cathode bias resistor calculator

Or, as every Marshall shows, 440V 3.4K in FIXED-bias can be an OK amplifier. Not realizing the cathode resistor was shared for both output tubes, he presumed the tubes were biased very hot. Cathode bias means non-adjustable and it varies up and down automatically with conditions (so it's 'not fixed'). In these calculations, 5% of the cathode current is assumed to be screen current. I was running that amp with a 560ohm cathode resistor on each tube and 100uf cathode bypass caps. Design for a cathode follower. Adjustments must be made while According to Patrick Turner, a PP amp's idle cathode bias - for optimal balance of both tube longevity and power output - should be set at 66% of its maximum Pd. Screen voltage has significant effect to grid voltage. 1, the designer is usually faced with choosing a cathode bypass capacitor in order to maximise gain. It is far and away the safest of the two methods, and can be successfully done with medium- and even low-quality test equipment. With multiple tubes sharing the same cathode resistor, this calculator assumes the tubes are fairly well matched. Cathode bias is also called "self" bias. Accordingly C3 has a value of 650uF. Measure the Bias by Cathode Resistor Voltage Drop. The green color represents the input signal. I have several applications and I would like to know the proper method of calculating the resistance value. In cathode bias a small value resistor is placed between the cathode and the ground so it develops negative voltage on the cathode thus achieving bias. . So we need some comon sense and widely available knowledge to bias your amp. This system of obtaining bias voltage is know as self-biasing since the tube's own screen and plate currents cause the voltage drop, which is used to bias the tube. This is the one I have. To understand this, we must first look at cathode bias. And a LOT more power output than the 25-35W possible from EL34 in self-bias. If you measure the voltage across that 10 ohm resistor you can use Ohm's Law to calculate the plate current and plate dissipation based on the plate voltage . However, the presence of a cathode resistor creates a local negative feedback loop. Number of Tubes that share a cathode resistor: Voltage Across Cathode Resistor: volts . CATHODE RESISTOR The "safest," but less "accurate" method is called the CATHODE RESISTOR method. PP Fixed Bias circuit design and calculator. I had a moment to glance at the schematic and see that the 10 ohm resistor you are referring to is in series with the plate (pin 3), not the cathode which is grounded. The red color is the cathode voltage. This calculator uses the power equation. It seems most likely he took a voltage rating on the cathode resistor, plopped that into an "Ohm's law calculator" and determined the current draw that way. If this is the case, there will be no voltage drop across the cathode bias resistor, and the cathode will be at 0 V. The grid is tied to 0 V, so V gk must be 0 V. This will cause the valve to conduct heavily, but as it does so, the anode current (which in a triode is equal to the cathode current) flows through the cathode bias resistor, causing . To calculate capacitance of the cathode bypass capacitor C3 we will use the following formula where f is the filter's cut off frequency which we select to be equal to 1 Hz. Grid voltage, in real circuis can be obtained using fixed or cathode bias. December 17, 2018 Tech. 4) Refer to the photo above (click it for a larger version) and locate the cathode resistor associated with the bias test point that had a faulty reading. This may be a question that reveals my ignorance but I am trying to calculate the proper base bias resistor to bias a transistor to saturation. If bias for 25mA each R=400V/0.05A=700ohm. In this 4th video in the Resistor Video Series, we discuss the purpose and function of the Cathode Bias Resistor. Well, before, with the 129R (measured) cathode resistor, the plate voltage (with the same wall voltage of 234Vac) was 349Vdc. Step 2: Triode mathematical model This calculator uses the triode equation of Norman Koren to calculate the anode current as function of grid-cathode voltage and anode-cathode voltage. The grid of the tube is also tied to ground through a resistor, which for practical purposes looks like a direct connection to the bottom of the cathode bias resistor. For the pentode: Total cathode current = anode current + screen current. Sep 17, 2010 #2. In this case the bias point indicates an anode current of about 8mA, and Vgk = -3.5V. As you hit a note the tube calls for more power and negative voltage on the cathode decreases briefly changing the tube's bias. Think of a cathode follower like a grounded cathode amplifier in that the grid modulates conduction through the tube and load resistor. Note how the capacitor is at the same voltage as the cathodes, which makes sense as the 10k resistor and the rightmost diode will charge up the capacitor. Solving for I we get. Regarding a 25W tube, 22.5W is 90% of its maximum rated Pd. cathode-current feedback and causes the apparent gain and non-linear distortion of the stage . Post. 12AX7 circuit, biased at 2 mA, with a plate load resistor of 100k, next stage grid resistor 470k. DC voltage measured between plate and cathode. Cathode bias resistor ( Rb) = Bias voltage / Idle current = 1V/1.35mA = 740 ohm At idle point, the tube will drop about 163 V as seen from the graph. If your amp is cathode biased or has 1 ohm bias setting cathode resistors then you can calculate bias current by measuring voltage drop across the cathode resistor.To do this set your multimeter for DC Volt measurement, clip the black probe to a chassis ground (so you can use just one hand to do . The relatively high cathode voltages needed to cathode bias a tube creates a large enough . The green color represents the input signal. This substantially increases gain. The red color is the cathode voltage. This calculator uses the power equation. From calculation of a pair of 6V6 at 21mA each, total current is 42mA. It is performed by reading the cathode current through each power tube. thereby maintaining the DC grid bias. cathode-current feedback and causes the apparent gain and non-linear distortion of the stage . Where this varies (general) is where there is back bias and the cathode has no resistor, the voltage being developed by all of the cathode currents passing through the back bias resistor ct to chassis and the OP tube grid being on . Dummies" method uses a 1 ohm (Ω) resistor between the cathode and ground. This is happening when there is no cathode bypass capacitor present. With just a single resistor in this position, the gain stage will amplify and the amp works just fine. The transistors are types ranging from 2N2222 to TIP-31. The most widespread method to bias a cathode is to insert a cathode bias resistor between the cathode and ground, and connect the grid of the tube to the ground via a grid leak resistor. The anode load impedance. It has a database of 36 tube types and will calculate the Class A Fixed, Class AB Fixed Bias and Cathode Bias Maximum Safe Bias settings. It seems odd, but the effects of that resistor (for example, from a noise standpoint) are similar to the effect of adding the resistor in series with the input (except the additional . Accordingly C3 has a value of 650uF. The bias resistor therefore needs to be: Rb = Vgk/Ia = 3.5V/0.008 = 437.5 ohms. When Headroom is inserted there appears Distortion estimates. However, from the anode characteristics graphs point of view there is no difference. The Abdellah amp actually sounds good with the KT88s biased pretty cold. Now, with the 158R resistor is 355Vdc. I know Fender Deluxe RI bias for 25mA and running at 400V. The 70% figure refers to fixed bias amps. As you hit a note the tube calls for more power and negative voltage on the cathode decreases briefly changing the tube's bias. This is not merely the anode load resistor value. 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. From calculation of a pair of 6V6 at 21mA each, total current is 42mA. We'll call this variable x. The tube amplifies the difference between the red and the green traces. This is the method that is best for hobby techs and do-it-yourselfers. But things get squirrely at low plate current, and typically the tube gets biased off the mathematical half-way point, hence the 0.6 in the equation. If your amp is cathode biased or has 1 ohm bias setting cathode resistors then you can calculate bias current by measuring voltage drop across the cathode resistor.To do this set your multimeter for DC Volt measurement, clip the black probe to a chassis ground (so you can use just one hand to do the measurement) and put the red probe on the . It has a database of 36 tube types and will calculate the Class A Fixed, Class AB Fixed Bias and Cathode Bias Maximum Safe Bias settings. With multiple tubes sharing the same cathode resistor, this calculator assumes the tubes are fairly well matched. The cathode-biased Class AB amp can't really make big power output the way a fixed-biased Class AB amp can, because the large plate current change results in a bigger bias voltage, which turns all the output tubes off. (Use the photos also to find your screen resistors to measure for 1000 Ohms on those too.) The value of the Cathode resistor depends on the type of output valves and the B+ supply . To understand why this works we'll need to know Ohm's Law and some very simple math. dissipation. {Ha! In parallel with 360 Ohm cathode bias resistor R3 this gives us a total resistance Rk of 245 Ohm. Neither you nor a tech can adjust the bias on these amps. This webpage offers two ways to calculate tube bias and plate dissipation. A resistor is placed between the tube cathode and ground. In this example let's pretend you got 20V at the cathode. . The DC blocking capacitor holds the charge constant as the signal level varies. P = I × V. P is the power rating of the power tube, I is the bias current, and V is the plate voltage of the power tube. Calculate Plate Dissipation In Cathode Biased Output Stage Based On Voltage And Resistance Readings. This is already available for the power stage loadline calculator. This gives cathode biased amp their awesome sustain. This loadline calculator is very fast tool to try different setups. For the EL34 the maximum value given is 700k, and we would probably use 470k. A large 10 Watt 470R resistor or similar value Resistor is placed in series with each Cathode. After discussion of the theory of its oper. The grid to cathode is assumed 35V, so cathode resistor is 35V/0.042A=833ohm. The voltage drop across this resistor provides the bias point for the gain stage. The Tube Bias Calculator will calculate power tube bias values of Plate Current, Dissipation and % of Max Dissipation using either measured voltage drop across a cathode resistor or measured plate current. And cathode bias resistor calculator number of tubes that share a cathode resistor creates a negative. Journal RIAA calculator Guide < /a > 5 nor a tech can adjust the bias resistor a... > Does the cathode resistor was shared for both output tubes, he cathode bias resistor calculator the tubes are biased. > variable cathode bias or self bias resistor therefore needs to be screen.. 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