Tube bias -- long cranky post

Paul G.

Friend of Leo's
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Mar 17, 2003
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Rhode Island
I see so many posts that make it clear that the concept of tube bias, and correct terminology is so confused I'm going to try and explain as simply as possible.

#1 -- In a vacuum, a conductor when heated, it does not hold onto its electrons. The electrons just kinda float in and around the conductor all loose and unhindered, sort of a cloud. If you add another conductor into the same vacuum envelope, but put a positive charge on it, the loose electrons will flow to the positive terminal as if they were mechanically connected. The donor conductor is called the cathode. The positively charged conductor is the anode. In tube terminology the anode is also called a plate.

#2 -- If the heated conductor is connected to an infinite source of electrons (ground) the current will flow like a dead short, cathode to anode. In tubes used in amplification there is (at least) one more element called the control grid. The idea is this grid is placed between the cathode and the plate. When AC is applied to the control grid (your guitar signal), when the grid is more positive, it will accelerate the electron flow, when it is more negative, it will inhibit the flow. (Like charges repel, opposite charges attract). In this way, a very small AC voltage will make wide swings in the current flow. We have amplification!! But there's a catch!!

#3 -- Remember the current is flowing as if there is a dead short all the time, so minimal voltage swing, and only while the signal is below ground. What is necessary is that the grid always be negative relative to the cathode. Because the important part is the relative part, conversely you can make the cathode always positive to the grid. This will inhibit the flow so you're not overheating/overloading the components. This is what bias is. You can superimpose AC on top of DC voltage, so the signal still can exist. Think of it this way: if you jump up and down on the first floor of your house, your movement is 10" at ground level. If you jump on the second floor your movement is 10" but your 8' above ground (+DC). You can jump in the basement, now you're still moving 10", but below ground (-DC).

There are many ways to bias tubes, but the two most common used in amplifiers are:

#4 -- Cathode bias. Used in all preamp circuits and most low-powered amps. It uses the least components (so it's cheap, both in parts and labor). Here's how it works: instead of connecting the cathode directly to ground, you connect it to ground through a resistor. As the plate sucks electrons out of the cathode, the resistor limits the current that can flow. This imparts a positive charge to the cathode because electrons are leaving faster than they can be replaced. If the tube tries to pull more current, the resistor will lower the voltage (Ohms Law), which will limit the current. The voltage and current will interact until an equilibrium is reached. That's why cathode bias is (more or less) self regulating. This bias is not "fixed" as it can change with current demand.

#5 -- Cathode bias is not terrifically efficient, so higher powered amps tend to use fixed bias on the power section.

#6 -- You can apply a fixed negative DC voltage to the grid. On some amplifiers the correct bias is maintained by buying tubes rated for similar current draw (i.e. Mesa Boogie), or adjusted by swapping resistors (i.e. Ampeg), or adjusted with a pot. In all cases this is fixed bias because you are applying a single "fixed" bias voltage to the grid. All fixed bias is adjustable, one way or another, but since the voltage sent to the grid doesn't change once set, it is fixed. This negative bias voltage can be a separate tap on the power transformer, rectified and smoothed to negative DC. Alternately, it can be tapped off of one leg of the high voltage output of the transformer, dropped through a resistor, rectified and smoothed to negative DC.

Of course, fly in the ointment, some amps use a combination of both(!!) cathode and grid bias. Yikes!!

Cathode bias (despite marketspeak) is not Class A. With the exception of single-ended amplifiers (one output tube), almost no guitar amplifiers operate in Class A.

There's tons more to amplification, and I omitted and simplified an awful lot even on this limited subject, but that's as simple as I can make the idea of bias.
 

Jon Snell

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Aug 31, 2015
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Jurassic Coast, Dorset. Great Britian.
Definition of a Class A amplifier, both valve and solid state;
For an amplifier to be classified as “Class A” the zero signal idle current in the output stage must be equal to or greater than the maximum load current (usually a loudspeaker) required to produce the largest output signal.
That means roughly 70% of current must be flowing even when no signal is present. Very inneficient and widely used by Fender in their smaller single ended amplifiers.

Many years ago, in my foolish youth, I did put together an FM amplifier (Class D ref Clive Sinclairs' idea from his X10 solid state amplifer) using a quad of 807 valves and push pull transformers but eventhough the valves and the transformers exceeded the required frequency response to 'switch' the output at 100kHZ, Class C was the best they could acheive. So I gave up relying on my thught that if it were a real possibility, someone else, with better skill levels than me, would have perfected it back in 1965.

It is useful to trim the bias using fixed and cathode bias especially in certain controlled output stages. Slightly less harmonic distortion as the signal hits the knee point of the valves maximum mU point but that is not realy suitable for audio. IF stages in older radios used it as an AGC tuning aid.

I am sure there are many hours pondered over regarding designs but I have found the best way to aproach these things is to keep it as simple as possible.
That makes for easier fixes if a failure occurs down the line and in the fulness of time.

Interesting topic Paul G.
 




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