Personally, i have never replaced a 6bq5 el84 due to failure. Fixed bias power tubes have their cathode connected directly to ground and a. 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. First it is important to clarify the fundamentals of how a guitar tube valve actually operates. The mini block tube amplifiers use a 5751 in the driver position and the el84 output circuit is biased into classa, ultralinear operation. Along the same line you gain about 15 extra volts if you use a fixed bias rather than a cathode bias.
Suppose we have a 6v6 working against an el84 in pushpull. A tube amplifier chassis contains lethal high voltagesometimes over 700 volts ac and 500 volts dc. The second important revision in the rh84 schematic is the replacement of the cathode resistor automatic bias with a ccs actually, current sink fixing the current draw of the tube. May 27, 2009 there is a lot of misunderstanding among guitarists regarding exactly how class a, class ab, cathode bias and fixed bias amps operate and differ from one another. For example, if a pushpull amplifier using el84 tubes is operated much above. El34 or el84, but inside of that range you need to set the actual bias voltage for the particular amp and for the new tubes being installed. When i started to set the bias, i couldnt get them to go much below 60ma and i was. Diy ecc802s srpp el84 6bq5 pushpull tube amplifier.
Im dividing voltage across biasing r by resistor value to get bias, then substracting the that current from the plate voltage and multipling by bias. This will be enough to bias the valve almost to cutoff, and we dont really need more than that. Sep, 2017 the original dynakit stereo 35 6bq5 el84 amp modified with dave gillespies enhanced fixed bias efb circuit is my favorite choice, especially for driving a pair of quad el57s. Drawing error the ecc803s heaters in the amp are actually wired as. Scott amplifiers employ fixed bias that is truly fixed. The bias cannot be monitored the same way as the previous fixed bias when using that circuit. Here is the circuit diagram of the classic 5w tube amplifier with el84 as the final amplification component. The original dynakit stereo 35 6bq5 el84 amp modified with dave gillespies enhanced fixed bias efb circuit is my favorite choice, especially for driving a pair of quad el57s. All the bias supply ends of the 120k resistors are bootstrapped to the cathode. The following post addresses this topic thoroughly. Dec 27, 20 red tube mainly due to the filtering cap on the negative bias leaks and short out the negative voltage.
An inexpensive, easy to build diy valvetube tester for quite some time i have been looking at developing a design for a diy valvetube tester that is easy to build and uses readily available, low cost parts. Homemade el34 6ca7 pp stereo tube amplifier from cathode to. Biasing an amp requires some knowledge of tube amp circuits and experience with high voltage. An interchangeable north american type is the 6bq5 the retma tube designation name for the el84. Vox ac15 have cathode biased output tubes el84 and these amps dont need biasing. The traynor circuit was a pretty close copy of a bf fender, but they used el84 s and ss rectifier. It uses a single, less than one dollar tl072cp op amp as the reverb driver and reverb. This amplifier was originally featured on the october 2003 issue of the highfidelity audio. The negative bias voltage is often designed to be adjustable by way of a bias pot. Hoffman sells an 18 watt transformer that would work great, but it currently shows as out of stock in the meantime, if youre ok with a ss rectifier, this aint a bad option. Measure mv across 1 ohm cathode resistors and directly convert to ma. What is still available is russian nos el84, called 6p14p.
The pushpull el84 tube amplifier schematic is from the book build your own audio valve. So my question is, what is the best way to reduce the voltage to an acceptable level. El84 ul pushpull stereo power amplifier with hashimoto transformers. Set el84 tube amp diy project sarris custom tube amps. Amps with a single el84 output tube are probably class a and will run normally at around 100% max.
Of course, the bias point must not be pushed above the maximum dissipation curve, and it is usual to bias well under it to avoid damage 70% of maximum dissipation. The el84 is a thermionic valve of the power pentode type. Sep 06, 2014 pushpull pp el84 tube amplifier schematic with ecc83 input stage. It is a thermionic valve of the power pentode type. 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. V2 and v3 are el84 in triode and in a differential long tail pair voltage amp. Just monitor closely on first powerup for any signs of red plating in the power tubes. This is a single channel amplifier mono of course with single input and single output. Adjustments must be made while the amplifier is under load either a speaker, or a dummy load power resistor. The pdf file below will give you a quick overview of bias amps license tiers standard and professional. Cathode biased amps are less efficient than fixed bias amps. The power supply design diagram already provided on above tube amplifier circuit diagram.
The cathode resistor that creates the bias voltage is typically not adjustable. The layout drawing shows where the tubes are positioned in the actual. I have a thing that plugs on all four sockets at once and lets me measure the actual current flowing for setting the bias on different tube amps. The et pro one bias probe is also available with our custom built 9 pin head. An amp builder is claiming that operating tubes at low voltage and high current.
Homemade el34 6ca7 pp stereo tube amplifier from cathode to fixed bias. I wanted something that was affordable, doesnt compromise on tone and is made in england. In fact, this is what we are doing whenever we adjust the bias on a fixed bias amp. Fixed biased amps require biasing to change power tubes as well as periodic bias checks to ensure they are working properly. There are rule of thumb formulas using the tubes wattage, the voltage of the particular amp and a percentage of operation that help a. Typical implementation of a fixedbias, ultralinear pushpull output stage with a pair of pentode tubes el84 6bq5. The jj electronic el84 is a solidly built el84 6bq5 with excellent tonal qualities. El84 power pentode components datasheet pdf data sheet free from datasheet4u. They are a somewhat hybrid of cathode fixed bias topology.
There is more information available about tube power supply design in the power supply design for vacuum tube amplifiers article. I will say most of the el84 amps ive owned would set it fixed no bias pot at anywhere from 14 to 11. Just understanding the principles of how your amp works is well worth the time and effort. I really didnt need another amplifier as i have lost count of how many are around the house, perhaps as many as 15 or 20. I can hear a slight increase in output volume when i switch over to fixed bias, though it is barely audible. This is a non center tapped pt that i will be using with a solid state bridge rectifier. I can use voltage division, however i dont know what the load current will be or the equivalent resistance. Fixed bias amps have a bias circuit that puts a negative voltage on the power tube grid. Not just my biased opinion try the following link for some interesting reading on the boogie bias subject. Its not difficult and it doesnt take long but if you dont know what youre doing you may hurt yourself amps can give lethal electric shocks even when unplugged, and no, were not just. Mine is cathode bias the traynor was fixed bias so the platetocathode voltage is actually a bit less than the original traynor. Zener diodes with fixed bias telecaster guitar forum. Grid bias is a fixed voltage, but the bias supply is often adjustable, so you can change the voltage at which the bias is fixed and optimize it for the tubes, essentially setting the idle speed. The dual el84, fixed bias power section delivers up to 28 watts in full power mode, and just 2 watts in low power.
It is not adjustable, it would need to be replaced with one of a different value to alter the bias, or better still convert to adjustable bias. Both tubes need to be drawing the same current, otherwise the net current through the transformer will not. Heater voltage vf 6,3 v heater current if 760 ma fax. Well be drawing the load lines for the second preamp stage v2a center left. The same amp in fixed bias would be about 40 watts. Sep 17, 2010 the 70% figure refers to fixed bias amps. You have to measure the voltage each time because the voltage and current will change with a bias pot change. I made my own layout from the version 8 schematic, using visio. Discussion in amp central station started by fenderguy53, may 6, 2009. Normally, bias adjustment is required only for output tubes that is, the tubes in the output or power stage of the amplifier. I guess i have to call it bigboy nfb at halfway, clipping start occuring at 200mv input. Originally designed for the output stages of audio frequency amplifiers, the el84 can deliver up to 1 7 watts.
We also sell a range of bias manuals written for specific amplifiers that you may be. That bias voltage is not out of the realm of proper for an el84 either, and by nature cathode bias runs hot. By that we mean theres not a bias control potentiometer to be found. The first circuit in the pdf shows an archetypal, small to medium power pushpull amplifier configuration that uses a triodepentode ecf83 in the frontend. The tube uses cathode bias, which makes the circuit simple and allows you to try different tubes without needing to. They are usually used for relatively small amplifiers, e. What you really want to know is the plate voltage, and then set current draw of the tubes by adjusting the bias voltage. If youre struggling to decide on which license to purchase, please see the comparison cha. A fixed bias pushpull amp has two advantages over a similar amp without bias adjustment. Fixed bias power tubes have their cathode connected directly to ground and a negative dc voltage applied to their control grid. It is used in the power output stages of audio amplifiers, most commonly now in guitar amplifiers, but originally in radios. With multiple tubes sharing the same cathode resistor, this calculator assumes the.
Jjs rugged construction of this tube offers high reliability without any compromise to the sound. I also have a stock jet city jca20h ltp, fixed bias, cathode follower before tone controls, master volume, positive feedback. Fixed bias in valve guitar amplfiers what exactly is it. This tube has solid lows with warm bright mids and shimmery highs. Im working on a schematic for a new project, push pull el84 power section with fixed bias. Aug 23, 2017 biasing is the process of setting a transistorsdevices dc operating voltage or current conditions to the correct level so that any ac input signal can be amplified correctly by the transistor. Grid bias is a fixed voltage, but the bias supply is often adjustable, so you can change the voltage at which. 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. With approximatley 335v at the plate of the el84s you might expect about 35ma. Boogies have a fixed resistor that sets the bias voltage. Actually never made any real circuit modifications before. That extremely high plate voltage didnt seem to bother them.
That limits us to cathode biased el84 or if using fixed bias it needs a. Bias tremolo addon circuit for small fixed bias power tubes. The fender 5f6a bassman and most marshall amps are fixed bias class ab amps. Assuming everything else is healthy, youre good to go. How to clamp down a el84 9 pin tube socket cool amp tools how a bias circuit works fender bias circuit conversions how to add a bias pot to a princeton and other amps fender cathode fixed power tube conversions filter cap hookup solutions bassman reissue filter cap hookup document how tos how to use the turret lug tool. The pairing is quite magical, and the original dynaco ultralinear output transformers have the damping factor for the quads. Yellow jackets tube converters allow el84 power tubes to be used in place of the. Bruce heran outlines the design and construction of monoblock pushpull el84 6bq5 valve amplifiers. Measures el84 current with optional noval proble set. If your amp has fixed bias with a bias pot make small changes when adjusting the bias pot and measure the plate voltage and plate current again. If your tube isnt listed then pick one with the same max dissipation rate which is shown to the right in the tube list. As you turn up the bias and flow more current through the power tube the demand on the power.
Yellow jacket tube converter technical information antique. Pushpull pp el84 tube amplifier schematic ecc83 input. I couldnt tell you what bias voltage will be since its meaningless. This is a fixed bias amp so not much to do after you measure it but it is a nice check to see where the tubes are operating or if you have one that is way mismatched. Atarashi designed this amplifier based on the el84 schematic diagram published by rogers in england.
The baby huey pp ultra linear el84 is a design by member gingertube. If you are uncertain that you are experienced enough to work on a live high voltage amp, please take your amp to a professional to get it biased. A switch found on some tube amplifiers that changes how the tubes are configured, and reduces the efficiency of tube operation, reducing the power output of the amplifier. Jan 19, 2018 the cathode biased 2 x 15 watt amplifier is made into a 2 x 33 watt fixed bias amplifier. Tube amp schematics, tube amp information, tube amp projects. Converts 6l6, el34, 6550, 7027, or 6v6 to el84 the yjs is for amplifiers with fixed bias power tubes. Amps with the cathode resistor you mentioned are cathode biased and generally run at a higher idle dissipation. In fixed bias amps we adjust the grid voltage to set the bias.
A triode tube has a control grid signal in, a plate signal out, and a cathode. The first, mentioned earlier, is the opportunity to adjust the tubes for the best compromise between sound quality and longevity. Vacuum tube el84, jj electronics antique electronic supply. And i know they are fixed biased which has always turned me off to em that and i dont like the high gain channel. In the meantime, if youre ok with a ss rectifier, this aint a bad option. Then its good to know how to bias tube amplifiers, or we need to keep paying a technician to bias tube amplifiers for us. After you try it you will find ltp quite stiff in comparison. Output tube biasing, an introduction wall of sound. Higher powers are obtainable with either very large power valves, or by using multiple valves in parallel, but beyond about 15w it is usual to use. For adjustable fixed bias amplifiers, the amp should be biased by a tech with experience. Home forums amplifier discussion forums amp central station biasing el84 s in a pro juniorblues junior. It utilizes two el84 output tubes in a pushpull configuration, and has switchable fixed and cathode bias on the power tubes, and an ultralinear pentode switch on the output transformer.
Measures the current of 4x any standard base octal power tubes 6l6, el34, 6550, etc. You should build the two similar circuit to make it stereo. Tube operation, cathodebias vs fixedbias, and class a vs. You know the amp has roughly 350 volts on the plates. Hhscott vacuum tubes fixed bias adjustment procedure. The et pro one bias probe is calibrated at 180 milliamps to plus or minus one tenth of one milliamp at 700 volts dc, so unlike other probes on the market the et pro one can be used on amps that exceed plate volts.
The telefunken el84 tk is a perfect replacement for any el84 6bq5 tube offering a reliable and great sound. Tube amplifier with el84 pushpull stereo madexp by. There is actually a wide range of valves that fit the bill, but the most common for guitar use are the el34 25w, 6l6gc 30w, or a pair of parallel el84s. Reverb and is best used for small power tubes like the 6v6 and el84 using fixed bias. Its a straightahead, giggable, nononsense valve amp. This project was an exercise in what would happen if i combined some interesting concepts. Fixed bias refers to the bias condition with respect to the signal cycle and means that. Those el84 adapters are designed to just plug in and work within a lower percentage of power output.
It varies with the characteristics of the tube and the load. Hoffman sells an 18 watt transformer that would work great, but it currently shows as out of stock. This will cause excessive current flow through the tube and get red hot. However as you can see it requires 300v 290v and 280v. Enter values in blue and click any calculate button select the tube type from the scrolling list.
Screen capture of the tube bias calculator webpage. From the schematic, the el84 only use 10v bias i can tell you, i almost never see a low voltage like this causing the cap to leak and short out. The el84 is smaller and more sensitive than the octal 6v6 that was widely used around the world until the 1960s. U a 300 v w a 12 w u g2 300 v w g2 2 w u g1 100 v i k 65 ma r g1 1 m. Obviously such a construction requires the appropriate speakers, particularly horn type designs, so that the output of the amp can reach an efficiency satisfying level. Once it was mounted i went to work wiring it up with the proper components that i specified off the schematic, and also confirmed on the pcb. Fixed bias amplifiers can be either adjustable or nonadjustable. I can guarantee that my 60 sqm living room are very well sonorized with half of that power. For years i have been using a valve tester that i had developed in the past. There are only two things to worry about in biasing a guitar amp. Then take your target idle current,say 70% 70% is considered the high sideyou can go coolerlower so multiply.
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