Cathode bias is also called "self" bias. This is possible because the grid is biased negatively compared to the cathode (the negative bias is what allows it to modulate electron flow). The first is cathode bias or self bias, just like with an ordinary gain stage. 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 . Screen voltage has significant effect to grid voltage. {Ha! This method measures the tubes' CATHODE current, NOT Plate current. The relatively high cathode voltages needed to cathode bias a tube creates a large enough . There are two easy ways to bias the valve. Accordingly C3 has a value of 650uF. CATHODE RESISTOR The "safest," but less "accurate" method is called the CATHODE RESISTOR method. I am planning to add computation of the cathode resistor in future releases. Cathode Bypass Capacitor. 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. 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 . This phenomenon is known as. cathode-current feedback and causes the apparent gain and non-linear distortion of the stage . 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 . In a grounded cathode amplifier, the current through the load . cathode-current feedback and causes the apparent gain and non-linear distortion of the stage . A cathode resistor in a typical triode preamp is bypassed within a large capacitor to eliminate a negative form of feedback is called the cathode degeneration, which significantly increases gain. The grid to cathode is assumed 35V, so cathode resistor is 35V/0.042A=833ohm. Believe it or not, this is called the Cathode-Resistor Method of biasing! Think of a cathode follower like a grounded cathode amplifier in that the grid modulates conduction through the tube and load resistor. TUBE BIAS CALCULATOR (for push pull class AB1/AB2) Select Tube Type and the Plate Voltage of the amplifier below. 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. I have several applications and I would like to know the proper method of calculating the resistance value. To allow a reasonable signal swing, it was decided to set the cathode potential at + 10 V; thus R3 would need to be about 1kΩ in value. This phenomenon is known as. Dummies" method uses a 1 ohm (Ω) resistor between the cathode and ground. The type of tube determines the power dissipation, which is required to calculate the idle current. Tube Bias Calculator. As you hit a note the tube calls for more power and negative voltage on the cathode decreases briefly changing the tube's bias. Origin 20 is cathode biased and no bias adjustement is needed. I was running that amp with a 560ohm cathode resistor on each tube and 100uf cathode bypass caps. Tube Dissipation Using Cathode Resistor Voltage Drop Please enter Tube Type and Plate-to-Cathode Voltage above before calculating Tube Dissipation. The push-pull design for output stages has persisted since times of antiquity. To understand this, we must first look at cathode bias. I did some computations on an EL34 with 450V B+ 5K load, with 750 ohm cathode bias resistor using a 560 ohm base resistor and 190 ohm resistor, the wattage across the 560 ohm resistor is 1.7W and the 190 ohm resistor is 0.56W totaling 2.24W across the 750 ohm cathode bias resistors (560+190). I plugged in the capacitor values I have in the circuit, and the Rs and Cs seemed to line up pretty well with the desired time constants -- except for the series resistor, which it showed should be 120k. 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. This is already available for the power stage loadline calculator. With multiple tubes sharing the same cathode resistor, this calculator assumes the tubes are fairly well matched. Accordingly C3 has a value of 650uF. Cathode bias resistor = Grid bias voltage/total cathode current. For replacement valves please visit https://www.ampvalves . 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. A close standard is 470 ohms and will do fine. This calculation is based on Langford Smith's formula from Radiatron Designers Handbook 4. We'll call this variable x. 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. In these calculations, 5% of the cathode current is assumed to be screen current. The transistors are types ranging from 2N2222 to TIP-31. In the case of the triode the anode current and the cathode current are one and the same thing; in the case of the pentode they are not. Here a 1-Ohm/1-3W resistor is connected between the cathode (pin 8) and ground on each output tube (ie: Bias-Probe, etc.). Ri The internal anode resistance. If you are speaking of a cathode resistor bypass capacitor in a common cathode circuit, then actually that cathode resistor and capacitor are in the signal path. This is the method that is best for hobby techs and do-it-yourselfers. When Headroom is inserted there appears Distortion estimates. Set Bias - Rob Robinette tip robrobinette.com. To calculate this resistor, use Ohm's law: R=-32V / 0.088A The value of the Cathode resistor depends on the type of output valves and the B+ supply . 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 . I know Fender Deluxe RI bias for 25mA and running at 400V. 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 voltage across the self bias resistor is the grid bias. ATRAD Audio. Having selected our operating point at 85V, we know anode, and hence, cathode current. Neither you nor a tech can adjust the bias on these amps. Number of Tubes that share a cathode resistor: Voltage Across Cathode Resistor: volts . I = P V. When biasing to 70% or whatever percent you're looking for you simply multiply the answer by that number divided by 100. This is R2 in the circuit below. This calculator uses the power equation. 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. This calculator uses the power equation. It doesn't have an HUGE impact on bias calculation, however I see your point, especially since the plate voltage is already on the high side. The value of cathode bias resistor calculated from this data is 60 Ω (0.6 V / 10 mA) and a standard value of 56 Ω was actually used. We'll call this variable x. Then turn off the amp and measure the actual resistance across the bias resistor. Yours is fixed (aka "adjustable" ;) ) bias. Cathode bias is mostly used in low cost valve amps that function from a 300V to 400V B+ supply. With just a single resistor in this position, the gain stage will amplify and the amp works just fine. And a LOT more power output than the 25-35W possible from EL34 in self-bias. Read more about this in the support info section about triode curves fitting and why the classic Child-Langmuir equation was not used here. So we need some comon sense and widely available knowledge to bias your amp. 1, the designer is usually faced with choosing a cathode bypass capacitor in order to maximise gain. In this 4th video in the Resistor Video Series, we discuss the purpose and function of the Cathode Bias Resistor. thereby maintaining the DC grid bias. This is what bias does, either by applying a negative voltage to the grid (fixed bias), or by putting a positive voltage on the cathode (cathode bias). 5. Solution: Pd = 450V * .050A = 22.5W. If bias for 25mA each R=400V/0.05A=700ohm. The red color is the cathode voltage. This substantially increases gain. Enter plate minus cathode in the box, and hit calculate. For the EL34 the maximum value given is 700k, and we would probably use 470k. 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. For cathode bias one tube resistor is simply:Grid Voltage divided by Current in Amperes. Theoretically, Class A would idle halfway to cutoff, or a bias of 1/2 * Screen Volts / Mu g1-g2. 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. In this specific case, total cathode current = 0.9 + 0.22 (mA), = 1.12 mA. Solving for I we get. My recommended bias measurement method is the "Output Transformer Resistance Method."You measure the output transformer's voltage drop and . 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. . 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. Cathode bypass capacitor calculator The cathode bypass capacitor in tube amp forms a filter that reduces the low frequency gain of the a common cathode amplifier. The DC blocking capacitor holds the charge constant as the signal level varies. PP Fixed Bias circuit design and calculator. Regarding a 25W tube, 22.5W is 90% of its maximum rated Pd. In this case the bias point indicates an anode current of about 8mA, and Vgk = -3.5V. Once charged, the capacitor only sees the 10k resistor connecting it to the cathodes, as the two diodes are no longer forward biased, so they effectively fall out of the circuit. For the pentode: Total cathode current = anode current + screen current. Solution: Let's say it's 475 for this example. The green color represents the input signal. First, select 6V6GT and measure you plate *and cathode* voltages. The value of the cathode resistor will vary depending upon the type of output tube used, and can range in value of 150 to 600 or so ohms. This is happening when there is no cathode bypass capacitor present. To calculate the value of a back bais resistor one needs to take the total current drawn by set into consideration. This gives cathode biased amp their awesome sustain. Cathode biased using 1k bypassed with a big capacitor + 470 ohms additional unbypassed resstor. How to Measure Bias. Making a cathode sit at some positive voltage is easy enough, as a cathode resistor or zener or constant-current source work well. With multiple tubes sharing the same cathode resistor, this calculator assumes the tubes are fairly well matched. 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. Control grid bias is always in respect of the cathode. 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. Posted March 6, 2018. A resistor is placed between the tube cathode and ground. Sep 17, 2010 #2. Push-pull Auto-Cathode-Bias. So if we're cathode-biased, we're looking for a "little ways into Class AB" if we venture beyond Class A at all. Calculate Plate Dissipation In Cathode Biased Output Stage Based On Voltage And Resistance Readings. 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. The red color is the cathode voltage. 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 . However, the presence of a cathode resistor creates a local negative feedback loop. The voltage drop across this resistor provides the bias point for the gain stage. THE CATHODE RESISTOR METHOD. (At this point the tube rp is about 62k and the gm is about 1600 umhos). If the anode resistor is 100k and the input to the next stage has a 1M grid bias resistor, then the AC load to the anode is actually 100k shunted by 1M, or 90.91k. A large 10 Watt 470R resistor or similar value Resistor is placed in series with each Cathode. 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 . Post. Output tubes are either biased with a negative grid voltage or with a positive cathode voltage. However, many amps (as examples: the Dynaco ST-70 and the Leak TL/25+) were designed to . When designing a simple cathode-biased triode, voltage-gain stage of the type shown in fig. The tube amplifies the difference between the red and the green traces. 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. 12AX7 circuit, biased at 2 mA, with a plate load resistor of 100k, next stage grid resistor 470k. The bias voltage is (or should be) the result of a very small current flow between the cathode and grid and through a high value resistor, and must be present whether there is a signal or not. The green color represents the input signal. I've just re-proved the _three_ SELF-BIAS conditions on Steve's chart: 430V 6.6K 470r cathode resistor, and 355V (315V is typo) 3.4K 130r cathode resistor.) Cathode bias means non-adjustable and it varies up and down automatically with conditions (so it's 'not fixed'). 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. Measure the Bias by Cathode Resistor Voltage Drop. The tube amplifies the difference between the red and the green traces. In parallel with 360 Ohm cathode bias resistor R3 this gives us a total resistance Rk of 245 Ohm. 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. For cathode bias actual operative B+1 is a sum of plate and grid voltage. How to Calculate Bypass Capacitor Value. 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. Normally, bias adjustment is required only for output tubes (that is, the tubes in the output or power stage of the amplifier); preamp tubes are usually biased automatically at a fixed point using cathode resistors. In parallel with 360 Ohm cathode bias resistor R3 this gives us a total resistance Rk of 245 Ohm. If bias for 25mA each R=400V/0.05A=700ohm. Note that the 'anode' is sometimes called the 'plate'; British vs . In these calculations, 5% of the cathode current is assumed to be screen current. BG159 - A question based upon my SE 807 build but the explanation should suffice for any cathode biased amplifier. If the cathode resistor is left unbypassed, negative current feedback will limit the available gain of the stage to some minimum value. Some smaller amps with 6V6 power tubes also have adjustable bias (Fender Deluxe Reverb, for example). Amps with the cathode resistor you mentioned are cathode biased and generally run at a higher idle dissipation. Design for a cathode follower. This loadline calculator is very fast tool to try different setups. dissipation. The 70% figure refers to fixed bias amps. However, from the anode characteristics graphs point of view there is no difference. So for example in a set that draws a total current of 80 ma, the resistor would need to be 12.5V (the bias voltage neede for the 6V6 valve) divided by 80ma(the current drawn by the set). The Grids are referenced to ground (chassis) by a 220k resistor. CALCULATE PLATE DISSIPATION (In Cathode Biased Output Stage)Based on Voltage and Resistance Readings. . December 17, 2018 Tech. 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. Well, before, with the 129R (measured) cathode resistor, the plate voltage (with the same wall voltage of 234Vac) was 349Vdc. 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. Recommended range for setting plate current and/or cathode current will be calculated. 2010-09-17T20:00. I = P V. When biasing to 70% or whatever percent you're looking for you simply multiply the answer by that number divided by 100. 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 . The grid-reference resistor (Rg1) can be found by consulting the data sheet for the maximum allowable value in cathode bias, which will usually be less for a power valve than a preamp valve. This webpage offers two ways to calculate tube bias and plate dissipation. It is far and away the safest of the two methods, and can be successfully done with medium- and even low-quality test equipment. 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. They don't need biasing when changing tubes as they automatically 'settle' to the correct bias current. Measure the Bias by Cathode Resistor Voltage Drop. 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. I know Fender Deluxe RI bias for 25mA and running at 400V. 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. The grid to cathode is assumed 35V, so cathode resistor is 35V/0.042A=833ohm. The bias resistor therefore needs to be: Rb = Vgk/Ia = 3.5V/0.008 = 437.5 ohms. This is not merely the anode load resistor value. So far it seems OK. -- I downloaded the Tube CAD Journal RIAA calculator. To understand why this works we'll need to know Ohm's Law and some very simple math. (4) To force the bias-point, the correct self-biasing cathode resistor is chosen, or better, several tubes are set up in a rig, and the resistor is selected by experiment to put the bias-point in the best compromise position between the acceptable range of tube samples. The Abdellah amp actually sounds good with the KT88s biased pretty cold. Yours is 50W - it is not a cathode biased amp, it is very seldom to see 50W amps being cathode biased. I can't remember the cathode voltage, but I think it was biased at around 80ma / tube at idle. In this example let's pretend you got 20V at the cathode. I hope a 1V bias is enough to keep it out of grid current. 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.
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