El84 Cathode Bias

Another component, called a control grid, is placed between the anode and the cathode, and it looks like a mesh or screen. Like the thirty watt Vox AC-30 it was intended to replace, the Vox output stage of the UL430 had four EL84 tubes in a traditional cathode biased, parallel push-pull circuit. Telefunken power tubes are some of the best and most reliable power tubes. The power supply is fairly simple and compact. Number of Tubes that share a cathode resistor:. ) Class A, all tube circuitry, 4 x EL84 output tubes, cathode biased, spring reverb. Modern players have the advantage of an extremely wide “acceptable” tone range to experiment within. Clear and clean sound. even Mullard EL84 old tube if you can get too. The results are really not too bad with the -3db point near 120 Hz. 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. Some add capacitance, some add regulators. This product has been discontinued or is no longer sold by SameDayMusic. 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. Bias current is measured by attaching an external ammeter to a set of test jacks, and activating the each tube, in turn, with a bias activation switch (shown at left). This displacement is caused by the effect of increased anode and screen-grid currents on the cathode-bias circuit. So to answer the original question, you can just replace the tubes without worrying about biasing the amp. The cap bypassing the current regulator needs to be at least twice the "bias" value (voltage drop from cathode to ground): for an EL84 22V is a good starting point among standard values. 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. for comparison purposes. 91) I agree with you that EL84 amps are great. Continued from (Electrostatic Cathode Ray Tube Project 1) datasheet is at the link. Everybody says it will, but I don't hear it, and I've tried it on probably 20 different amps…. The heating/cooling cycle inside an AC30 easy bake oven seems to loosen up the mechanical structures within. Cathode bias will subtract the bias voltage from the ultimate peak-to-peak output swing and that we can do without. Regarding Deep-Switch on the 20/20 of the Marshall EL84 - there is no cathode bias capacitor on V1, the deep circuit is just made by a filter in the negative feedback loop - just as the presence control. This displacement is caused by the effect of increased anode and screen-grid currents on the cathode-bias circuit. 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. MM, I believe that this amp is Class "A"--- cathode biased via a common resistor/bypass cap from the cathodes of the EL84 to ground. TECH INFO 6V6 - 6L6/KT66 AMPLIFIER PROJECT CIRCUITS · 6SN7 HEADPHONE AMPLIFIER · OUTPUT TUBE BIAS SUPPLY FROM HIGH VOLTAGE. We also know that using a cathode resistor drops the power output. Input sensitivity is >=500mV for 10W output. It's really nice sounding. This connection is broken and and a wire taken form the supressor pin to the moveable tap on the potentiometer'. In other words, there are no adjustments to make and no modifications necesary, you simply plug the Yellow Jackets into the amp's output tube sockets, (in cathode-bias amplifiers) screw the ground wires under one of the output tube socket mounting screws, plug the provided EL84's into. So a 100ohm potentiometer had to be included. The de Lisle Nickel Box is a cathode biased 5+ watt single-ended design that features an EF86 preamp tube, 12ax7 driven tone stack, and an EL84 output mugsy , Apr 28, 2009 Comments. In Reverse Bias, the anode terminal of the voltage source is connected to the N-type pin and cathode terminal of the voltage source is connected to the P-type pin of the diode. The last few amplifiers that I've designed used pentodes and lots of NFB to get low distortion. the correct tube called for by the amp maker) power tubes in a cathode biased amp, pop them in, turn the amp around backwards (so you can see the tubes), turn out the lights, and start bashing away on some sustained, loud power chords. From what I learned you'd like a voltage rating for the caps that is 1/3 more than the actual voltage there for a 50% extension of life so an EL84 SE is typically 10 volts or less for the cathode bias so a 16 volt would be fine. The fixed bias phase inverter is a design that I've never seen on an amp outside of the Carmen Ghia and Mazerati. The Bassbreaker 007 has one EL84 in a single-ended, cathode-bias arrangement. How Vacuum Tubes Work by Eric Barbour - [Inside a Tube] - [Why are Tubes Still Used] - [Using Tubes] The BASICS. The subject of output tube biasing has many otherwise rational men arguing amongst themselves like little school children. Fit for ECC83 12AX7 ECC82 12AU7 ECC81 12AT7 12BH7 6CG7 ECC99 ECC88 6922 E288CC tube. A self-biasing amplifier, also know as a cathode bias amplifier, has a cathode resistor that sets the bias current of the tube. Cathode-biased power tubes have a bias resistor connected from their cathode to ground. In this type of bias, the characteristic curve of the diode is shown in the fourth quadrant of the below figure. 1x ECC832/12DW7 reverb drive/recovery tube. When I had my BB15 I checked the bias and they run those tubes very hot they were around 42 mA, tried to see if there was a bias adjust in that amp and it was a PITA just to see the circuit boards. i would agree with caev's assessment on the tubes. Cathode biased amps are self biasing as you know. Do you just measure the el84's only and don't worry about the preamp tubes. , plus you can mix different output tube types and blend their volume levels. This resistor must be oversized to dissipate more than 3W. Output Stage The output stage consists of two output pentodes, type EL84, in a cathode-biased, push-pull circuit. Higher B+ (up to 380 to 400VDC) can be used, just reduce the plate current accordingly, to 30 mA or less. But then, this amp is not pure cathode biased in stock form as they have a negative voltage bias supply circuit that feeds the control grids of the 6V6 valves. Buy with confidence as the condition of. I also had an old Raytheon that we still use in my son's Model 1 Kalamazoo, because it sounds so nice. gain stage and cathodyne phase inverter • Volume and Tone controls • 2 position Notch switch with black face mode • 84 - 60’s British • 2xEL84 18W Class A cathode biased power amp • NOS 5879 pentode gain stage. This Yellow Jacket converter is a specialized adapter which permits the use of EL84/6BQ5 power tubes in place of 6V6 type tubes. It can accept a wide variety of tubes, including EL34, KT66 and KT88, etc. If left unchecked, the problem will only worsen and can eventually lead to serious (expensive) damage to the output transformer, or power supply. Unless you take the plate voltage into consideration, a current specification is meaningless. The Morgan JMI is a 20-watt, cathode biased, EL84 all-tube amplifier in the style of the old British “top boost” design. The tube simply biases itself, due to the amount of current it draws from the plate supply. An interchangeable North American type is the 6BQ5 (the RETMA tube designation name for the EL84). Then connect a cable from the cathode to a voltmeter ( 1mV = 1mA). Biased Opinions. Bias current is measured by attaching an external ammeter to a set of test jacks, and activating the each tube, in turn, with a bias activation switch (shown at left). Higher B+ (up to 380 to 400VDC) can be used, just reduce the plate current accordingly, to 30 mA or less. If fixed bias is near cutoff the stage might not oscillate. It employs 2 cathode-biased 6V6's to produce a modest 14 watts of power. 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. Audio tube amplifiers: Cathode Bias or Fixed Bias, Triode Connected Amplifier, Cathode Loaded Output Circuit, Transformer coupling, The EL84 / 6V6 PP Dynaco A. The current of the two sections flows in this resistor, the result is a smaller value that it would have been if directly grounded the usual way. Fender amps made prior to 1963 carried this type of Bias-based Tremolo circuit - but it was labelled "Vibrato" on Fender amplifiers. (Sorry if this is a. In cathode biased amps you can generally plug and play for that hot creamy “woman tone” that so many desire. The tubes are AC heated as I think that DC heating make the amp sound too dull. In other words, there are no adjustments to make and no modifications necesary, you simply plug the Yellow Jackets into the amp's output tube sockets, (in cathode-bias amplifiers) screw the ground wires under one of the output tube socket mounting screws, plug the provided EL84's into. When pushed hard into the land of the square wave they remind me a lot of a good 6V6. By owning a bias probe you're not at the mercy of a tech anymore and you are now in control of your tone. Telefunken power tubes are some of the best and most reliable power tubes. The cathode resistor is an energy. i would agree with caev's assessment on the tubes. But then, this amp is not pure cathode biased in stock form as they have a negative voltage bias supply circuit that feeds the control grids of the 6V6 valves. That square wave has a period of 40ms, frequency of 1/40ms=25hz. These are the GT 4-7 range, and the Fender "white" painted tubes. The de Lisle Nickel Box is a cathode biased 5+ watt single-ended design that features an EF86 preamp tube, 12ax7 driven tone stack, and an EL84 output mugsy , Apr 28, 2009 Comments. Is the cathode pin3 150r, the plate pin9 100r, are the screen resistors pin2 8k2 for the el84's. Triode type for most cathode-biased 6V6, 6L6 and EL34 amps. 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. When I had my BB15 I checked the bias and they run those tubes very hot they were around 42 mA, tried to see if there was a bias adjust in that amp and it was a PITA just to see the circuit boards. This amp features. Yellow Jacket® Converters not only rearrange the pin locations of the tubes, but also provide the necessary current limiting on the screens and cathode as well as blocking the amplifier's grid bias voltage, while configuring the EL84 in a Class-A, Self-adjusiting cathode-bias circuit. That's easy: just take the screen-to-cathode voltage and divide this by the triode mu of the valve, i. The Bias Rite Cathode Current Monitor is a self contained unit designed to allow the user to set the desired idle cathode current value with the G1 bias control. 12) Posted by Sidegetter on 2005-12-18, 14:52:17 (65. The high transconductance factor simplifies the design of driver stages and offers convenient self-biasing (cathode biased) operation. JJ EL84 PDF - Buy Used and Save: Buy a Used "JJ Electronics Amplifier Tube (T-ELJJ-MP)" and save 52% off the $ list price. The data sheets seem to indicate that the 6sj7 remains fairly linear more or less to cutoff. So, here is a rundown of how these tubes differ in sound. What is it supposed to do and what does it actually do in the opinion of the forum members. Using a matched set of properly biased tubes will yield a clear smooth sound that is fairly warm and transitions into a smooth breakup with that singing tone that EL84’s are known for. Most auto bais is quite reliable however. 5V Chinese/Russian valves 15 V Golden Dragon and NOS After modification BTW a new Doge 6210 is coming out which will use a circuit board instead of point to point wiring. If you really want to use the EL86 in place of a EL84 you'll need to adjust the bias on the EL86 otherwise the plate current will be way too high. Check with your amp tech to be sure your amp's voltages don't exceed the maximum ratings for an EL84. With cooler grades the dissipation is around 80 to 90% and the tone is still warm and juicy. Cathode biasing is very commonly found in lower powered amplifiers and even more so in amps that use EL84's. From what I understand of the cathode resistor method, you are measuring a combination of plate and screen current thru the resistor. I used paralleled 15K/5W Beyschlag carbon film ones. Normally they are used in a very simple circuit that features no negative feedback and are run in the cathode biased mode which gets close to them being run in class A, which makes for a. The Bias Rite Cathode Current Monitor is a self contained unit designed to allow the user to set the desired idle cathode current value with the G1 bias control. Some power tubes are also operated with cathode resistors so that a separate grid bias supply voltage isn't required. This gives cathode biased amp their awesome sustain. I have done that in a fixed bias single ended amp with a 6L6 tube, without any major effort. 3) Choose a desired cathode bias voltage and divide by the cathode current for the resistor value. Sovtek EL84M - $35. These tubes are run as cathode followers, directly coupled to the grids of the KT90′s. 8w cathode biased, 10w fixed adjustable bias Nice breakup at a lower volume, natural power amp overdrive. An interchangeable North American type is the 6BQ5 (the RETMA tube designation name for the EL84). 3 V Golden Dragon and NOS Voltage across 200ohm cathode resistor EL84 6. This product has been discontinued or is no longer sold by SameDayMusic. It's a unique sound, subtly different from any common preamp config. The high transconductance factor simplifies the design of driver stages and offers convenient self-biasing (cathode biased) operation. Cathode-biased amps, on the other hand, which are often billed as "Class A amps," have a bias level that is set at the factory with a fixed resistor. DIY All DC coupled EL84 SE Amplifier The two stages of the Aikido driver are DC coupled and I thought it would be nice to have the final stage DC coupled as well. For use with octal tubes EL34, E34L, 6CA7, KT66, KT77, 6L6, 6V6, 5881, 6550, KT88, KT90. How to Bias a Tube Amplifier. As per usual when putting in new, properly spec'ed (i. The JJ tubes also sounded very good. This is what is causing the variation. Tubes on the other hand now need to be biased properly for extreme performance, the tubes need to be perfectly matched. The Mini Dirty Shirley is a cathode biased amp, so no tube biasing required. Unlike the AC-30 and UL730, the UL430 power amp stage included a negative feedback loop. This sets the grid voltage negative in relation to the cathode and establishes the proper operating current. If the anode voltage is too low, the phase splitter bias will be too high, and the stage will operate away from the linear position of the valve characteristics with a resulting increase in distortion. If the tube draws more current, the voltage drop across the cathode resistor increases, which increases the negative grid bias that the tube sees, thus reducing the plate current. We have some users run into problems using Psvane EL84-TII over long term in this type of design. 375V, hence 34mA per tube at the cathode. Forward Bias. 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. All tubes can be set to a fixed bias or with mod, an auto bias of sorts. Includes a pair of Yellow Jackets converters and a pair of EL84 tubes 50% to 70% drop in output power Perfect for getting killer power tube break up at lower volumes or taming amps that are too loud For use with amp that range from 20 to 100 watts Comes with a ground wire for cathode-biased amps. 95 pair/duet A military-spec version of the standard Sovtek EL84. But the web provides such an accessable and immense resource for such a purpose. The tubes that readers ask about most often are the EL84, EL34 and 6L6. Using cathode bias can serve as auto bias. Click here to buy complete sets of tubes for your Z amps from The Tube Store. Long time discontinued but worth the effort to source some NOS. Which Amps Are Cathode Biased and don't need rebiasing when you change the power valves? For reasons I won't bore you with, it's only low power amps which have cathode biased. Input sensitivity is >=500mV for 10W output. Adjustments must be made while the amplifier is under load - either a speaker, or a dummy load (power resistor). 6BQ5 / EL84 tube data Key: Rp=8k PP fixed bias. What you really want to know is the plate voltage, and then set current draw of the tubes by adjusting the bias voltage. Used Telefunken EL84 Vacuum tubes for sale on + second hand hifi sites & shops. Crate V33-212 V-Series Guitar Combo Amplifier (33 Watts, 2x12 in. Being prone to mechanical noise in a combo amp, the EL84 tube is not recommended for amps with poor air circulation. The 12AX7, EL84 & 6AQ5 Tubes are NOS Tesla and the Output Transformer is a Mercury Axiom, hand-picked by Patrick from Mecury Magnetics. 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. Add a 150 kOhm/1W resistor tied from the plate of 12AX7A to the plate of 12BY7A. Contains the following capacitors, used as main filters, decoupling filters, bias filter, footswitch suppression, and cathode bypass capacitors in the Fender `65 Twin Reverb Reissue amp, produced from 1991 to the present and the Fender `68 Custom Twin Reverb amp, produced from 2009 to the present. Also here, I designed it using the approach described in my book. Boost Footswitch for Channel 1 that disables the Treble and Bass controls and provides gain and volume boost and front panel LED indication. The high transconductance factor simplifies the design of driver stages and offers convenient self-biasing (cathode biased) operation. Each cathode is "looking at" the other cathode. By Rob Robinette. Seperate with a common mode cathode resistor for each tube pair and bypass. This occurs because the free electrons around the cathode are repelled by the negative potential at the anode, as shown in Fig. So, here is a rundown of how these tubes differ in sound. TAD ToneBone converters do permit to replace 6L6GC, 5881, EL34, KT66 and 6V6GT for EL84/6BQ5 tubes and operates them in pure CLASS A. In cathode biased amps like the VOX they get a premium workout and will suffer some effects from heating and cooling. Billy Rocker: 16-watt cathode-biased 6V6 or EL84 output tubes operating in class A push-pull with 5Y3, 12AY7, and 12AX7/7025 tubes. EL84 vs EL34. Plate Dissipation (Watts) Note: For calculating plate dissipation in cathode biased circuits, subtract the measured cathode voltage from the measured plate voltage and use that as the plate voltage in the calculator above. Harsh blurpy attack. For fixed bias, the equivalent would be to use a B+ of approx 350VDC, and a grid bias of -13 to -15VDC. Amps with the cathode resistor you mentioned are cathode biased and generally run at a higher idle dissipation. Bias current is set by adjusting a 20-turn trimmer pot with a small screwdriver. Super Goldtone is the next level – if you’re ready for it. Working chassis to fit in your Blues Junior cabinet $1099. PSVANE Vacuum Tube EL84-T Mark II T-Collection Grade Matched pair PSVANE EL84-T II 100% Brand New The price is for 1 pair(2 pcs), if you buy 2pair(4 pcs), we will send the Factory full matched tubes to you. The EL34 has separate g3 (pin 1) and cathode (pin 8) terminations, whereas the EL36 does not; in the EL36 both g3 and cathode appear at pin 8, while pin 1 is “n/c”. 35ma anode current. For your interest, all preamp valves are cathode biased. 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. In this type of bias, the characteristic curve of the diode is shown in the fourth quadrant of the below figure. For a typical output stage working from a 3l0V HT supply and using EL84 valves, the rise in cathode current -and thus in cathode bias voltage- with a sinusoidal signal voltage is about 40% at full drive. Checking the bias current in a cathode biased amplifier is easy, just measure the voltage across the cathode resistor and divide by the resistance value to obtain the cathode current. This resistor must be oversized to dissipate more than 3W. The Special 8/8 “Double Eight” features a unique dual output-stage design, with two cathode-biased 6L6GC tubes connected in parallel, and with a separate Master Volume control for each tube. Either 2 EL84 or 2 6V6. The gm under these bias conditions is about 1400 umhos (710 ohms). com Make of Guitar Tube amplifier kits Our Pcb based kits are the fastest way to built your amp We have. COM Equiv to US$559. With a Kalamazoo Model One or Two, the B+ should be in the neighborhood of 250 to 275 volts. You can also use individual cathode bias resistors on each tube. Traynor ran their EL84/6BQ5's at 420v,but had lower current and dropped the screens down to sane levels. When they say capacitor-coupled bias supplies can turn into charge pumps if they're not sufficiently loaded, they aren't kidding! God bless lightbulb limiters. 2 watts to 0 output. As per usual when putting in new, properly spec'ed (i. Conveniently light weight and portable, the Komet 29 is powered by four of cathode biased EL84 / 6BQ5 output tubes. The 6BQ5 is the weak link here, allowing no more than 100 volts differential between cathode and heater - which is a lot more than we plan to use. The drawbacks are the EL84's shorter life span and higher distortion. The stage is self-biased, so negative DC grid bias is created by raising the cathode voltage relative to ground via the 47Ω cathode resistor. WELCOME TO PHAEZAMP. As I have no bypass cap in the preamp either, gain is a little low for my moderate output EV mics (vol will hit flat out with 6V6), fine for the hotter mics in cathode bias. Amps with the cathode resistor you mentioned are cathode biased and generally run at a higher idle dissipation. The bias current is set purely by the input bias voltage. The cathode resistors for the output tube are of the new type in TO220-package (partly visible left of the octal socket). When I noticed that I set it to the more normal -7v, but the stock design with 2. same circuit can be use for EL34 single ended amplifier only 2 parts need replace ,Power Trans and Out put. The amp has a true plate voltage (-cathode voltage) of 318V. It MIGHT be OK if you add a 5-10K resistor in series with the screen. The two pairs of EL84 output valves are configured as push pull, cathode biased, class A power amplifiers. Current tube line up October-2019 Carmen Ghia. The AC30 push-pull power amp includes four EL84 pentodes, two for each phase. A classic 50 peavey is to compete with fender hot rod deville using 6l6 bottles and 60 watt so maximum output from the 4 x El84 is wanted. If the tube is protective biased, it has to be biased at a safe dissipation but must not be near cutoff. By owning a bias probe you’re not at the mercy of a tech anymore and you are now in control of your tone. Once again, Michael employs a very simple tone cut that follows our less is more for lower wattage amplifiers. 6L6 Tubes are the Classic American high-power valve. Remove the power tubes and Insert the power output tubes into the bias probe sockets. DIY All DC coupled EL84 SE Amplifier The two stages of the Aikido driver are DC coupled and I thought it would be nice to have the final stage DC coupled as well. I have likely built more EL84 amps than ANYONE, and this one is the champ $729. The bypass cap values are large (100uf input, 220uf output) so electrolytics are currently being used. 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. If your amplifier features cathode biased output tubes also, then you don’t need to bias that amp – ever. Some years ago a co-worker put JJ's in a small Mesa amp. Which Amps Are Cathode Biased and don't need rebiasing when you change the power valves? For reasons I won't bore you with, it's only low power amps which have cathode biased. - fixed adjustable-bias amplifiers - fixed non-adjustable-bias amplifiers (no bias adjustment pot) - cathode bias amplifiers. They give you simple Class-A operation from any amp. Adapter Yellow Jacket Cathode Biased Converts 6L6 EL34 to EL84 6B This Yellow Jacket converter is a specialized adapter which permits the use of EL84/6BQ5 power tubes in place of 6L6 or EL34 type tubes. It's also common for cathode biased EL84 amps to run at 100% plate dissipation. Immediately following are equations for calculating cathode voltages, currents and resistances. The EL84 is normally a pentode, which means it has 5 elements inside the tube: anode, cathode, and three grids in between. Anyway, since the grid bias voltage should remain around -7. NOTE: In the video, plate voltage is measured. 5W will do) for a test. This allows a very stable bias injection scheme (to the grids of the EL84′s) as well as a very low drive impedance to the KT90 grids. an IF amplifier, so in effect, this can be used as a positive voltage source. With multiple tubes sharing the same cathode resistor, this calculator assumes the tubes are fairly well matched. You can add individual 1 ohm resistors from the cathode of each tube to the common bias resistor, but you must then measure across the 1 ohm resistors, not from the cathodes to ground, to determine the voltage drop, and thus the cathode current. Normally they are used in a very simple circuit that features no negative feedback and are run in the cathode biased mode which gets close to them being run in class A, which makes for a. As I have no bypass cap in the preamp either, gain is a little low for my moderate output EV mics (vol will hit flat out with 6V6), fine for the hotter mics in cathode bias. His amp has cathode bias for the output tubes so we couldn't adjust it. 05" to reflect a total current of 50 mA. Some fixed bias, either cathode or grid, would solve this issue. Fit for EL34 KT88 6L6 6V6 5881 6550 KT66 KT100 KT120 KT150 7027 tube. In cathode biased amps, the grids will be held at ground, but for a fixed bias amp you should have -20 to -60 volts here, depending on what kind of output tubes the amp uses. This list is our current tube type, location and bias points for all of the Dr. You have to determine how much gain you can stand to lose. These might seem stupid questions to you but I'm still trying to understand this stuff before i build anymore. The amp has a true plate voltage (-cathode voltage) of 318V. Some fixed bias, either cathode or grid, would solve this issue. Is the cathode pin3 150r, the plate pin9 100r, are the screen resistors pin2 8k2 for the el84's. A classic 50 peavey is to compete with fender hot rod deville using 6l6 bottles and 60 watt so maximum output from the 4 x El84 is wanted. EL84 Power Tubes provide 15/30W of Power in Class A modes. It employs 2 cathode-biased 6V6's to produce a modest 14 watts of power. combinations. the correct tube called for by the amp maker) power tubes in a cathode biased amp, pop them in, turn the amp around backwards (so you can see the tubes), turn out the lights, and start bashing away on some sustained, loud power chords. Like the thirty watt Vox AC-30 it was intended to replace, the Vox output stage of the 730 had four EL84 tubes in a traditional cathode biased, parallel push-pull circuit with no negative feedback. I've used EL84 in triode for. Recapping Guitar Amps. They give you simple Class-A operation from any amp. 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. Probe body is made of machined PA6. Along the same line you gain about 15 extra volts if you use a fixed bias rather than a cathode bias. Now the two cathode windings are used, while resistors R21 and R22 function as well, enabling you to measure the quiescent currents. In short – the cathode bypass capacitor “handles” any variations in the cathode current preventing the cathode voltage to vary as well. 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. The EL84 tubes are biased at 40mA, There should be ~ 11v on the 270 ohm resisters. For adjustable fixed bias amplifiers, the amp should be biased by a tech with experience. , plus you can mix different output tube types and blend their volume levels. Four 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. WELCOME TO PHAEZAMP. The tube drawing 22mA, cathode resistor for proper biasing will be 170V/22mA or 7. It doesn't add gain like preamp bypass caps. EL84 Single Ended AmplifierD. When I noticed that I set it to the more normal -7v, but the stock design with 2. 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. Granger Amplification Custom Shop Bias Probe (Current Measuring) [BIAS_PROBE_MA] - Measures true bias current. A regulated heater supply is a nice option for quiet operation. The cap bypassing the current regulator needs to be at least twice the "bias" value (voltage drop from cathode to ground): for an EL84 22V is a good starting point among standard values. Normally, bias adjustment is required only for output tubes (that is, the tubes in the output or power stage of the amplifier); preamp tubes are usually biased automatically at a fixed point using cathode resistors. These might seem stupid questions to you but I'm still trying to understand this stuff before i build anymore. DC bias voltage from reaching the control grids of the EL84s, 4) Giving the control grids of the EL84s their own ground reference resistor for gain control, 5) Giving the EL84 its own cathode-bias (self-bias) circuit to enable it to run safely and squarely in the Class-A range. So what one does is add a resistor from the junction of the LED anode and tube cathode and send that to B+ for some bias - LED's are turned on harder in their linear region and no operating point changes because they act as constant voltage devices Cheers!. So I decided to go to the other extreme: how low distortion could I achieve with no negative feedback of any kind. Anyway, since the grid bias voltage should remain around -7. When removing the jumpers, the cathode current can be determined via the exposed pins through. The circuit and its equivalent are shown in Figure 24. 375V, hence 34mA per tube at the cathode. The Special 8/8 “Double Eight” features a unique dual output-stage design, with two cathode-biased 6L6GC tubes connected in parallel, and with a separate Master Volume control for each tube. A very popular choice for the studio or small club (or any venue when using a microphone). WELCOME TO PHAEZAMP. Biasing an amp requires some knowledge of tube amp circuits and experience with high voltage. Decreasing the bias has the opposite effect. You can add individual 1 ohm resistors from the cathode of each tube to the common bias resistor, but you must then measure across the 1 ohm resistors, not from the cathodes to ground, to determine the voltage drop, and thus the cathode current. Running a pair of these big bottles in a cathode-bias, Class-A configuration results in the very loud 35 watt power section of the original tube-rectified Supro Thunderbolt. If left unchecked, the problem will only worsen and can eventually lead to serious (expensive) damage to the output transformer, or power supply. 6CW5/EL86 will probably go nuclear in a 6BQ5/EL84 circuit - screen voltage will be over its limit. Long time discontinued but worth the effort to source some NOS. The quiescent current of each power stage is set by R71 and R73, which are bypassed by. Build your own bias adapter with our TubeDepot Bias Scout kit and you'll save money every time you set the bias on your amp. When sustained sine wave is applied to the control grid, the bias across the cathode resistor will readjust itself as a result of the increased plate and screen-grid cur- rents. The cathode biased quartet EL84 output section is responsible for a beautiful natural compression that supports the player. WARNING: A tube amplifier chassis contains lethal high voltage--sometimes over 700 volts AC and 500 volts DC. Best AUDIO TUBES listed at: 6BQ5 = EL84 / 7189 PENTODE TO BUY 6BQ5=EL84 TO BUY 7189 and 7189A 6BQ5/EL84 Near Equivalent = 6CH6/EL821 It is NOT a plug in equivalent for the 6BQ5, but a near equivalent in specs. With the addition of a Gain Control you are able to drive the Custom 20 into an awesome natural overdrive without the customary Hiwatt. So, here is a rundown of how these tubes differ in sound. Thus, in this circuit, where the cathode rests at +40V and its grid at +20V, the triode can never be cutoff. The EL84 tubes are biased at 40mA, There should be ~ 11v on the 270 ohm resisters. Fixed bias amps have a bias circuit that puts a negative voltage on the power tube grid. We also know that using a cathode resistor drops the power output. The cathode resistor is an energy. Tubes must be biased correctly to operate as designed. If you are trying to get the maxium power out of a EL84 tube fixed bias, controlled by a 10 turn screwdriver adjustable pot is the way to go. (Sorry if this is a. I like the 6V6 mod however. 91) I agree with you that EL84 amps are great. The EL84 Class-A cathode biased circuit can be set to either 6 or 12 watts. EL84 datasheet specifies that maximum grid to cathode resistance, with fixed bias, is 300K Ohm. I can hear a slight increase in output volume when I switch over to fixed bias, though it is barely audible. As you hit a note the tube calls for more power and negative voltage on the cathode decreases briefly changing the tube's bias. A classic 50 peavey is to compete with fender hot rod deville using 6l6 bottles and 60 watt so maximum output from the 4 x El84 is wanted. Those “small” tubes visible between the KT90′s are EL84′s (aka 6BQ5). It's really nice sounding. Tube Dissipation Using Cathode Resistor Voltage Drop Please enter Tube Type and Plate-to-Cathode Voltage above before calculating Tube Dissipation. Here is a PDF of the complete (scan amplifier) schematics, as of 20150225. Grid 2 normally has its own node on the power supply, but in triode mode, it is connected to the anode. Audio tube amplifiers: Cathode Bias or Fixed Bias, Triode Connected Amplifier, Cathode Loaded Output Circuit, Transformer coupling, The EL84 / 6V6 PP Dynaco A. PSVANE Vacuum Tube EL84-T Mark II T-Collection Grade Matched pair PSVANE EL84-T II 100% Brand New The price is for 1 pair(2 pcs), if you buy 2pair(4 pcs), we will send the Factory full matched tubes to you. For a typical output stage working from a 3l0V HT supply and using EL84 valves, the rise in cathode current –and thus in cathode bias voltage– with a sinusoidal signal voltage is about 40% at full drive. Bias current is measured by attaching an external ammeter to a set of test jacks, and activating the each tube, in turn, with a bias activation switch (shown at left). The subject of output tube biasing has many otherwise rational men arguing amongst themselves like little school children. Insert tube in the socket of the probe. 91) I agree with you that EL84 amps are great. But the web provides such an accessable and immense resource for such a purpose. Higher B+ (up to 380 to 400VDC) can be used, just reduce the plate current accordingly, to 30 mA or less. Famous fixed-bias amps include larger tweed and blackface Fenders, Marshalls JTM45 and Plexi, most high-gain amps made today and many others. Calculating Cathode Bias Resistor. 1 - Octal cathode current: Divide the cathode voltage (H-J) by the value of R17 (470Ω). Hi Joe - there are a few ways to bias the tubes and currently the most popular is fixed bias where you can adjust the voltage, some use cathode bias (which is more common than one thinks) and then we have auto bias. com Make of Guitar Tube amplifier kits Our Pcb based kits are the fastest way to built your amp We have. Meticulously assembled with only the best sounding and most reliable parts. Which Amps Are Cathode Biased and don’t need rebiasing when you change the power valves?. 6BQ5 / EL84 tube data Key: Rp=8k PP fixed bias. I have read not to use these el84 in cathode bias amps just like many new production tubes and 12ax7 made today also do not last a week in cathode follower positions in preamps !.