Cathode Resistor Bias Method
Often cathode bias amps are run close to 100% dissipation rather than the max 70% zone that people like for fixed bias. Unfortunately, it doesn't work like that. 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. One gotcha is that increasing the cathode resistor will run the tubes colder at idle, in effect shifting the output's bias more towards class B. Establishing cathode bias. Triode common-cathode amplifier calculator This tool demonstrates a step by step calculation of the amplification factor of the basic triode voltage amplifier and plots the amplitude and phase vs. Many people actually refer to them as "self-biasing" for this reason. Adjustable "Fixed" Bias. With the key open, reduced plate current flows up through resistor R1 making the end connected to grid resistor Rg negative. The cathode bypass cap also gives the stage more gain. Obviously this has the same effect as a bias resistor with a perfect bypass capacitor, so carries tonal considerations. The YJC is for amplifiers with cathode biased power tubes. 6 Stability Factor 9. This way, current is limited to the tubes with a big resistor on the cathodes of the power tubes. From what I understand I need to figure out the plate current, then plug that into the handy weber bias calculator, then adjust the bias to the suggested value and see how things sound I read more about the cathode current method of determining the plate current What I haven't been able to figure out is a list that goes something like:. When this current flows through this small resistor, a voltage is developed. This method measures the tubes' CATHODE current, NOT Plate current. Cathode Bias with a Constant Current Source A few years ago, I received many e-mails asking for help on how tube-based, push-pull amplifiers that used a constant-current source at the output tube's connected cathodes worked. 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. To change the value of the cathode resistor, refer to the steps in the Mod 2: Swap Negative- Feedback Circuit Resistors to Adjust Headroom section. 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 only way to properly evaluate requires a spectrum analyzer which I don't have. For this example I will choose 14v bias for early breakup. Our preamp tube bias resistors have been measured & selected to properly bias your preamp tubes. The voltage V2 across RG2 provides the necessary bias. It follows that increasing the value of the cathode resistor causes the tube to bias cooler, lowering the gain of the tube and thus increasing clean headroom. This re-sistor is critical without it, my amplifi-er oscillated at 115MHz, with 100V RMS on the triode plate! The triodes are cathode biased with a 330Ω resistor. In conclusion, do not forget to properly bias the TL431 via an external resistor. The cathode bypass cap also gives the stage more gain. With the key open, reduced plate current flows up through resistor R1 making the end connected to grid resistor Rg negative. When biasing a tube, fundamentally what you're doing setting up the circuit so that the control grid of the tube is at a negative voltage with respect to the cathode. 4 shows the effects of the resistor. So I did the same test but with a bypassing capacitor (decoupling) of 27µF : The power output is the same as previously but the waveform show no obvious differences between CB and FB. Base Resistor Method. When a cathode resistor is used, it is called cathode bias. If a cathode resistor bypass capacitor is used on the input stage's bottom triode, then the two resistor voltage divider ratio must be changed from 50% to match the new AC noise divider ratio imposed by the input stage. This method involves grounding the cathode, and using a resistor in the power transformer centre tap to generate a negative voltage. 1 1 resistor fixed bias Rg1 0,3 (MΩ) Cathode to heater voltage Vkf 100 (V) *When the heater and positive voltages are obtained from a storage battery by means of vibrator, the max. 1-31 Figure 1-22. Looking at the schematic, I suspect it's the 47K next to the diode. Analog Electronics: Fixed-Bias Configuration Topics Covered: 1. This is the reason why you can just plug in new preamp tubes. Cathode Bypass Capacitor. The current through the resistor can be found by adding the plate and screen-grid currents which will flow at the value of plate voltage used. If the tube draws too much current, bias voltage increases and turns the tube more-off. Interpret the results: A result between about 0. Leaky/shorted cathode resistor bypass capacitor. Fixed bias: The DC grid potential is determined by connection to an appropriate impedance that will pass DC from an appropriate voltage source. Purpose: BJT should be in active (or MOS should in saturation) at all times. At 25°C, i. The output stage draws a high current compared to e. When current comes through this resistor, a voltage develops on the cathode. 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. —Cathode bias. Calculating Cathode Bias Resistor. i've done work on my other amps, but none of them are cathode biased. As current flows through the tube and cathode resistor, it creates a voltage drop across the resistor and therefore raises the cathode relative to the grid by that voltage. Since the control grid bias is a function of the plate current, several iterations may be necessary during the. ← what most amp manufacturers use in their amps. 3) Choose a desired cathode bias voltage and divide by the cathode current for the resistor value. The cathode tied resistor is much higher resistance and thus generated a greater voltage drop (and sets up the idle bias) and more heat. Current is flowing at all times. 2) Cathode bias allows the tubes to be biased hotter. Inverting Amplifier (DC Coupled): In this case, the voltage divider biasing method is the best to use, unless you happen to have a voltage follower bias already in your circuit, in which case you can merely. The grid circuit puts the grid at zero volts dc relative to negative side of the plate voltage supply,. To calculate the amplification factor, standard values for the parameters "S", "mu" and "Ri" are often used. Cathode Bias with a Constant Current Source A few years ago, I received many e-mails asking for help on how tube-based, push-pull amplifiers that used a constant-current source at the output tube's connected cathodes worked. For this example I will choose 14v bias for early breakup. The effects of the input bias currents will then be negligible. With him we walked through several methods of biasing to sort an amp out - and we settled on Cathode Resistor Method to bias that amp (as known matched tubes were measuring not matched with other methods). The bias controls the anode current as well as the screen current. The grid will be connected to 0v through a resistor so it has a negative voltage relative to the cathode. 13 Stability Factor for Potential Divider Bias 9. Again a tube manual can be used to determine the approximate value. Hence on low power the cathode resistor value is doubled - the effect I notice is that it has much less headroom and doesn't sound as nice on full power. Agreed, however my point - and reason for advising against that method in my initial post - was that there are other, more accurate methods out there (such as using bias probes and the 1 ohm cathode resistor method) that allow you to measure the bias current in real time without stopping to re-calculate at every increment. 75 watts] [remember to include the screen current when calculating the bias voltage] So the operation in AB with cathode bias and a pair of 260 ohm resistors does not exceed the 25 watt rated plate dissipation. Usually this will be the cathode of the self-biasing output stage. Touch the red (positive) lead to the wire on the anode side, and the black (negative lead) to the wire on the cathode side. all the schematics call for it as well. 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. The pull up resistor (Ra) and the pull down resistor (Rd) should. The cathode bypass cap also gives the stage more gain. Base bias the simplest way to bias a BJT transistor. Inverting Amplifier (DC Coupled): In this case, the voltage divider biasing method is the best to use, unless you happen to have a voltage follower bias already in your circuit, in which case you can merely. —Cathode bias. The bias circuit does not change with a change in current (it is oblivious to what is going on in the amp unlike the resistor in the cathode bias circuit) so if it is set for -20V on one tube giving X amount of current you may have to change it to -18 for the next tube to get X amount of current. 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. It wouldn't hurt to swap that resistor out for a 150R/5W, and you'll probably not notice a lot of difference in tone. Cathode bias produces less power, so you are able to crank it up a bit more. 13 Stability Factor for Potential Divider Bias 9. … What is a Cathode Ray Oscilloscope? A Cathode Ray Oscilloscope (CRO) is an instrument generally used in a laboratory to display, measure and analyze various waveforms of electrical circuits. I removed the 1 ohms bias resitors to ground, the bias feed from the standby switch, the NFB wire and grounded the 220k junction. Using a volt meter, measure the voltage from cathode (pin 8) to ground on the output tubes. The increased current WILL FLOW THROUGH the capacitor and charge it. First, you must accurately measure the resistance of the current sensing resistor(s) within the output tubes' cathode circuit. At 25°C, i. The most conventional and in my opinion also one of the best biasing methods is good old cathode bias. I started replacing electrolytic capacitors and noticed a big drop in the hum when I replaced the 25 uf cathode bypass electrolytic on the 12AX7 audio amplifier. So if 38 milliamps is flowing through a 1 ohm resistor, 0. Measuring the voltage drop across a 1-ohm resistor in the cathode gave a reading of 31. 4 shows the effects of the resistor. As I was getting ready for bed last night, I started thinking about the method I use for measuring and setting bias in my Fender Blues Deluxe Reissue (Strange things sometimes pop into my brain at 2:00 am. Two methods of BLOCKED-GRID KEYING are shown in figure 1-24. Fixed Base Bias or Fixed Resistance Bias. This will introduce more crossover distortion when the amp is cranked. The cathode biasing method is self-regulating, to an extent, because increases in cathode current create a larger voltage drop across the cathode resistor, which in turn, creates a larger negative grid-to-cathode voltage, which counteracts the increase in current. Agreed, however my point - and reason for advising against that method in my initial post - was that there are other, more accurate methods out there (such as using bias probes and the 1 ohm cathode resistor method) that allow you to measure the bias current in real time without stopping to re-calculate at every increment. i just popped a sovtek, and the JJ's i have have small pins. If grounded, then the entire vacuum chamber serves as the anode and accumulates deposits during the sputtering process. There are two types of grid-leak bias commonly in use: SHUNT TYPE and SERIES TYPE. During operation, mismatches between the two output tube sections will be significantly reduced by this combination biasing method. The following are five examples of transistor Base bias configurations from a single supply ( Vcc ). When measuring plate current there will be 1. Finally, adjust the bias for the correct calculated voltage. When a cathode resistor is used, it is called cathode bias. 4 shows the effects of the resistor. The grid will be connected to 0v through a resistor so it has a negative voltage relative to the cathode. The grid circuit DC return is connected to the other end of the resistor, causing the DC grid voltage to be negative relative to the cathode. 4K 130r cathode resistor. I don't mean to disparage cathode bias, but sometimes the language amp manufacturers conjure up does tend to cloud the fact that this method of biasing does not allow user adjustment. The cathode resistor and final amplifier screen grid form a load for the screen regulator tube. OK to eliminate cathode bias resistor? (1/2) > >> Hi, I'm trying to fix up an old Heathkit Twoer. OK, redplating occurs when the bias is off and too much current flows through the tube causing the plate to glow red. Patent us high voltage series biased fet amplifier for drawing. From what I understand of the cathode resistor method, you are measuring a combination of plate and screen current thru the resistor. Using ohms law, (E= IxR), you can calculate the correct voltage. Forward Biased: When the Positive (Red) test lead is connected to the anode (+) and negative (Black) test lead is connected to the cathode (-) of diode. In the ideal circuit the voltage at the non-inverting input is zero even when there is a finite input bias current. Transistor Biasing (Bias is the state of the circuit when there is no signal) 1. Here a 1-Ohm/1-3W resistor is connected between the cathode (pin 8) and ground on each output tube (ie: Bias-Probe, etc. Cathode bias amplifiers use a cathode resistor to generate a positive bias voltage on the cathode while the grid is grounded through a 'grid leak' resistor. 66 "On cathode-biased amp, the bias adjustment is not necessary beacuse there is a large resistor on the cathode of the tube (hence the name cathode-biased). The following are five examples of transistor Base bias configurations from a single supply ( Vcc ). The circuit usually consists of biasing resistors in a voltage divider network whose values are determined through circuit analysis. To change the value of the cathode resistor, refer to the steps in the Mod 2: Swap Negative- Feedback Circuit Resistors to Adjust Headroom section. resistor and bias voltage methods are shown. "Cathode bias", "self bias" (same as cathode bias) and "fixed bias" are all bias methods or specific circuit approaches used to get the tubes to idle at the desired current. With the 6,8M grid leak the bias voltage is -650mV with the plate sitting at 77V. The increased current WILL FLOW THROUGH the capacitor and charge it. You will see a yellow orange glow usually at the top or the bottom of the tube. The cathode biasing method is self-regulating, to an extent, because increases in cathode current create a larger voltage drop across the cathode resistor, which in turn, creates a larger negative grid-to-cathode voltage, which counteracts the increase in current. If there's a short, the cathode resistor will burn up before something expensive. 12 Voltage Divider Bias Method 9. By using the cathode resistor method you can easily bias your amp. 1mv on pin8 measuring 24v resistor checks at 583ohm pin3 364v on 2nd 6v6 pin 8 measures 28v resistor checks at 743ohms pin3 364v Looking at that I know I need to change the cathode resistors as they are out of 5%spec. Looking at the schematic, I suspect it's the 47K next to the diode. This substantially increases gain. 1 volt, the grid to cathode space exhibits non-linear resistance. Amplifier Deck Modifications. Cathode Resistor Okay, that was a bit heavy, sizing the cathode resistor is much easier now we've decided how we want to operate our valve. Oh, if you have the cathode resistor bias method, then the 5881 are safe if your plate voltage is 430V and your cathode voltage is about +50V, because the tubes will see 430V - 50V = 380V. o Bias point impacts the small-signal parameters. 298K, from the formulas above you get:. The following are five examples of transistor Base bias configurations from a single supply ( Vcc ). With the key open, reduced plate current flows up through resistor R1 making the end connected to grid resistor Rg negative. 8 volts means the diode is working. Base Bias of a BJT Transistor. Across the 10 OHM cathode resistor is a convenient measuring point. Some amps (e. Some people send me an email asking about how to measure cathode current and resistor rating. Since this current does not flow through the resistor we have a steady cathode voltage. This is the method that is best for hobby techs and do-it-yourselfers. 10 Circuit Analysis of Emitter Bias 9. When the cathode & Anode is connected to the negative & positive terminal of power supply respectively, the diode starts conducting and is said to be in Forward bias. Since we know that a diode is essentially nothing more than a one-way valve for electricity, it makes sense we should be able to verify its one-way nature using a DC (battery-powered) ohmmeter as in the figure below. One gotcha is that increasing the cathode resistor will run the tubes colder at idle, in effect shifting the output's bias more towards class B. Cathode bias is achieved by inserting the proper value resistor either on the filament transformer center tap or at the center terminal of a hum bucking pot. 3: In series with a cathode load resistance/tail resistance. A 'cathode bias' amplifier has a resistor going from the cathode to ground. Measuring by way of the ‘transformer shunt method’, the indicated plate current (or what is purported to be the plate current) will be lower than if you insert an ammeter in series with the plate connection, or use a 1-ohm resistor in the cathode of the output tubes. Usually, two values are given, one for cathode bias and one for grid bias; the value for grid bias is invariably lower (2. Many of these resistors have drifted quite a bit over the years. I'll replace the 680s with a pair of 820 ohms resistors to hear the difference and also measure the voltages. Gmgx in the shack december i can bias fets as expected measured idle current and then increased fet voltage until each drew an additional ma board with. 3v (in case of germanium diode) or 0. One way of implementing an external bias to a stage is to 'lift' the cathode by connecting it to some suitable positive voltage. So flaws show up sooner. Cathode bias is a great option for such an output stage, because the tube bias is equal to the voltage dropped across a resistor as the result of tube current, and bias stays pretty consistent (there are some fine points about how/why it could change). Besides, a minor overvoltage (of say 20%) usually is not that critical as dissipation overload or excess cathode current. When pin 2 volts exceeds pin 3 the output goes low. The only difference between the illustrated circuit and the one used to demonstrate triode operation is the elimination of the battery, E cc, and the addition of circuit components R k, the cathode-biasing resistor; C k, the cathode ac-bypass capacitor; and a grid resistor (whose purpose will be explained later). When a cathode resistor is used, it is called cathode bias. I just played around with this resistor to. I will show you how to do it and actually this way is not only applicable for cathode resistor (but commonly used for that purpose). 10 Ω cathode resistor are calculated by an approximate method. As for the wattage value of the resistor after the 0C3, read the voltage across the resistor, divide it by the value of the resistor (I=V/R) to see what the actual current through the resistor is. The anode, which can be the substrate, can be grounded, floating, or biased. These have been explained in detail in our books and many other texts. The effects of the input bias currents will then be negligible. Cathode bias amps are the least work intensive, as they do not really require biasing at all in a general sense. Calculation of input current in fixed-bias circuit. 65 subtracted = 50. It wouldn't hurt to swap that resistor out for a 150R/5W, and you'll probably not notice a lot of difference in tone. These have been explained in detail in our books and many other texts. Looks like the triode could've been easily biased using a cathode resistor with a bypass cap and do exactly the same job. Finally, adjust the bias for the correct calculated voltage. When the positive polarity is at the anode – the diode is forward biased and is conducting. Cathode bias produces less power, so you are able to crank it up a bit more. —Cathode bias. The electrochemical. The output stage draws a high current compared to e. The first thing you need to do is remove the chassis. Cathode bias: Cathode bias is different than fixed bias. All crt's used a 1 megohm resistor in series between the cathode and the negative end of the power supply. Voltage across the cathode resistors is 40V and plate to cathode voltage is 380V. The resistor is usually wired in parallel with a capacitor, and no negative voltage is applied to the grid. The only difference between the illustrated circuit and the one used to demonstrate triode operation is the elimination of the battery, E cc, and the addition of circuit components R k, the cathode-biasing resistor; C k, the cathode ac-bypass capacitor; and a grid resistor (whose purpose will be explained later). As current flows through the tube it develops a voltage across the cathode resistor making the cathode positive in respect to the grid of the tube. 66 "On cathode-biased amp, the bias adjustment is not necessary beacuse there is a large resistor on the cathode of the tube (hence the name cathode-biased). This method of biasing was rather popular with Western-European manufacturers wherever low negative bias was required. This type of bias is sometimes referred to as self bias, or a few other “marketing speak” terms. Auto-bias is usually achieved by putting a resistor in series with the cathode of a tube and referencing the grid to ground (usually through a large resistor). The YJC is for amplifiers with cathode biased power tubes. 4 as a bias feed for the drain. At small carrier signal levels, typically not more than 0. The most conventional and in my opinion also one of the best biasing methods is good old cathode bias. The calculations focus on the clamping action of the coupling capacitor CG due to grid current, so they assume that the cathode bias voltage or fixed bias supply voltage remain. Antonyms for biased. It is VERY subjective. Believe it or not, this is called the Cathode-Resistor Method of biasing!. During operation, mismatches between the two output tube sections will be significantly reduced by this combination biasing method. In fact, we CAN’T bias them!. In my datasheets the same recommendation is made for KT66, 6550, and KT90. The best way to calculate the value of the cathode resistor is by the use of a load line. The dc-30 schematic has high low power switch. Hence on low power the cathode resistor value is doubled - the effect I notice is that it has much less headroom and doesn't sound as nice on full power. The following are five examples of transistor Base bias configurations from a single supply ( Vcc ). I made the bias pot available from the back of the amps too. The plate resistor is chosen first, typically around twice the value of the internal plate resistance of the tube for most linear operation, although higher values. Across the 10 OHM cathode resistor is a convenient measuring point. Immediately following are equations for calculating cathode voltages, currents and resistances. Throw in the fact that cheapo multimeters aren't very accurate when reading super low voltages in the mV range and you have a real recipe for disaster. There are many methods of achieving grid bias. Setting Driver Stage Bias. newbie question here I have a cathode biased amp using 510 ohm resistors the 6v6's are redplating, with a bias probe showing 41. Types of bias You can now see that the bias of the amplifier affects its tone. For the bias voltage methods, either voltage divider or voltage follower circuits can be used. 3) Choose a desired cathode bias voltage and divide by the cathode current for the resistor value. 2: LED bias. This method of biasing was rather popular with Western-European manufacturers wherever low negative bias was required. 298K, from the formulas above you get:. When biasing a tube, fundamentally what you're doing setting up the circuit so that the control grid of the tube is at a negative voltage with respect to the cathode. 04 W; there-fore at 0. Using a volt meter, measure the voltage from cathode (pin 8) to ground on the output tubes. ) long and with a total area of 1. 6 Stability Factor 9. This type of bias is sometimes referred to as self bias, or a few other "marketing speak" terms. Ik = Ek / Rk cathode current equals cathode voltage divided by cathode resistor value (Remember to convert milliamps (ma) to amps (A) by dividing ma by 1000. The bias current is set purely by the input bias voltage. The additional gate resistor RGl from gate to supply voltage facilitates in larger adjustment of the dc bias point and permits use of larger valued RS. Finally, adjust the bias for the correct calculated voltage. The grid is connected via a high tube of resistance (usually 500k Ohms or greater) to earth or signal ground. Cathode Bias vs Fixed Bias. Some fixed-bias amps dispense with the cathode resistor altogether, either by switching it out (shorted) during normal usage, or bias is measured through the plate circuit. This is often called self-biasing. The rule-of-thumb method that allows the correct choice of decoupling capacitor to be made is as follows. This is normal and is the heaters doing their job (valves are essentially lightbulbs with extra bits in ). This, as we already know will lead to increased cathode current. Cathode bias (self-bias, automatic bias) - The voltage drop across a resistor in series with the cathode is utilized. 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. A large 10 Watt 470R resistor or similar value Resistor is placed in series with each Cathode. Using ohms law, (E= IxR), you can calculate the correct voltage. How does bias point (i. The decoupling capacitor C1 needs to have an impedance comparable to R2 at the low frequency limit, lets say at 20Hz, thats 1 / 2p *300 * 20 ~ 33MF (so this will be an electrolytic). I selected a pentode rather than a triode because a pentode by its nature is a current sink and the control grid will not have to do as much to make it into one. I'll replace the 680s with a pair of 820 ohms resistors to hear the difference and also measure the voltages. The cathode bias or self-bias technique connects the grid to ground and elevate the cathode voltage above ground. 66 "On cathode-biased amp, the bias adjustment is not necessary beacuse there is a large resistor on the cathode of the tube (hence the name cathode-biased). Class A Bias. Sometime you see a capacitor in parallel with the plate resistor, much like the cathode resistor bypass cap. 5V and the cathode resistor is 390ohm. Cathode bias amps are the least work intensive, as they do not really require biasing at all in a general sense. Various passive biasing schemes are shown in. The hollow cathode assembly is connected to ground via a 100kX resistor causing the cathode to acquire a nega-. 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. In a cathode biased amp, this involves adjusting the cathode resistor. 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. Ohm's law is our friend. To reduce bias you could: (1) Have others repeat the experiment (2) Label your groups so you will not know what they are (3) Do not include opinions in a experiment (4) collect your data in which. If the cathode bias resistor is fitted, the lower ends of R 2 and R 3, and the lower terminal of the pick-up, are returned to chassis. I use the following bias probe to measure cathode current and plate voltage in each power tube:. The cathode biasing method is self-regulating, to an extent, because increases in cathode current create a larger voltage drop across the cathode resistor, which in turn, creates a larger negative grid-to-cathode voltage, which counteracts the increase in current. It also usually requires a high quality cap to bypass the cathode resistor. Likewise, if you up the cathode (bias) resistor some, you intend to close the tube's throttle some, and it will have less current, which in turn causes less voltage drop, and you end up at pretty much the same bias current as before. There may also be a resistor in series with the grid as a "stopper" to prevent oscillation. —Cathode bias. Believe it or not, this is called the Cathode-Resistor Method of biasing!. Because shunt type grid-leak biasing is the simplest, we will discuss it first. In this scheme the plate current is returned to ground through a dropping resistor which is placed between cathode and ground. These films are used to produce miniature resistor networks, encapsulated in surface mountable plastic houses or chip carriers. Base bias ensures that the voltage fed to the base, VBB, is the correct voltage, which then supplies the correct current so that the BJT has enough base current to switch the transistor on. If there's a short, the cathode resistor will burn up before something expensive. 9 Emitter Bias Circuit 9. Setting Driver Stage Bias. with Monolithic Bias Resistor Network This new series of digital transistors is designed to replace a single device and its external resistor bias network. As current flows through the tube and cathode resistor, it creates a voltage drop across the resistor and therefore raises the cathode relative to the grid by that voltage. The solution is to connect a capacitor from the cathode to ground. Cathode bias produces less power, so you are able to crank it up a bit more. Transistor Biasing with Collector Feedback Resistor. If you have not been trained to work with high voltage then have an amp technician adjust the amp's bias. The only difference between the illustrated circuit and the one used to demonstrate triode operation is the elimination of the battery, E cc, and the addition of circuit components R k, the cathode-biasing resistor; C k, the cathode ac-bypass capacitor; and a grid resistor (whose purpose will be explained later). In a cathode biased amp, this involves adjusting the cathode resistor. Voltage across the cathode resistors is 40V and plate to cathode voltage is 380V. One gotcha is that increasing the cathode resistor will run the tubes colder at idle, in effect shifting the output's bias more towards class B. Tim VK5ZT. Various passive biasing circuits will be discussed along with their advantages and disadvantages. of the circuit and elements within to the signal is different that the response of the circuit and its elements to Bias (or to Bias + signal): o Different transfer function for the circuit. Although I have heard of people using 1/2 watt resistors in this position, I recommend using a higher wattage device. 6 Stability Factor 9. The network consisting of resistor R14 and zener diode D3 provides this grid bias voltage. (In triode anode and cathode currents are equal). 3) Choose a desired cathode bias voltage and divide by the cathode current for the resistor value. 1, we see a typical Fender bias-balance supply with the new cathode-bias option and switching added. (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. For our simple example of a single resistor, these conditions hold. In addition, the following guidelines should be met. 35ma anode current. Purpose: BJT should be in active (or MOS should in saturation) at all times. While not as accurate as the output transformer shunt method (detailed below, after the cathode resistor procedure) it is far and away the safest of the two methods, and can be successfully done with medium- and even low-quality test equipment. I know I wondered at the time, because it seemed written in stone that you have to bypass the main cathode resistor if cathode bias. But but, How is the grid DC grounded?. All crt's used a 1 megohm resistor in series between the cathode and the negative end of the power supply. The grid is connected via a high tube of resistance (usually 500k Ohms or greater) to earth or signal ground. Because the value of the gridleak resistor needs to be very high for the bias voltage to develop. This is often called self-biasing. Note that a completely open circuit only produces ~24 volts cathode bias on the 3cx1200A7 tube! A typical 3-500Z produces about 30-35 volts cathode bias with no resistor. 5 watt 1 megohm resistor using 25 1/2 watt 1 megohm resistors soldered together in a series-parallel configuration. 75vdc drop across the 5 ohm cathode shunt if 350ma of actual cathode current is flowing. That is an interesting question for which I have no answer. I chose the Cathode-Resistor Method because it's easy to do, and it's recommended by Fender on the Hot Rod's schematic (see Note 5). i just popped a sovtek, and the JJ's i have have small pins. Varying the negative bias on the grid will modulate the current flowing towards the anode. 5K, 12W bias resistor bypassed by a 100uf/100V electrolytic cap [connected in parallel; watch for. This will introduce more crossover distortion when the amp is cranked. The additional gate resistor RGl from gate to supply voltage facilitates in larger adjustment of the dc bias point and permits use of larger valued RS. This is referred to as fixed bias, though it is usually adjustable, and is more usually used for fairly high power push-pull output stages. As quote on page. Ohm's law is our friend. 12AX7 cathode and anode resistor values - gain (volume) vs distortion V1 is important to your amp's gain and distortion characteristics - how to get that sound The Canadian Guitar Forum. In a properly biased amp, the plate current x plate voltage should be in the neighborhood of 70% of the max plate dissipation for an EL84, or 12W. With the 6,8M grid leak the bias voltage is -650mV with the plate sitting at 77V. Antonyms for biased. I made a 12. First, you must accurately measure the resistance of the current sensing resistor(s) within the output tubes' cathode circuit. The cathode resistor method This is the method that is best for hobby techs and do-it-yourselfers. This means that the cathode resistor is tied to a more negative voltage of something more than twice the bias voltage, so it can pull the output tube grid negative. Even tubes of the same type and manufacturer will run hotter or cooler in the same amp. Although I have heard of people using 1/2 watt resistors in this position, I recommend using a higher wattage device. Transistor Biasing Methods In this article, we will go over the different ways in which a bipolar junction transistor (BJT) can be biased so that it can produce a stable and accurate output signal. The dc-30 schematic has high low power switch. Also the 120R/5watt cathode resistor is allowing that amp to run really hot, bias wise.