By Steve Weiss

Tubes - The Mystery Unraveled (Part Two)

This month we continue our discussion on tubes. I invite all who missed last months article to click on the Metro Music Mayhem archive link to read it as it will get you up to speed on what we will cover this month.You can also access any other of my past articles if you keep scrolling down the page. I promised this month to talk about those magic buzzwords, Class A and Bias, so here we go.....

First a reminder of how a speaker operates will aid in understanding what a Class A tube amplifier output stage is actually doing. Every speaker has a coil of wire (voice coil) that is suspended inside a circular gap in a magnet. Putting a signal from an amplifier into this coil causes it to move around inside the magnet gap due to the signal inducing a magnetic charge on the coil causing it to attract or repel the magnet. The voice coil is attached to a paper cone and moves in step with it pushing and pulling air and creating sound. When there is no signal present the voice coil is adjusted to sit at a point allowing it the freedom to move equally in either direction. If the voice coil couldn't do this it would create a bad sounding distortion since an audio waveform contains complementary parts that push or pull the speaker cone equally in either direction from its resting point. If the speaker cone can't follow the waveform exactly distortion is created since what is being heard is different than the signal going into the speaker.

Moving on let's talk about an amplifiers power output stage. This is where we come up against the first of the magic buzzwords "Class A". I hear many guitar players speak of Class A with much reverence. Problem is, I never meet any who can explain to me what it means. An amplifier class refers to how the signal sent to the speaker is created. Class A single ended amplifiers contains one output tube that creates the power going to the speaker all by itself. To do this the tube must be amplifying 100% of the time.This is an inefficient process that creates a lot of waste heat and doesn't allow a lot of audio power to be generated. You won't see more than 15-20 watts at the most coming from a Class A power stage.So the people telling you that Vox AC30's and old Plexi Marshalls are class A are very wrong.

So if Class A is so inefficient why do we even use it? The answer is Tone. Each amplifier Class imparts a different type of harmonics on the guitar. Class A accentuates warm sounding even ordered harmonics. Additionally the lower powered Class A power stages can be pushed into distortion without creating ear-splitting volume so the guitar player can more easily get output stage distortion(please refer to my earlier articles in this series for an explanation of this).

In order to create a more efficient way of producing power we can utilize Class AB. In this class we use two or more tubes in what is called a Push-Pull circuit. Here, for every cycle of audio one tube turns on and moves the speaker forward from its center point and back to center and then the other tube turns on and causes the speaker to move backwards and then back to center. If more than two output tubes are being used they are arranged in even multiples on each side of the Push-Pull. This is a much more efficient way to create power and allows us to easily hit 50 watts for two tubes or 100 watts for four as each tube is not required to handle the entire signal cycle.

If you desire a loud and clean sound especially when playing with other musicians Class AB will play louder than Class A. Class AB when pushed into distortion is certainly louder than Class A and for a large stage where a louder sound is required it is usually the way to go.When it comes to tone Class AB contains a different set of harmonics than what is created in Class A. Class AB amps can also be adjusted as to where during the signal cycle one tube turns on and the other turns off. This can really effect the distortion sound of the circuit. How this is adjusted is where our other buzzword comes in- Biasing.

BIAS-What does this magical adjustment really do? To begin with it is a measurement of how much power a tube is dissipating when it is at a no signal condition called idle. Let's look first at Class A. We said before that to make sound the amplifier must both push and pull the speaker to create sound. Therefore, we need opposite electronic signals input into the speaker to both push and pull it. Audio is alternating current (A/C) and changes polarity with each cycle of sound. Obviously this is perfect to drive a speaker. But the voltage inside a tube guitar amp is positive Direct Current. So how do we take this one positive voltage and convert it to varying A/C? I suggest you look back at last month's column for the explanation of how a tube works because we are going to build on that concept now. Remember that a signal on the control grid of a tube will control the flow of electrons from the cathode to the plate. Make the grid more positive and more electrons get though, more negative and less electrons get through. Suppose instead of a signal we cause the tube grid to stay at a specific DC voltage.

It will cause the tube to have a specific non varying current flow through it. As we said before current times the voltage in the tube equals power, so this static condition of the tube causes a specific amount of power to be dissipated by the tube. This static voltage is called the bias voltage. Now if we input an A/C audio signal it will add and subtract to this dc bias and cause the tube to conduct more or less current in step with the input signal. This is input to the output transformer and since the transformer blocks DC from appearing on its output, all that appears at the output going to the speaker is an A/C audio signal.

So it should be apparent that if we bias the idle of the tube so that it sets the tube's current flow at idle somewhere about halfway between its maximum and minimum possible current flow we will be able to amplify both sides of the waveform equally without running out of headroom on either side. If we slide the bias away from this optimum point either up or down it will cause one side of the signal to distort before the other. This asymmetry when the power stage is pushed into distortion sets up a different set of harmonics. You'll hear a different tonality between the negative side distorting first, the positive side distorting first or both sides distorting equally. This asymmetry is sometimes desirable. Harp players are examples of players known to favor asymmetric sounding distortion.

Moving on to the Class AB push-pull circuit we now are going to split the signal into two halves so in the case of a two tube output stage one tube pushes the speaker and one pulls it.In this setup the last stage of the pre amp is configured as a phase splitter. This tube outputs two identical signals that are the opposite of each other (out of phase). When they are input to the output tubes it causes one of the output tubes to conduct for the positive half of the waveform and the other tube for the negative. But it is not that cut and dry. There is not an instantaneous transfer of the signal where one tube turns on and the other tube turns off. There is some amount of each cycle of the waveform where both tubes are conducting. This is where the A comes from in Class AB. As I mentioned before the transition between tubes can be varied. Looked at another way it is possible to vary the amount of output there is before each tube transitions from class A to AB. This can be easily exemplified by pulling one output tube out of a two tube push pull amp. If you play loud, it will sound distorted because the speaker is moving much more in one direction than the other. But if you bring the volume down you'll hear the full undistorted waveform. This is because up to a certain point of amplification each of the tubes does amplify the entire waveform. Where this transition is set can make an amplifier sound different as the distortion is different sounding based upon where the transition from Class A to AB is set on the amp's power scale. This is one reason that people favor 50 watt Marshalls over 100 watt. The distortion point is closer to the class A transition point at lower volumes.

We can liken biasing a Class AB stage to the idle on your car. When it is set too low the engine could stall. If it idles too high the engine wastes a lot of gas. With tube biasing we find an optimum point below which your sound will be very brittle (stall) and above which will cause your tubes to overheat(waste) and prematurely age.

So now let's talk about how much you will actually hear the effect of bias in your sound. There is no one answer to this. It is dependent on the combination of many variables. These include amplifier power, volume amp is played at, genre of music, amount of pre amp distortion used etc. I can give some general examples so you can get an idea of where your needs fall in regards to bias. If you are a player using a heavy amount of pre amp distortion and are also not pushing the amp's power stage to its limits(master volume low), you can turn the bias control back and forth all day long and probably never hear a difference. This is for two reasons. Firstly the copious amount of distortion coming from the pre amp masks any difference in power amp sound change by bias. Secondly, if you are not pushing the amp into distortion any harmonic distortion influence by bias settings is nonexistent. In this type of sound it is better to set the bias a little cold and lengthen the life of the tubes by running them cooler. Just not so cold that when you play clean the amp sounds brittle. Biasing the stage really hot will do nothing for you except bring me more business in tube sales. It's the blues and classic rock players that may benefit more from bias. When your tone is built around distorting your output tubes as we previously mentioned, bias can affect the type of distortion you achieve, although I believe that in most cases and with most amps slight variations in bias do not have as much tonal influence as people seem to believe.

Another term we see affiliated with bias is matching.When output tubes are matched it means the tube vendor has tested many tubes and grouped them into sets whose performance characteristics are similar to each other. One benefit of this is that at least the tubes are being tested before you buy them so the really bad ones can be rejected. Of course what performance levels of working tubes should be accepted comes into question. Groove Tubes position was to take all the tubes and rate them on a scale of one to ten. This allowed them to have a lower rejection rate, although It is the opinion of many techs that anything outside of their 4 or 5 rating should of been rejected and that the claims of different ratings for different sounds were just so much snake oil. How important is matching? Again as we said above, it depends on the type of playing style the tubes are used in. Remember also that tubes are not entities unto themselves. The circuit they operate in contains many components, so if the component tolerances have drifted between the two halves of a push-pull circuit your matched tubes can become unmatched very fast. Some amps contain a bias balance control in addition to its bias adjustment control. This will let you slightly vary the bias to each half of the Push-Pull circuit setting an optimum match point for the tubes installed and their related circuitry. When the output stage is matched it helps with bass response, makes the distortion on power chords more lush and improves the picking dynamics for lead playing (the sound will feel spongier and different picking intensities will vary your distortion sound.

Next month we will pick up from here and make some points about different bias schemes (fixed, cathode etc) and talk about the amp's power supply. See you then.

 

Steve Weiss is the owner and main technician of Steve Weiss Electronics Inc. He is experienced in the repair of analog and digital musical equipment. This includes everything from Vintage Tube Amps and Pro Audio equipment to Digital Keyboards There is also a guitar repair shop staffed by some of the areas top guitar repair techs. He is authorized for warranty work on most major brands. Steve Weiss Electronics is located inside of Sam Ash Music at 5460 West Sample Road Margate, FL 33073 954-975-3390 Ext 272. Steve has also spent 25 years on the road as a performing guitarist and is the designer of Primal Guitar amps that can be seen at Primal Audio.com Steve can also be reached at stevew@metromusicmayhem.com
Sam Ash Quikship Corp.

 

 
blog comments powered by Disqus
 
 
 
Bookmark and Share
© Copyright since 2011 - Legal Notices