as time goes on. This is detected sonically in a system that has a dull or rolled off sound. Start by replacing the input or preamplifier tubes first, then move on to the power output tubes. The small tubes are less expensive to replace and after several years of service it’s time to replace them anyway. Some tubes may get noisy. This will give you static or popping sound that comes from the speaker. If it is in one channel then swap the tubes from the left to right channel. If the noise changes channels then replace the noisy tube. Most noises of this type are tube related. Start with the input tube and work your way to the output tubes. Remember, swap one section at a time and be aware of which channel the noise is actually coming from. Stop your source component from playing and begin changing tubes. If the unit has been on for several hours the tubes will be very hot. You will want to use a towel to pull the tubes out. Turn the unit off, make the change, and then turn it back on.
Power output tubes can short out. Our amplifiers have a protection fuse in the circuit for this purpose. If this fuse blows, you know you have a problem with the power output tubes. It always helps to have a spare power tube available for this type of troubleshooting. This could be expensive for some amplifiers. In a mono block push-pull amplifier, swap the output tubes one at a time with the other mono-block until the problem occurs in the opposite channel. If you have a stereo unit, the tube fuse may be shared by both channels. In this case, you will need to substitute a new tube in the circuit to find the problem. This is easy for single-ended amplifiers, they have only one output tube. If the unit is tube rectified and is blowing the A.C. fuse, it is most likely a problem with one of the rectifier tubes.
If the bias is constantly getting lower and lower, the output tube is getting weak. As tubes age, they gradually reduce emissions and eventually stop conducting. With adjustable bias units you can continue using your tubes by resetting the bias to its original value.
When replacing tubes, the question is should you replace them in matched pairs? Power tubes in a push-pull circuit should be replaced in matched pairs because they work together to provide the final output. For single-ended amplifiers it depends on how many years the tubes have been in service. Generally, we would replace one tube at a time except for the CAD-300SEI (stereo amplifier). Other tubes like input, driver and preamplifier tubes should always be replaced in matched pairs.
Are matched tubes used in Cary Audio components?
We are using carefully matched input tubes in our preamplifiers and power amplifiers in production. We match tubes by the internal physical construction of the tubes and by electrical specifications for gain or impedance, etc. Some of them will have different labels on them from time to time but this is not a sign that we have made an error in production. Read on about our selection and tube matching during manufacture, please…
Many New Old Stock (NOS) tubes are available that have a myriad of brand names on them. For US made vacuum tubes, the majority were actually made by either GE, Sylvania, Tung Sol or RCA. There were other US based makers (like Philips) but these brands were the largest makers in the types we typically use for input or driver section tubes.
Our output tubes are new production tubes and are installed in matched sets. Because they are new production we are able to have them matched for us by our suppliers. As a result they will be all one brand. With regard to the many NOS tubes utilized by us, we are buying them in large quantities from various suppliers and doing the matching here at Cary Audio. We are very familiar with the characteristics that are most important for tube life and optimum performance. Rest assured that we have 100% Quality Control (QC) on your new Cary Audio component. In addition, we have done 100% QC on the included tube set with your new Cary Audio component.
During production we assemble a new component, test it in the QC department with the tubes to be shipped with it, ‘burn it in’ on our burn in rack, test it again in the QC department and then pack it for shipment. All Cary Audio components have been burned in from 24 to as much as 72 hours prior to shipment and have been tested twice for performance against our rigorous test standards for each model. We do this so that you will have a great sound experience with our audio components.