Biasing tube amplifier calculator
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The push-pull design for output stages has persisted since times of antiquity. It was one of the very earliest circuit designs, and has persisted until the present day, with modern solid-state linear amplifiers still overwhelmingly using it.
With tubes, a typical topology is given by the circuit below. Click to magnify. The anode plate voltage on either side comes through the primary of the output transformer. This design uses a pentode tube, which has a screen grid.
This is attached to taps on the output transformer to run in Ultra-Linear mode, increasing efficiency and reducing distortion. In this design, the cathodes are tied to ground through a very low value shunt resistor. The resistor is simply there to provide a small voltage drop from which the current through the tube can be measured.
It plays no other role in the circuit, other than being a fuse if the tube red-plates. Because the cathode is at or very close to ground potential, this requires the control grid to have a negative DC bias voltage applied to it, to regulate the flow of current through the tube.
If there was no negative bias applied to the tube, it would go into full conduction, the plate would glow red hot, fireworks will happen and that would be Bad, mmkay? So we need to contrive to feed a constant negative voltage into the grid, along with the signal, to achieve the desired regulation. In this case, our adjustment range is from How to set up the bias adjustment resistor values The bias voltage needs to be adjustable. Both tubes need to be drawing the same current, otherwise the net current through the transformer will not be zero, which will lead to magnetisation of the transformer core.
This is a most undesirable situation and left unchecked, it will cause quantum fluctuations in the space-time continuum. Well ok maybe not that bad, but the transformer will saturate unevenly and distort the sound. The usual approach is to use a voltage divider network with a potentiometer, as above. Couple of points about this design. The next question is — what value resistors will be needed? This is where some trial and error in the calculations is needed.
From there, you can calculate the values for the resistors and potentiometer to give you the range you need. This is a screenshot from a spreadsheet I made that can help with this calculation.
You can download this Excel worksheet here. Put in the numbers in the red. In the case of an EL84, that value is K. However that power rating is across the entire length of the track. Adjust the potentiometer for maximum negative voltage wiper closest to the left, in this schematic. Power on the circuit and let it stabilize. Measure the voltage across the cathode resistor. Then adjust the potentiometer until the desired current is flowing through the tube. What is the desired current through the tube?
Glad you asked. It means the current through the tube will be low, and the sound may take on a thin, glassy, brittle aesthetic, and the distortion will increase. For an EL84 we see the maximum plate dissipation is 12 watts.
So we want to see 0. Very halpfull, thanks for your job. But why 10 Ohm resistor on 0,24V is as big as 1W power? The resistor plays an important role as a fuse in the event of a bias failure.
In that case, the tube would conduct as much power as the PSU could throw at it, red-plating in the process. In this case the 10R resistor could be expected to overheat and eventually fail. Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. Values of one ohm or ten ohms are typical of this arrangement. The more negative the voltage goes, the lower the current through the tube In this implementation, if the potentiometer fails, it will fail safe.
The most common mode of failure with potentiometers is the wiper lifting off the track. If this happens, effectively the voltage at the grid of the tube will go full negative, reducing the current through the tube to almost zero.
This is far more desirable than the voltage reaching zero and the tube immediately red-plating. This is where a spreadsheet can be mighty useful.
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Cathode bias
A: To bias an amp is to adjust the idle quiescent current draw through the power devices tubes or transistors. The most meaningful measurement is the amount the current flowing flow the tube from cathode to plate. This combined with the plate voltage allows you to calculate the power dissipating in the tube. Excessive power results in a broken tube, and a broken amplifier follows that.
Auto Biasing Tube Amp
Auto Biasing Tube Amp And thanks to Infinium's Auto-Bias design, you simply replace the indicated tube " without a trip to the repair shop, saving you even more money. This is the main reason that tube amplifiers sound better. The advanced engineering within VKi eliminates the traditional headache associated with most vacuum-tube power amplifiers—specifically, the constant need to set and readjust the proper bias of the output tubes. A self-biasing amplifier, also know as a cathode bias amplifier, has a cathode resistor that sets the bias current of the tube. Fit the new set of tubes. Auto-bias, no adjustments needed. Infinium Series amps take all the guesswork out of tube replacement by illuminating an LED next to a tube that is reaching the end of its usable lifespan.
Bias Calculator
All Rights Reserved All text and images on this website may not be used without express permission. All prices and product information subject to change without notice. Shopping Cart Shopping Cart. There are two different kinds of bias probes, one type that uses a resister in the circuit and measures in millivolts and the second type that does not use a resister and measures directly in milliamps.
Welcome To the Hoffman Amplifiers Forum
The bias voltage keeps the quiescent current in the power output valves at the safe design value. In most amplifiers the HT voltage is not regulated -it is free to rise and sag in accordance with load current and mains voltage fluctuations. To maintain roughly the same operating point and dissipation in the power valves, the bias voltage must therefore go proportionately more negative as the screen voltage goes more positive. In most valve amps the bias voltage can be adjusted over a small range to accommodate differences between valves and changes due to aging as they get older their gm falls and less negative voltage is needed for a given operating point. Sometimes the bias is user adjustable, but generally it is trimpot inside the amplifier which must be adjusted by someone with at least a rudimentary understanding of what they are doing. If the bias is reduced to zero volts then the valve will run red hot and perhaps be destroyed , while if it is made very negative then it will reach cut-off.
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By joining Download. To calculate the tube's Max Safe Bias setting you simply pick your tube from the pick list and enter the Plate-to-Cathode Voltage plate voltage is always measured between the tube's plate and cathode. To calculate bias by measuring the voltage drop across a cathode resistor enter your Tube Type and Plate-to-Cathode voltage at the top of the page, enter the Number of Tubes that share the cathode resistor, the Voltage Drop Across the Cathode Resistor and the Cathode Resistor actual resistance. To measure the voltage drop across a resistor set your multimeter to volts DC and touch each leg of the resistor with a probe disregard any minus sign. Do not attempt to set an amplifier's bias unless you are competent with test gear and very familiar with high voltage and the dangers involved. Free YouTube Downloader.
Tube Bias Calculator
Did you buy my book? If you are not familiar with loadlines, their purpose, and their use, I suggest you checking my guide: Section 3. The underlying model and equations, for tracing tube characteristics curves, are those discussed by Norman Koren, in his web site.
TUBE BIAS CALCULATOR app for iPhone and iPad
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The Amp Garage
I use this method to bias my amps. Below is an exerpt, read the whole thing here. In fact, go to Randy's site and read all the tech stuff there. You'll be a lot smarter when yer done. For example, the aforementioned EL34 tube has a plate dissipation of 25W, so at V class AB operation, it should be biased no higher than 0. At V class AB operation, it should be biased no higher than 0. Note that a class A amplifier does not necessarily have to be run at the maximum ratings.
The cathode resistor in a typical triode preamp is bypassed with a large capacitor to eliminate a form of negative feedback known as cathode degeneration. This substantially increases gain. When the capacitor is large enough, it acts as a short circuit for audio frequencies, eliminating the negative feedback, but as an open circuit for DC, thereby maintaining DC grid bias.
Wonderful topic
Wonderful, very useful information
In this something is. Thanks for the help on this question. All just great.