AFD100 Marshall 'Slash' Amp Tube Bias

The AFD100tube biasing feature is completely automatic? It's always a bug bear when changing tubes. Is this bias system REALLY that good? I investigate below.

Marshall AFD100 'Appetite' SLASH Amp Tube Bias and why it matters to you

One things for sure - biasing a tube amp is a pain. Sure you don't have to do this on all amps but on this one and many more you do. If you just put in some new tubes and hope? You will either sound crap or you wont be hoping for long. You will be more hopping - hopping mad that you blew the tubes or your Transformers or the board or the whole darn thing. Can you really afford that to happen? Of course not.

Like the Power Scaling issues discussed on these pages its a fact that marshall and others know of these issues that users have with tube biasing. If you don't know what tube biasing is then read the bias section at the bottom of this page from tubeampfaq index. I am not the author of that section they are. You could REALLY be bogged down with all the technical gibberish surrounding tube biasing so Marshall have set out and developed what they claim is automatic biasing of the amp when you change tubes (no more gibberish!).

Heres how:


Click for a larger image.

Here is a unique method of biasing your amp when you change tubes. Santiago got this basically spot on in this amp. All you need to do is change the tubes (almost ANY octal - 8 pin tube) and then hold the FX loop switch on while you turn on the power and after a few minutes the amp is biased.

Its not REALLY that simple.

Look at that adjuster on the back panel above. Depending on which tubes you put in the amp depends on the position of this adjuster. Marshall supply a table at the back of the manual which shows where to position this control BEFORE you go through the auto routines as just described. Its a little fiddly, but a massive and world leading feature that is sure to be included on other Marshall amps moving forward.


There are other benefits of this bias system too. You can MIX TUBES - for example you can have 2x6550 and 2xEL34 or even 6L6GC in there. Carry out the re-bias and that's it - you are ready to rock. As well as this there are four bias lights on the panel above which light up if a tube cannot be biased or has some other failure inside the tube. The system will 'cut off' the tube so you can continue playing to the end of the gig. This is not new and other manufacturers have achieved similar results in this area (check out Randall's Lynch Box).

This section from TUBEAMPFAQ on the net

When should I bias my amp and how do I do this?

 What is "bias"?

"Bias" in this context refers to the amount of voltage held on the grids of the output power tubes. This controls the amount of current the output tube(s) conduct exclusive of the signal current, or, looking at it another way, the amount of overlap where both tubes are conducting simultaneously.

I will talk about the output tube current since the terms "under biased" and "over biased" are confusing with tube amps. A technician who works with only tube amps will usually refer to the voltage which sets the operating current in the tubes. In these amps, the bias is a negative voltage, so "over biased" to such a technician would mean that the tubes are held in a condition of too little current, just backwards from the solid state terms most of us are familiar with. "Under biased" would mean that the tubes have too little negative voltage on their grids and are conducting too much current simultaneously.

The idle current in the output tube and the degree to which the output tubes overlap in conduction is what you're trying to adjust, not how many volts go on the grids; you just have to use the grid volts to change the current and conduction angle.

The whole topic of bias is tied up with the "Operating Class" the power amp is designed for. There are only three classes useful to us in tube amps, Classes A, AB1, and AB2. Class A means that the output tubes are biased so that both tubes are always conducting. Even on maximum signal peaks, the tube driven most "off" will still be conducting some current. In both class AB's, the bias is set so that on a signal peak, one of the tubes can be driven completely off for some part of a signal cycle. In class AB1, no grid current flows into the grid of the tube, and in class AB2 some grid current is driven into the grid of the tubes. There is a class B, where both tubes never conduct current at the same time, only alternately.

The point of all this is this: The Class of the amplifier is determined by how much bias current is present. If there is a lot of bias voltage, the grids are held 'way negative, then only the tube which is driven by the positive going half wave of the signal at any moment is conducting. This is class B. It sounds ugly because the point where the signal crosses over from positive to negative and begins to drive the other tube is not reproduced cleanly, and creates [surprise!] crossover distortion. You can look at the output signal with an oscilloscope and see crossover clearly as you make the bias voltage too negative for both tubes to conduct at the same time. As the bias voltage is made less negative and allows both tubes to conduct a little, the crossover notch diminishes swiftly, and you are in class AB2; a little less negative, and they both conduct more, and you have class AB1. If you go further, you get to the point where both tubes always conduct, making the amp work in class A, which has the least crossover distortion of any of these operating conditions.

Too little simultaneous conduction in the output devices puts them in the most nonlinear region of their transfer characteristic, so crossover distortion is high; but as you increase the amount of simultaneous conduction, the power used and dissipated by the outputs goes up, perhaps to a disastrous degree. You are trading standby current and power dissipation in the output devices off against distortion. If both outputs are biased almost totally off at idle, crossover distortion is very bad. As the simultaneous conduction is increased, crossover goes down rapidly, until it gets smaller than the residual THD of the amp itself, and becomes much less audible. There is a fairly broad sweet spot where the crossover distortion is comparable to the THD and the idle current and idle power dissipation are reasonably low. This is the region you're looking for.

Lots of bias, both tubes conduct all the time - and eat a lot of power, get hot, other Class A kinds of things. Little bias, both tubes overlap less, get less hot, put out more total power - and produce crossover distortion, which sounds especially unpleasant.

Power tubes individually 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. Groove Tubes grades tubes from 1 to 10 so that any two "3"'s for instance are close enough to sub for any other "3", so you don't need to re-bias if you keep buying the same number from them.

Note that you may not want matched pairs, depending you your taste.

When should I bias my amp?

You should re-bias the amp whenever you change power tubes or modify the power amp circuits.

Each 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 or tinker with the circuit, you need to make sure the tubes are set back into operation in a way that sounds good and does not cook the tubes.

Amps typically provide only one adjustment point for bias, assuming that you will have bought matched sets of power tubes.

It is possible to modify your amp to "match" unmatched tubes by setting the bias voltage and AC drive level of each tube individually.


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