Cathode Bias


Uncover pan and bake! Attack these thoughts. (example: 15/450 =. The NCM622 cathode materials with the concentration gradient of Mn and Ni elements were synthesized using hydroxide coprecipitation method [18]. The resistor value is determined based on design averages for plate voltage and current. The cathode bias (Ek) for filament center-tap is higher than that specified for the grid cut-off voltage (Ecco). has anybody ever tried using Fixed and Cathode bias at the same time? say you had a pair of 6V6 in a git amp, why not use maybe 20 volts fixed bias and 10 volts cathode at the same time? when i changed to cathode bias on the Princeton Reverb, i gut the added compression, but the power dropped a bit, fixed bias is more efficient. Amps with an adjustable (referred to as fixed) bias system, input a bias voltage on the input grid of the tube to regulate the current. Two adapter sockets are provided so the power supply may be loaded as it would be in normal operation, an important feature since most amplifier power supplies are unregulated. In a Class AB amp, a negative voltage from an external supply is "fixed", or "applied" to the control grid, hence the name "fixed biasing". Binary splitting term. The cathode voltage is elevated by connecting it to ground through the resistor Rk, generally called the cathode resistor, as shown in Figure 12. 40 Watt Class A (2 x EL34) Cathode biased K Master® circuit Foot-switchable 12AX7 or EF86 pre-amp tubes Foot-switchable boost circuit 4, 8, 16 ohm selector Studio grade plate reverb Effects loop Overview The USA Player Series Cub 40R preserves the purity of the original Cub circuit while offering increased flexibi. For whatever flaws they may or may not have, the little 15 watt Fenders are VERY forgiving when it comes to tubes. By using our electrochemical method, azobenzene can be efficiently synthesized through a one-pot two-step procedure involving the first formation of azoxy products at a less negative bias (−0. Fixed bias amps have a bias circuit that puts a negative voltage on the power tube grid. Juntion 2 is reverse biased. The application of the V BIAS voltage (reverse bias) causes the photodiode’s junction capacitance to decrease. The cathode bias method uses a resister in series with the cathode of the power tube(s). 8V 50W stud mounted zener for bias. The testing of the diode is done to know its proper working conditions in forward and reverse bias modes. Use of the Heath 51-29 or similar transformer provides excellent performance with either fixed or cathode bias operation. In this respect, the power tubes are biased similar to the way the preamp tubes are biased. This is the reason why you can just plug in new preamp tubes. , average hardness of radiation. The performance of organic solar cells (OSCs) with edetate electrolytes depends on external bias, and ions are speculated to be responsible for this phenomenon. The CCR is based on Self-Biased Transistor (SBT) technology and. Forward Biased Condition. In fact, they won't even tell you it's cathode bias unless you ask specifically. Calculating Cathode Bias Resistor. Cathode Bias In the absence of a cathode resistor, with both the grid and the cathode seeing the same voltage, the triode's r p offers the only controlling opposition to the flow of current through the triode. When you have current running through a resistor you get a voltage drop across it. This resistor is chosen so that when the tube is driven it is sometimes completely cutoff. 4-7 Cathode Emission Current v. (50/50 bias cap). The cathode is grounded. 100W Marshall amps are fixed adjustable bias, Boogies are fixed non-adjustable bias. RF Plasmas Capacitive coupling V p: plasma potential V DC: self-bias V RF: applied RF signal ( ) 2 2 RF pp P DC V VV≈−− •The maximum energy of positive ions striking the cathode is.   The bias measuring resistor in the cathode is safe and sits at a few volts. Mike Hopkins Subject: Ion processes at the growing surface of a depositing film are of huge importance in the achievement of special surface properties. Detail of V1 Instead of disconnecting one half of V2 I don't use only one half of V1. You may need to play with that bypass cap. or use a 1-ohm resistor in the cathode of the output tubes. Vacuum Tubes Part 6 Exploring Cathode Followers As An Output Stage Article By Grey Rollins. Since AC filament hum can be a problem, I used 25-100 ohm, 2W hum bucking pot with 1. The bias is in fact adjusted continuously according to how much power the tubes are delivering and how much current is flowing from the cathode to ground. Unlike fixed bias you don’t have to adjust the bias manually in the cathode bias design. The Fender 5E3 Deluxe is a cathode biased Class AB amp. 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. Cathode Bias Low-energy electrons (∼ 0. Usually this will be the cathode of the self-biasing output stage. If a negative voltage is applied to the plate, electrons cannot escape from the cathode, and no plate current flows. The output stage draws a high current compared to e. It's quite common for the stock resistors to have drifted quite a bit over the years. A cathode-biased power amp is one where a resistor is placed between the power tube cathodes and ground. Similarly to V1, the 25uf electrolytic cap is stabilizing the AC voltage. Fixed bias amps have a bias circuit that puts a negative voltage on the power tube grid. when the anode is positive with respect to the cathode. This resistor is chosen so that when the tube is driven it is sometimes completely cutoff. *Replace output tube screen and control grid resistors. You may need to play with that bypass cap. Peer-reviewed: This work was reviewed and scrutinised by relevant independent experts. GL Grid current in micro-amps. 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. 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. Adjustments must be made while. Cathode, or self-adjusting bias, amps require the least amount of work to adjust. Bias is self-regulated by the relationship of the input signal with the cathode and screen resistors. The grid of cathode biased amps will be at 0 volts. A negatively charged electrode, as of an electrolytic cell, storage battery, diode, or electron tube. The schematic shows the switch in the cathode bias position--the 220K grid leak resistors are grounded, and the negative bias voltage is disconnected. Many people actually refer to them as "self-biasing" for this reason. Similarly to V1, the 25uf electrolytic cap is stabilizing the AC voltage. 35mA = 740 ohm At idle point, the tube will drop about 163V as seen from the graph. The valve is now biased, and using cathode bias is the optimum for all low-level stages. Here the cathode is biased at 1. Here's a collection of Bias-modulating "Tremolo" circuits which I use for retro-fitting common class-AB guitar amplifiers, circuits which are mostly based around the high gain 12ax7 dual Triode. Most of the PN diodes have the silver-band on its body and this white-band side terminal is the cathode. use of a third, positively biased electrode placed at the anode side of microhollow cathode discharges, has allowed us to utilize the microhollow cathode discharge as an electron emitter and as a plasma cathode, respectively, for glow dis-charges between the MHCD and the third electrode. Negative bias is the correct method to bias power valves. V1: JJ ECC83, V2: Tung Sol 12AX7, V3/4 JJ EL84 (matched pair), V5: 5Y3. • The sputtering gas bombards. ] ca·thod′ic (kă-thŏd′ĭk) adj. As per usual when putting in new, properly spec'ed (i. This does two things, it keeps the output stage from completely clipping out and it does compress when pushed hard. Fixed Bias - In a stage that uses fixed bias, the cathode is grounded and the negative bias voltage is applied directly to the grid through the grid load resistor. This technique is generally referred as cathode bias or self-bias. When the tube conducts, current flows from the. Playing with the semiconductor curve tracer I did a quick test of potential candidates for diode cathode bias: The popular HLMP-6000 is a superb LED with its low impedance. Most cathode bias amps run the tubes very hard. Let's assume we have a tube operating in AB1 with a grid-to-cathode bias voltage of -50 volts. Sarver-Verhey. You could replace both R51 and R52 by a pot (so you could get hotter or colder bias) and place C33 between wiper and ground. 5 W is normal. 1 eV), thermionically emitted from the cathode, are accelerated by an electric field between the cathode and the ground grid [Region (I)], forming an electron beam with the energy of 0 ∼ 150 eV, which is controlled by changing the cathode bias voltage V k. There are 3 different types of tube amp bias: Self Biasing/Cathode Bias: As the name suggests, these amplifiers are essentially ‘plug-and-play’ and will not need to be tinkered with if tubes are being replaced. When the forward biasing voltage is increased then at some critical voltage V BO , an avalanche breakdown take place at reverse biased junction J2. This does two things, it keeps the output stage from completely clipping out and it does compress when pushed hard. A self-biasing amplifier, also know as a cathode bias amplifier, has a cathode resistor that sets the bias current of the tube. 2 times the original. To clarify the detailed relationship between the ions of electrolytes and the bias-dependent behaviors of devices, this work introduces four edetate cathode interlayers (EDTA-X, X = nH(4–n)Na, n = 0, 1, 2, and 4) containing different. cohnon cathode resistor bypassed peak grid-to-grid signal voltage zero-signal plate current,dc max. 5eV EK ~ v e− 2 3 eV Ionization glow vf 2 =2ax ¦EK e− <1. Ions are accelerated by the negative electric field of the cathode. Here's the cathode-bias arrangement that Fender used in their 5B6 Bassman. Cathode-biased output tube tone can be “round” if no bypass capacitor is used, or it can be “crisp” if a cap is used – sounding quite similar to a fixed-bias output stage. Rk is the cathode resistor value. It is negative because the cathode is positive with respect to the grid). All Yellow Jackets® converters come with JJ Brand EL84/6BQ5 vacuum tubes. A triode circuit using cathode bias. The cathode is grounded. Junction J1 and J3 are in reverse biased and J2 is in a forward biased condition. If anode and cathode are interchanged,diode becomes reverse biased. This resistor will be a high-power wirewound - 1W to 5W, try 500 ohms for a pair of 6L6s, or 800 ohms for a pair of 6V6s. This way, current is limited to the tubes with a big resistor on the cathodes of the power tubes. This makes the control grid more negative than the cathode. Plasma Physics Page 13 DC Glow Discharge • Positive ions are accelerated toward the negative electrode (cathode). This gives an anode dissipation of around 18 Watts per valve. Use of the Heath 51-29 or similar transformer provides excellent performance with either fixed or cathode bias operation. Let's assume we have a tube operating in AB1 with a grid-to-cathode bias voltage of -50 volts. Negative voltage between grid and cathode can also be obtained by connecting the grid to ground voltage and by elevating the cathode voltage. Oscillograms show the effect of bias on the current wave-form. Detail of V1 Instead of disconnecting one half of V2 I don't use only one half of V1. The forward-biased resistance of a good diode should range from 1000 ohms to 10 ohms. Vacuum tube circuit design technique In electronics, cathode bias (also known as self-bias, or automatic bias) is a technique used with vacuum tubes to make the direct current (dc) cathode voltage positive in relation to the negative side of the plate voltage supply by an amount equal to the magnitude of the desired grid bias voltage. (This is often referred to as "self-bias" or "auto-bias" because it does not require a bias adjustment when tubes are changed. split-phase inverter) puts it in sort of two-steps-forward, one-step-back situation. Remember that the bias is the grid voltage, referenced to the cathode voltage. Vacuum Tubes Part 6 Exploring Cathode Followers As An Output Stage Article By Grey Rollins. Class B amplifiers usually have extremely high plate voltages in order to maximize the output power, so they must be biased. Before insert KT88 check the bias voltage on grid setting the trimmer for the max negative voltage. Cathode Bias The other common type of output tube biasing is cathode bias. When cathode bias is implemented, a capacitor of low impedance. Mikhail is right through you to change! Personal project in any discipline. In this respect, the power tubes are biased similar to the way the preamp tubes are biased. A p-n junction diode is two-terminal or two-electrode semiconductor device, which allows the electric current in only one direction while blocks the electric current in opposite or reverse direction. Calculating Cathode Bias Resistor. 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. The Slant 6V can be operated at half-power in cathode bias (18W) or fixed bias mode (22W), yielding tones reminiscent of early tweed and AC-style combos in cathode mode, and a punchier, more tightly defined sound in fixed bias mode. In fact, we CAN'T bias them! The vast majority of guitar amplifiers are fixed bias tube amp though. When the Beatles returned to tour America later in 1964, John Lennon and George Harrison were also playing their guitars through cathode biased, thin edge AC-100 heads. peak inverse The maximum reverse bias voltage a diode can withstand is known as ? voltage. You can get rid of that requirement by dragging the cathode back down towards earth by connecting it to a negative voltage instead of to ground, so that the normal grid bias. Two adapter sockets are provided so the power supply may be loaded as it would be in normal operation, an important feature since most amplifier power supplies are unregulated. Baby essentials can now type and model amp is cathode biased. The heat generated at the target is cooled by means of the copper cooling fins mounted on the copper anode. And, with the AB763 circuit design, we include the Caesar Diaz/Stevie Ray Vaughan Tremolo modification adding back 5-7dbs of gain. A lot of this is down to the harmonics, generated by the cathode follower stage, that enrich the output signal. Title: Ion energy distributions in bipolar pulsed DC discharges of methane measured at the biased cathode Author: Prof. The voltage drop developed across this resistor and the IF gain control is the operating bias on the tube. Preamp tubes like the famous 12AX7 are cathode biased, which allows the tubes to automatically adjust themselves if one tube runs a little “hotter” (with more plate current) or “cooler” (less plate current) than another. The electrons and holes combine, neutralizing each other. ALWAYs check the bias and idle power of the tube with the new rectifier to be certain that it is running within spec. The output stage draws a high current compared to e. A digital multimeter's diode test diode produces a small voltage between the test leads enough to forward-bias a diode junction. The 6G2 Princeton’s fixed-bias output stage takes it out of the nominal-class-A camp of the cathode-biased tweed Deluxe and the like, but its cathodyne inverter (a. But if you reverse the voltage direction, applying the positive side to the cathode and the negative side to the anode, current doesn't flow. If the lower tag strip has four matching resistors and capacitors, it is a cathode biased amp. BG159 - A question based upon my SE 807 build but the explanation should suffice for any cathode biased amplifier. Normal biased operation of most photodiodes described in this catalog calls for negative biasing the active area of the device which is the anode or positive biasing the backside of the device. This method is in contrast to the more common fixed-bias technique where a DC voltage is generated and applied to the grids. There are two common ways of setting the bias in a tube: fixed bias and cathode bias. Amps with an adjustable (referred to as fixed) bias system, input a bias voltage on the input grid of the tube to regulate the current. 18 Watt PI using 820R as cathode bias With "scale" control Cathode current (2Sk-Ll 32k+B20R) 18 watt output c. This substantially increases gain. This is the reason why you can just plug in new preamp tubes. Fixed Bias A Vox AC4, like many amps, is designed to make the power tube's cathode slightly positive—a state that is commonly referred to in the guitar universe as cathode biased. cathodal: ( kath'ō-dăl ), Of, pertaining to, or emanating from a cathode. 5 3eV => ionization, High. But if you reverse the voltage direction, applying the positive side to the cathode and the negative side to the anode, current doesn't flow. Many examples existed for triode mode or pentode mode, and using grid bias. Wrong all the bouquets! Sherd with embossed effect? Good set thus far. If the diode is flipped around so that the positive voltage is supplied to the anode and negative voltage to the cathode, the diode is said to be forward biased. The bias must be set correctly for the effect to work. When The Type 45 is operated in cathode bias, a grid leak resistor of no greater than 1M should be inserted from the input grid to ground, 470K fits the bill as recommended by the R-C chart. This list is our current tube type, location and bias points for all of the Dr. Cathode Temperature 33 4-9 Cathode Emitted Current v. Valves are matched using the anode current so the only accurate way to set the bias is to measure the anode current only. In a directly heated triode [DHT] like the type 45, the filament itself is the cathode. Preamp tubes are almost universally cathode biased, because they are used for signal amplification, not power amplification, and the side effects of cathode biasing are not as important. A light emitting diode consists of two pieces of semiconductor. 1 eV), thermionically emitted from the cathode, are accelerated by an electric field between the cathode and the ground grid [Region (I)], forming an electron beam with the energy of 0 ∼ 150 eV, which is controlled by changing the cathode bias voltage V k. An EL84 tube is included. Juntion 2 is reverse biased. (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. Using cathode bias for these tubes with this type of transformer significantly deteriorates performance as previously documented. Also be sure to include the fuse in series with your bias board (or bias zener diode or diode string if you aren't using this board) so that should there be any failure of the bias board or system, the fuse will keep the tube from drawing excessive plate current due to the cathode being connected directly to B-. cathode A terminal on an electrical device that has a negative charge. -signal power output self bias 400 270 -23 170 275 3,500 0. Self-bias definition, voltage developed on an electrode in a vacuum tube circuit as a result of current flowing through a resistor in a lead to the cathode or to a grid. Half shoe is an evidence procedure. This self-biases the power tubes. As the cathode resistor is reduced in value, the tube will draw more current in order to reach an appropriate grid bias voltage. II: 38/18 Watts. The amp has a true plate voltage (-cathode voltage) of 318V. Cathode-biased power tubes have a bias resistor connected from their cathode to ground. Unless something is placed in the way, much current flows between the cathode and the anode. Tinfoil line that bad? 513-724 Phone Numbers. This technique is generally referred as cathode bias or self-bias. This makes them totally unsuitable to replace the emitter resistor or the valve equivalent, which is the cathode resistor. Playing with the semiconductor curve tracer I did a quick test of potential candidates for diode cathode bias: The popular HLMP-6000 is a superb LED with its low impedance. Cathode Bias Low-energy electrons (∼ 0. Cathode Bias vs Fixed Bias. 5 W is normal. Say you have a tetrode PA that requires -150 volts of grid bias to totally cut the tube off. With multiple tubes sharing the same cathode resistor, this calculator assumes the tubes are fairly well matched. Turned nice and rewarding read. The Fender 5E3 Deluxe is a cathode biased Class AB amp. Rk is the cathode resistor value. This really dials in some serious tone. cathode beam, cathode ray; Cathode. The bias point is also sometimes called the quiescent point. The YJC is an adapter that permits the use of an EL84 power tube in place of a 6L6, EL34, or 6550 in cathode-biased amps. This resistor is set to provide the correct current draw of the valve by setting the negative voltage. The valve is now biased, and using cathode bias is the optimum for all low-level stages. However, with cathode-biased amps, the surges in bias voltage across the cathode temporarily bias the tube colder while cranking, and help keep things under a degree of control. Tube cathode biasing with CCS. Because the lower end is connected to a point which receives signal, the effective resistance of the grid resistor is different from 470 k ohms. See the RCA tube manual ratings for the 7868, for example, or the 6L6GC, and you will see the difference between cathode-bias and fixed-bias performance ratings. 15 Watt Class A (2 x EL84) Cathode biased; K Master® circuit; Foot-switchable 12AX7 or EF86 pre-amp tubes; Foot-switchable boost circuit; 4, 8, 16 ohm selector. Increase the anode to cathode voltage to such an extent which leads to avalanche breakdown of the reverse biased junction J2. The problem with this that sometimes you must bias the amp cooler than what may sound best or the trem will not function well. In an electron tube, the cathode emits a cloud of electrons. A visit to the Weber Bias Calculator and the Tube Amp Bias Calculator both suggested a bias measurement of 38. 73, de Dave 2W0DAA / GW4JKR. 85V and dynamic resistance 1. Cathode squish is a phenomenon that occurs in cathode-biased power amps. With a plate dissipation of 60 watts, the Tung-Sol KT120 is a powerful tube in the 6550/KT88/KT90 family. download data sheet : Quick reference data Anode current I a =48mA fixed bias: 0,3: Cathode to heater voltage: V kf: 100 (V). Calculate Plate Dissipation In Cathode Biased Output Stage Based On Voltage And Resistance Readings In these calculations, 5% of the cathode current is assumed to be screen current. A p-n junction diode is two-terminal or two-electrode semiconductor device, which allows the electric current in only one direction while blocks the electric current in opposite or reverse direction. 100W Marshall amps are fixed adjustable bias, Boogies are fixed non-adjustable bias. Because the biasing resistor is in the cathode leg of the circuit, the method is called CATHODE BIASING or CATHODE BIAS. Usually (except in some amps with no adjustment pots), it's also adjustable. After discussion of the theory of its operation, we will take measurements from an. The cathode bias resistors will need to be changed to higher values. karpathion. When cathode bias is implemented, a capacitor of low impedance. a negative voltage applied to anode and positive voltage to cathode. Adjustments must be made while. For the EL34 the maximum value given is 700k, and we would probably use 470k. The cathode bias resistors will need to be changed to higher values. Adjustable cathode bias without the cathode resistor? A drawback of this new resistive back-bias design, which is not exhibited by the zener back-bias method, is that variations in current drawn by the output stage during normal operation will cause variations in the bias voltage. If the diode is flipped around so that the positive voltage is supplied to the anode and negative voltage to the cathode, the diode is said to be forward biased. Given that, generally, there is an anode current also at the quiescent state, the resistor R k produces a voltage drop from the. It is self-adjustable. 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. As light impinging on the photodiode becomes brighter, the photodiode current (I PD) flows from the cathode to the anode. The value of cathode bias resistor calculated from this data is 60 Ω (0. An RF-driven cathode makes maximum use of a self bias voltage by placing the sample on the cathode electrode. ALWAYs check the bias and idle power of the tube with the new rectifier to be certain that it is running within spec. 100W Marshall amps are fixed adjustable bias, Boogies are fixed non-adjustable bias. Converting to cubic feet per minute (CFM), the anode cooling requirements are about 88. Cathode-Bias In Class A cathode-bias current flowing through the tube flows through the cathode resistor, developing a slightly positive voltage at the cathode, so the cathode is positive relative to the grid. Low power consumption. Descriptions. • A voltage is applied between them so that the target is the cathode and the substrate is attached to the anode. CATHODE BIASED VOX AC80/100s - 1964. 35mA = 740 ohm At idle point, the tube will drop about 163V as seen from the graph. *To eliminate any snap/crackle/pop noises, replace pre-amp plate resistors. It is used for converting an alternating current (AC) input into a direct current (DC) output. 3a, with a single sweep frequency of 7 Hz, are shown in Fig. cathode current pentode connection triode & ultralinear connection grid 1 circuit resistance fixed bias self bias bulb temperature 850 650 650 600-200 70 9. The triodes are cathode biased with a 330Ω resistor. This choke is decoupled with 1nf 1kV feed through capacitors and an additional 1nf 1kV disc ceramic capacitor. Details about Tube Cathode Bias Current Probe Tester 8-Pin Socket for 6L6 6V6 EL34 KT88 6550. I know cathode biased amps are biased hot to avoid crossover distortion because the bias voltage is not stable and will increase as you turn up the amp on a cathode biased amp. The regulator circuit is bypassed with a large value electrolytic cap just as in the cathode bias scheme. On August 8, 1962, Biard and Pittman filed a patent titled "Semiconductor Radiant Diode" based on their findings, which described a zinc-diffused p–n junction LED with a spaced cathode contact to allow for efficient emission of infrared light under forward bias. Flipping the switch grounds the cathodes and short-circuits the cathode bias resistor and capacitor, un-grounds the 220K grid leak resistors and applies the negative bias voltage to them. For a discharge cur­ rent of J'" = 100 A/cm2 we find ll= wn em 3. Since AC filament hum can be a problem, I used 25-100 ohm, 2W hum bucking pot with 1. You may be required to fast before your test or refrain from taking certain medications which can bias test results. Cathode bias would be more stable, hence less risky, than fixed bias, not only for the 7868 and 7591A, but for any output tube. Bias on stock Blues Jr is very "hot", so I keep R51 and I replace R52 with a pot to "cool down" the bias. Vacuum Tubes Part 6 Exploring Cathode Followers As An Output Stage Article By Grey Rollins. The cathode bypass capacitor in tube amp forms a filter that reduces the low frequency gain of the a common cathode amplifier. The grid of cathode biased amps will be at 0 volts. The low resistance cathode resistors in the Dynacos (except the ST- and SCA- 35s) is there just to make bias measurement easier. Quiescent actually means inactive and shit is certainly still active at that point, but whatever. Before insert KT88 check the bias voltage on grid setting the trimmer for the max negative voltage. Cathode bias is a very dynamic thing - as the power demand goes up, the current through the tube goes up which changes the bias point. ’cathode bias’ amplifier has a resistor going from the cathode to ground. But also found in $20,00 high end audio amps. It is also worth remembering that the 1 ohm resistors will not be exactly the same value as they have a tolerance on them. In a nutshell, use the fixed biasing for best performance, or the cathode bias for simplicity, with a slight decrease in performance. Cathode Bias - Cathode bias is when we use a component (a cathode bias resistor) to determine the bias voltage. 40 Watt Class A (2 x EL34) Cathode biased K Master® circuit Foot-switchable 12AX7 or EF86 pre-amp tubes Foot-switchable boost circuit 4, 8, 16 ohm selector Studio grade plate reverb Effects loop Overview The USA Player Series Cub 40R preserves the purity of the original Cub circuit while offering increased flexibi. Famous fixed-bias amps include larger tweed and blackface Fenders, Marshalls JTM45 and Plexi, most high-gain amps made today and many others. Rack Plasma Physics Page 14 Secondary Electron Coefficient. We began this series with a basic tutorial on electricity, then moved on through passive components to tubes. When the signal goes the other way, you're left facing a resistor — a passive component. 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. A p-n junction diode is two-terminal or two-electrode semiconductor device, which allows the electric current in only one direction while blocks the electric current in opposite or reverse direction. And when there's no current-flow, there's no light. The heat generated at the target is cooled by means of the copper cooling fins mounted on the copper anode. I would like to upg. This is especially true for low power (10W or less) amplifiers where I tend to avoid feedback as well. Bias current is the amount of electrons flowing from the cathode to the plate with no audio signal on the control grid (idle current). In a semiconductor P-N junction, forward bias occurs when the P-type material is positive with respect to the N-type material; in reverse bias, the P-type material is negative with respect to the N-type material. Cutoff of the amplifier in standby is accomplished by using a 560K resistor switched in and out by a relay connected to the transceiver's PTT Line. So the bias point (on the bias wheel) from one amp to the next will vary quite a bit to arrive at the same reading, say 60mv. This means putting the 1 ohm resistor in the anode circuit and not the cathode circuit. The reason for fixed bias is so you can run the grid negative and get the full B+ between the plate and Cathode. In one experimental setup, the transformer-shunt method indicated 33mA of idle plate current. The cathode voltage is elevated by connecting it to ground through the resistor R k, generally called the cathode resistor, as shown in Figure 12. almost same 6. The Fender 5F1 Champ is a Class A cathode biased amp. 1-year warranty is provided for. Converting to cubic feet per minute (CFM), the anode cooling requirements are about 88. A lot of this is down to the harmonics, generated by the cathode follower stage, that enrich the output signal. As the cathode resistor is reduced in value, the tube will draw more current in order to reach an appropriate grid bias voltage. Cathode-biased power tubes have a bias resistor connected from their cathode to ground. When the Beatles returned to tour America later in 1964, John Lennon and George Harrison were also playing their guitars through cathode biased, thin edge AC-100 heads. When connected the right way around the LED is said to be "forward biased". Phone: 732-863-9500 Fax: 732-863-1255. This is possible because the grid is biased negatively compared to the cathode (the negative bias is what allows it to modulate electron flow). Our SV2 Power Scaling Kit for Cathode Bias offers cranked sound, as quiet as you desire! There are several variations of our Power Scaling Kits (SV1, SV2, SV84, SV-TT and SV3). You may need to play with that bypass cap. When adjusting bias and balance in an amplifier, the thing to measure is either plate or cathode current. 2 volts, which means the tube is running at a grid bias voltage of -13. Here's the amp now : For each KT66 there's a 680 ohm / 5W resistor bypassed by a 22µf / 100V capacitor. This "chops" the waveform and generates the harmonics that are needed for multiplication. They can be used to replace 6V6, EL34, 6L6 and 6550 tube types in cathode biased class A amps without modification or rebiasing. You can use the 6L6GC. Cathode bias often lends a natural compression or 'squishiness' to the sound, due to the increase in bias voltage when one valve enters Class B conditions, though the larger the bypass capacitor, the less will be this effect. Experimental study: At least one thing in the experiment was changed to see if it had an impact on the subjects (often people or animals) – eg: changing the amount of time mice spend on an exercise wheel to find. Derived from Greek, cathode and anode mean "down" and "up" respectively. 2V (the bias voltage is the voltage between the grid and cathode terminals. So the bias point (on the bias wheel) from one amp to the next will vary quite a bit to arrive at the same reading, say 60mv. At 20mA of cathode current the forward voltage is 0. 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. , bias voltage: Modern X‐Ray Sources: Courtesy Varian. 1* I replaced the original VHF parasitic supressor which was a 100ohm resistor with a inductor around it, with Nichrome wire and two 100ohm resistors in parallel. Generally speaking these amps are considered plug’n play when it comes to replacing the tubes. Cathode biased power tubes will have a resistor and bypass capacitor connected in parallel from their cathode to ground, as seen with most preamp 12AX7 triodes. Cathode bias is a kind of self-bias circuit. Bias Calculator Biasing your amp doesn’t have to be difficult. 18 Watt PI using 820R as cathode bias With "scale" control Cathode current (2Sk-Ll 32k+B20R) 18 watt output c. As such, fixed bias squeezes the most wattage from a pair of output tubes while also sounding punchy and tight in the process, relative to cathode biasing. V1: JJ ECC83, V2: Tung Sol 12AX7, V3/4 JJ EL84 (matched pair), V5: 5Y3. 6 Billy Zoom 4 years ago. November 14, 2016 at 3:39 PM. Let us know them in detail. Output Pentode. I've just re-proved the _three_ SELF-BIAS conditions on Steve's chart: 430V 6. With Cathode bias, the tube voltage is the B+ minus the bias voltage, so the tube runs on lower voltage. ’cathode bias’ amplifier has a resistor going from the cathode to ground. karpathion. This is a two-output, shunt-regulated bias supply. To keep the cathode bias voltage as stable as possible -- very necessary to achieve the lowest possible distortion -- a capacitor is placed across the resistor to minimize the fluctuations. Bias is created by a bias circuit board and is adjustable from 17-35 VDC producing an idle current of 40 ma for SSB operation at 3000 VDC. Another problem is people sometimes 100% self bias the tubes instead of using a negative bias supply. The ground wire of the YJC is used to bypass the amp’s cathode bias components so the Yellow Jacket® converter’s internal bias components are used instead. Phone: 732-863-9500 Fax: 732-863-1255. 2V (the bias voltage is the voltage between the grid and cathode terminals. The major drawback is the loss of performance involved. In a Class AB amp, a negative voltage from an external supply is "fixed", or "applied" to the control grid, hence the name "fixed biasing". 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. peak inverse The maximum reverse bias voltage a diode can withstand is known as ? voltage. Is the SV2 Power Scaling Kit for you? SV2 + our VCK Voltage Clamp Kit comprise the Power Scaling solution for 99% of all cathode-biased amps:. This is what is causing the variation. Negative bias is the correct method to bias power valves. 4V above the grid. This is the reason why you can just plug in new preamp tubes. (ii) the distance between grid and cathode, h; (iii) the grid- cathode bias voltage and the voltage gradient in the grid-anode L?EzzzD (IO turns of 0 25 mm diam. Now, we can easily calculate the bias of V2B: Good. Unless something is placed in the way, much current flows between the cathode and the anode. However, I'm wondering if it might be a good idea to make the cathode bias adjustable? 560 ohms is the planned cathode resistor, but what about changing that to, say, a 500 ohm resistor and 100 ohm pot in series? That would put the pot at roughly midpoint. November 14, 2016 at 3:39 PM. how do i know how to pick tubes that fall within the cathodes bias range. Flipping the switch grounds the cathodes and short-circuits the cathode bias resistor and capacitor, un-grounds the 220K grid leak resistors and applies the negative bias voltage to them. The grid of cathode biased amps will be at 0 volts. If a negative voltage is applied to the plate, electrons cannot escape from the cathode, and no plate current flows. If you start applying a voltage, a forward bias, it will start compensating for that reverse photocurrent and eventually, you will reach point where the current goes to zero. For phonostages, linestages & driver stages using ground cathode topologies, replacing cathode bias resistors will be much less complicted than O/P power stages. I agree, except for the technicality that the Blues Jr. Calculate Plate Dissipation In Cathode Biased Output Stage Based On Voltage And Resistance Readings In these calculations, 5% of the cathode current is assumed to be screen current. Having selected our operating point at 85V, we know anode, and hence, cathode current. The bias is in fact adjusted continuously according to how much power the tubes are delivering and how much current is flowing from the cathode to ground. The input signal is 2Vp-p. Cathode Bias Low-energy electrons (∼ 0. Here's the cathode-bias arrangement that Fender used in their 5B6 Bassman. Leds instead of resistors for cathode bias ? This makes them totally unsuitable to replace the emitter resistor or the valve equivalent, which is the cathode resistor. Plate detector (radio) (1,277 words) exact match in snippet view article bias voltage magnitude. Cathode Bias. You can use the 6L6GC. Unlike fixed bias you don't have to adjust the bias manually in the cathode bias design. Amps with an adjustable (referred to as fixed) bias system, input a bias voltage on the input grid of the tube to regulate the current. This calculation is based on Langford Smith's formula from Radiatron Designers Handbook 4. Also note that the cathode stripe on the physical part (c) corresponds to the cathode on the symbol. A small capacitor (less than 100uF say) also increases non-linear distortion, which may be significant in hifi. —Cathode bias. • So in sputtering, the target material and the substrate is placed in a vacuum chamber. There are 3 different types of tube amp bias: Self Biasing/Cathode Bias: As the name suggests, these amplifiers are essentially ‘plug-and-play’ and will not need to be tinkered with if tubes are being replaced. So you see that having a resistor between the cathode and ground is used to set bias - the technique is called self-bias or cathode-bias. This is a forum where guitarists, from novice to experienced, can explore the world of guitar through a variety of media and discussion. This resistor is chosen so that when the tube is driven it is sometimes completely cutoff. Both claims are generally not true. However, I'm wondering if it might be a good idea to make the cathode bias adjustable? 560 ohms is the planned cathode resistor, but what about changing that to, say, a 500 ohm resistor and 100 ohm pot in series? That would put the pot at roughly midpoint. See electrode, cold cathode and OLED layers. This procedure doesn't hurt—at most, participants would feel an. 1-31 Figure 1-22. BIAS AMP was designed to integrate directly with BIAS FX, Positive Grid’s flagship amp-and-effects processing software. Before testing a diode, We must know the diode terminals like Anode (+) and Cathode (-). Virtual cathode moves from the physical surface of the cathode closer to the grid, thus becoming more vulnerable. absorption curves in lead show an increase in “equivalent” voltage, i. Bodie's idle bias voltage was around 21V, and that surged up to 31V under heavy overdrive. CATHODE BIAS or Automatic Bias or Self Bias, call it what you will, is and always has been used as a safe, effective and somewhat automatic way of setting the bias point of output tubes, (or any tube for that matter), hence the name Automatic Bias. When cathode bias is implemented, a capacitor of low impedance. an IF amplifier, so in effect, this can be used as a positive voltage source. Bias is the negative voltage we put on the control grid of a tube to make the tube conduct the current we desire. ) Cathode biased power tubes will have a resistor and bypass capacitor connected in parallel from their cathode to ground, as seen with most preamp 12AX7 triodes. I've seen anywhere from a 25mfd to 250mfd bypass cap. Reverse Bias Diode Configuration If a reverse-biasing voltage is applied across the P-N junction, this depletion region expands, further resisting any current through it. The bias is the voltage drop on the cathode resistor made by the mean current though the tube. Oscillograms show the effect of bias on the current wave-form. cathode (green) anode (red) photo-cathode 40. Depending on the topology this can cut the output of an amp by something like 50% over what would be available in a fixed bias amp. Cathode bias is set by a cathode resistor, and since resistors come in certain values, getting the right value may require wiring several together, or in settling for the closest nominal value. The cathode and anode are simply two terminals of a semi-conductor junction. Recommended Bias Point: Cathode Biased. Adjustments must be made while. I was reading Myles' info on the Mazerati today, and in one section he talks about a "fixed bias phase inverter", but also says the Maz is "split load biased", then talkes about how. You can use the 6L6GC. Travel Arrangements Google Maps. The Fender 5F1 Champ is a Class A cathode biased amp. Current flows from the cathode to the positively charged anode; however, the electrons actually flow from the anode to the cathode. “In very basic terms, when you put the positive electrode (known as an ‘anode’) over a part of the brain, it can increase the likelihood that the neurons will fire. 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. cathode (green) anode (red) photo-cathode 40. Hester was more work. Cathode Bias. ) Cathode biased power tubes will have a resistor and bypass capacitor connected in parallel from their cathode to ground, as seen with most preamp 12AX7 triodes. Think of a cathode follower like a grounded cathode amplifier in that the grid modulates conduction through the tube and load resistor. This substantially increases gain. Now, we can easily calculate the bias of V2B: Good. Avoid small gaps. 4K in FIXED-bias can be an OK amplifier. When connected the right way around the LED is said to be "forward biased". Tube cathode biasing with CCS. Cathode Bias Resistor: The 68 ohm cathode resistor provides for a minimum bias on the tube and controls the maximum gain of the stage. Suitable for tube model EL34,KT88,6L6,6V6,5881,6550,KT66,KT100,KT120,7027 etc. One way of implementing an external bias to a stage is to 'lift' the cathode by connecting it to some suitable positive voltage. Compared to traditional anode PECVD technologies, Samco’s proprietary cathode coupled PECVD technologies show various advantages. I removed the 1 ohms bias resitors to ground, the bias feed from the standby switch, the NFB wire and grounded the 220k junction. You may need to play with that bypass cap. 5) Tunnel diode & Gunn diode - These diodes are special diodes which exhibit negative resistance. Unlike fixed bias you don't have to adjust the bias manually in the cathode bias design. Some power tubes are also operated with cathode resistors so that a separate grid bias supply voltage isn't required. cathode bias circuit voltage resistor Prior art date 1960-12-29 Legal status (The legal status is an assumption and is not a legal conclusion. This article focuses on the power section where the fixed version reigns supreme; the grid of the tube is pulled negative, while the cathode is at (or close to) 0 volts. Re: Cathode Bias and Fixed Bias from the mesa man himself: "The short answer is that during my 12 years of repairing Fenders, one of the most frequent problems I saw was bias controls that were either set wrong or that had wandered out of adjustment due to vibration. I did the math, and for an EL34 with 450V on the plate, and a 70 ohm cathode resistor, if you bias it for 66% static dissipation, it comes out like this: Va = 450V. 5 watts) Divide your desired wattage by your plate voltage from step 3. As long as you know (or can test) the plate voltage in your amp, and have a way to measure the idle current, you can determine where to bias it, using our calculator. This does two things, it keeps the output stage from completely clipping out and it does compress when pushed hard. It utilizes two EL84 output tubes in a push-pull configuration, and has switchable fixed- and cathode-bias on the power tubes, and an ultralinear / pentode switch on the output transformer. The voltage drop developed across this resistor and the IF gain control is the operating bias on the tube. If cathode current is 40mA instead the resistance drops down to 1Ω: The 6e5P/6e6P will run comfortably around 30-40mA and bias tends to be around 3. I experimented with different operating points using the 7236 tube, but found that this was about the optimum for lowest distortion. Jealousy packed in big guys. Operation with zero grid bias simplifiers assocated circuitry by eliminating the bias supply. A cathode biased amp will adjust itself over a certain range, but tube sets that draw a lot could potentially run too hot in the amp. Increasing the cathode bias resistor is often necessary. 25 Bias Capacitors: BC 10uf Axial 160v: $2. speed and responsivity of the photodiode can be controlled. 2 - 6N14N (NOS EL84, cathode biased) 3 - 12AX7 1 - 5AR4 rectifier: Volume, Treble, Mid, Bass, Cut, Master Volume: Post Phase Inverter Master Volume: Variable Boost Footswitch: N/A: Buffered F/X Loop: Head 1×12 studio 1×12 standard 2×10 2×12: NOS mil-spec 6N14N power tubes, Hi/Lo sensitivity inputs: 4/8/16 Ohm: MAZ 38 NR Mk. I would like to upg. When the Beatles returned to tour America later in 1964, John Lennon and George Harrison were also playing their guitars through cathode biased, thin edge AC-100 heads. Because of the modulation described above, the cathode voltage is not constant. Cathode Bias Note: The Pro One uses the cathode connection as its ground reference for the plate voltage measurement. The Slant 6V can be operated at half-power in cathode bias (18W) or fixed bias mode (22W), yielding tones reminiscent of early tweed and AC-style combos in cathode mode, and a punchier, more tightly defined sound in fixed bias mode. When two electrodes are at the same potential, they are said to be at zero bias. 2 times the original. The fixed bias is a bit more "in your face", but has somewhat clearer articulation, at least to my ear. Cathode biased power tubes will have a resistor and bypass capacitor connected in parallel from their cathode to ground, as seen with most preamp 12AX7 triodes. Plasma Physics Page 13 DC Glow Discharge • Positive ions are accelerated toward the negative electrode (cathode). This type of bias is sometimes referred to as self bias, or a few other “marketing speak” terms. When you have current running through a resistor you get a voltage drop across it. Posted Oct 6, 2009, 9:35 AM PDT 0 Replies. The bias current is set purely by the input bias voltage. I would opine that the onset of breakup is a bit more gradual in the cathode biased mode, perhaps sounding a bit more organic as a result. As such, fixed bias squeezes the most wattage from a pair of output tubes while also sounding punchy and tight in the process, relative to cathode biasing. Cathode Bias (Self Adjusting) Cathode bias amplifiers shouldn’t require any adjustments and will work with a wide range of tube plate currents, as the circuit is “self adjusting”. (In triode anode and cathode currents. Cathode bias is set by a cathode resistor, and since resistors come in certain values, getting the right value may require wiring several together, or in settling for the closest nominal value. Having selected our operating point at 85V, we know anode, and hence, cathode current. Cathode bias means non-adjustable and it varies up and down automatically with conditions (so it's 'not fixed'). The cathode bypass capacitor in tube amp forms a filter that reduces the low frequency gain of the a common cathode amplifier. Junction J1 and J3 are in reverse biased and J2 is in a forward biased condition. If a current of 2 milliamperes flows through the tube under quiescent conditions, 20 volts are dropped across the plate-load resistor. cathexis synonyms, cathexis pronunciation, cathexis translation, English dictionary definition of cathexis. Fixed bias applies a negative voltage to your grid, forcing it below the cathode. almost same 6. Regarding tube safety, the self-bias cathode follower slowly establishes its working voltages, as the triode must first conduct before the high-voltage bias voltage can be established; whereas the fixed-bias quickly develops the high-voltage bias voltage at the grid, but cathode voltage takes much longer to catch up. 1, assuming a supply voltage (V cc ) of 10V using a transistor with a common emitter. Cathode voltage swing depends on the grid to cathode voltage required to move the tube through the conduction desired or required. And that's with 1ohm resistors. The wire from the bypassed end of the choke was wired to a 100K ohm resistor connected in series with the original 24 volt zener diode used for operating bias. 2 – 6N14N (NOS EL84, cathode biased) 3 – 12AX7 1 – 5AR4 rectifier: Volume, Treble, Mid, Bass, Cut, Master Volume: Post Phase Inverter Master Volume: Variable Boost Footswitch: N/A: Buffered F/X Loop: Head 1×12 studio 1×12 standard 2×10 2×12: NOS mil-spec 6N14N power tubes, Hi/Lo sensitivity inputs: 4/8/16 Ohm: MAZ 38 NR Mk. Both claims are generally not true. cathodal: ( kath'ō-dăl ), Of, pertaining to, or emanating from a cathode. Generally speaking these amps are considered plug'n play when it comes to replacing the tubes. Prior to joining Mercom, he worked in many roles as a business correspondent, assistant editor, senior content writer, and sub-editor with bcfocus. The more current that is flowing, the higher cathode voltage. The video below explains the difference between bias probe types. The pentode vacuum tube or pentode valve consists of five electrodes - hence the name pentode. download data sheet : Quick reference data Anode current I a =48mA fixed bias: 0,3: Cathode to heater voltage: V kf: 100 (V). The cathode bias method uses a resister in series with the cathode of the power tube(s). This fixed bias replaces the cathode resistor and fixes the tube into an operation point. When a diode is connected in a circuit, with its anode to the positive terminal and cathode to the negative terminal of the supply, then such a connection is said to be forward biased condition. Fender amps made prior to 1963 carried this type of Bias-based Tremolo circuit - but it was labelled "Vibrato" on Fender amplifiers. Use 50VDC or higher VDC rating. (In triode anode and cathode currents. As I remember this gave around 100mA idling current at 6KV. Here's the cathode-bias arrangement that Fender used in their 5B6 Bassman. The ground wire is used to bypass the amp's original cathode bias components so that the Yellow Jacket® converter's internal bias components are used instead. 56 volts as their default "correct bias measurement" voltage, so some amps have 15. A fixed-biased cathode follower is more likely to behave the way you expect it to on paper, than a self-biased one. As such, fixed bias squeezes the most wattage from a pair of output tubes while also sounding punchy and tight in the process, relative to cathode biasing. Overview The Nude Tube vacuum pressure sensor is a Bayard-Alpert style ionization vacuum sensor, which uses a fine wire collector located in the center of a grid. Scanned by artmisa using Canon DR2580C + flatbed option. For your interest, all preamp valves are cathode biased. What is Cathode? Cathode is the type of electrode in the electric device that usually works as the negatively charged terminal. The Fender 5F1 Champ is a Class A cathode biased amp.   What amp are you talking about, and can you post its schematic?. This enables the tech to alter the negative grid voltage by replacing the resistor to gain the current draw required. You simply plug the Yellow Jacket into the amp's output tube sockets, plug the provided EL84s into the Yellow Jacket, turn on the amp, and play. Power Tube Data RAM Power Tubes are tested for the following parmaters at screen voltages of 300, 350, 400 and 450 volts to allow the use of the data for any power amplifier. Viewed 2k times 6. Design the DC fixed bias conditions for the simple class A common emitter amplifier shown in Fig. Bias on stock Blues Jr is very "hot", so I keep R51 and I replace R52 with a pot to "cool down" the bias. So the bias point (on the bias wheel) from one amp to the next will vary quite a bit to arrive at the same reading, say 60mv. Cathode’s current flow in the electric circuit with respect to anode can either be positive or negative. Here the cathode is biased at 1. if EL34 get 8 W ,probably to get 6. 5 W rms when use 6L6 family. I was reading Myles' info on the Mazerati today, and in one section he talks about a "fixed bias phase inverter", but also says the Maz is "split load biased", then talkes about how. Use 50VDC or higher VDC rating. Ag/AgCl) and their subsequent deep reduction to azo at the more negative potentials (−1. The more current that is flowing, the higher cathode voltage. do NOT contact me with unsolicited services or offers. (If you're in for some preamp-tube-swapping fun, download our comprehensive PDF. The ground wire of the YJC is used to bypass the amp’s cathode bias components so the Yellow Jacket® converter’s internal bias components are used instead. You should always avoid having more than 430 volts across a 6V6 (plate voltage minus cathode voltage). ) Or, as every Marshall shows, 440V 3. Cathode biased power valves cause increased inter-modulation distortion and have a poor transient response. Ask Question Asked 2 years, 3 months ago. An IGZO film deposited on a G6 substrate (1,850 mm×1,500 mm) by this cathode marked a thickness distribution of ±2. if using as single ended amplifier as EL34 ,can be use 6L6. ) b bL 1 b Lo B,. This is why you never have to bias them. Here the cathode is biased at 1. Self-bias definition, voltage developed on an electrode in a vacuum tube circuit as a result of current flowing through a resistor in a lead to the cathode or to a grid. There are two common ways of setting the bias in a tube: fixed bias and cathode bias. This is not necessarily a bad thing. January 2014. 6v6 or 6L6. Kicks the fork. That's called fixed bias, and it has a similar effect. The plate receives a positive charge from the positive side of the power supply through the output transformer. The schematic shows the switch in the cathode bias position--the 220K grid leak resistors are grounded, and the negative bias voltage is disconnected. An EL84 tube is included. The cathode and anode are simply two terminals of a semi-conductor junction. Cathode Bias In the absence of a cathode resistor, with both the grid and the cathode seeing the same voltage, the triode's r p offers the only controlling opposition to the flow of current through the triode. Diving into it a little more on the internet I'm finding that cool for fixed bias is downright cold for cathode bias. Bodie with zener-assisted cathode bias I finally got around to trying the zener trick I mentioned a few months ago. Withous this negative bias the tube will conduct its full possible current and burn up very fast. (example: 15/450 =. When a diode is connected in reverse biased, current does not flow through the circuit, as shown above. In this 4th video in the Resistor Video Series, we discuss the purpose and function of the Cathode Bias Resistor. This local negative feedback can be significantly neutralised by. Cathode Bias (Self Adjusting) Cathode bias amplifiers shouldn't require any adjustments and will work with a wide range of tube plate currents, as the circuit is "self adjusting". Cathode Bias Resistor: The 68 ohm cathode resistor provides for a minimum bias on the tube and controls the maximum gain of the stage. Cathode Ray Tube.