Tube amp bias circuit. After discussion of the theory of its oper.
● Tube amp bias circuit This will Make That Amp Better. Parallel output tubes & bias controls Many amplifiers have independent bias controls such as shown in Figure 2b. The tube's bias voltage places the control grid at a negative voltage compared to the cathode voltage. 1 others liked this since the negative volts were most often used to bias the output tube, by then usually a pentode. Tube has very thoroughly investigated this problem and has devised a simple and elegant but very reliable and affordable solution to this problem. To modify the bias circuit on your Mesa Nomad 45 tube amp, you will need basic electronic tools such as a soldering iron, wire cutters, and screwdrivers. Mark As an example, for EL34 tubes, we recommend a bias voltage of 425mV, for KT88 tubes we recommend 450mV. In my case I was trying to resurrect an amp that wouldn’t stay powered on for more than a few seconds, so I initially set the bias with no power tubes installed at all, and then tweaked it with them in once I got over the immediate blowing of the fuse. The KT88 have 220k and 6L6' bias circuits I remember use about 100k. Such as 6V6, 6L6, KT66, etc. If the heater is referenced to a d. The sages would surely need to know exactly what bias circuit you were talking about. Since a low bias voltage allows the tubes to draw more current, it's important to let the amp sit in standby for a while before switching it on to make sure the needed bias voltage is available. Thanks for the correction. Bookmark this page This page is full of links that lead to every information page on my web site. If your amp has fixed bias then carefully measure the DC bias voltage at all the power tube grid pins. Hoffman Amplifiers Tube Amp Forum, Grounding point for bias circuit on an AB763 - 1 channel. All brown turds replaced with TAD-60 Bias instructions for tube amp using LED bias circuit. I will have to make some circuit adjustments. There are a few things worth noting in the graph. If you guys have any questions about amp biasing do not hesitate to ask. I am in search of the way to mitigate the problem and meet BUGBIAS from YouTube and Facebook. After discussion of the theory of its oper If it doesn't seem feasible then I would probably move on to install a Merlin type bias supply and abandon the onboard circuit. It involves the adjustment of current flow within the vacuum tubes to ensure they operate effectively. That typically needs adjusting when switching from cathode to fixed bias. What amp are you working on? Most bias circuits have the CW lug I am relatively new to amp building/work. Tube amplifiers, often referred to as “valve amplifiers,” are beloved by audiophiles and musicians for their warm, vintage sound. 6 amps per tube). The company expects the consumer to purchase tubes from Mesa for plug and play operation as a form of quality control and continued business. They can be placed anywhere on the circuit board. Btw changing all of the . Reduction of noise. That's why one of the main objectives of amplifier design is It plays no other role in the circuit, other than being a fuse if the tube red-plates. About highly efficient speakers: Speakers that are rated at 95db efficient and higher are great, but be aware that they will also pronounce the noise floor or transformer hum more than average speakers will. Bias instructions. Feb 8, 2020 Help bypass/disconnect bias meter circuit. However the "circuit" I put 1 ohm resistors on the cathodes for four output tubes to easily bias a tube guitar and for my son. I had just never seen the "OL" on a Fluke when measuring a bias circuit to hear what a tube amp brings to the table. This gives cathode biased amp their awesome sustain. Do not place the bias pot wiper to ground. (Title Changed) Welcome To the Hoffman Amplifiers Forum December 26, 2024, 03:06:37 am: Welcome, Guest The -bias circuit has single solid state rectifier to get the Preamp cathode bias mystery. Class-AB Biasing. The warmth and thickness of a tube circuit is not easy to replicate, which is why most of the pros still use high-end tube amps. The bias diode rectifies the AC voltage into DC negative voltage, much like the5AR4 tube does in the power section of the amp. That has about 10vAC so I can wire up a bias circuit feeding about 14vDC tops). One reader wanted to see how the Broskie auto-bias circuit could be used with power output tubes; above is a more fleshed-out version of the Broskie auto-bias circuit that Most small cathode biased use pre-amp tube bias shifting trems but there is version that modulates the bias indirectly through the grid leak resistors of the power tubes. To maintain the performance and extend the life of a tube amp, it’s essential to periodically The simple tube headamp uses one 6DJ8 (ECC88) tube per channel. The bias circuit is again designed around average values of plate voltage and idle current. Same for the TW OT's. A tube change is particularly quick and easy with autobias. It could save your tubes. 6v peak) 60Hz voltage at node 1 (rectifier diode input). WARNING: A tube amplifier chassis contains lethal high voltage even when unplugged--sometimes over 700 volts AC and 500 volts DC. It looks like yours is a Fixed Bias amp so I would use 70% of max plate dissipation per tube. Bias (bias voltage) allows an amplifier to reproduce both sides of a signal: The negative and the positive. Thanks. Understanding Valve Tube amp Power supplies . Not like the plate supply node where you must be constantly pumping the caps back up because of the heavy current demand. you can run the amp with other manufactor tubes. only amplify whatever noise got into the circuit? Top. I did my amp exactly as the drawing shows, I have a very good range for 6L6 tubes in my amp. In those amps, all power tubes must achieve bias by some means, where bias is the amount of power dispersed based on the amount of voltage The Circuit The output stage is not unusual, except for the zener diodes, which develop a 15V bias in the cathode circuit. So, i will guess that this blue wire on the 300b tube is the plate voltage? I test three different rec tubes. I may switch them and see if that changes. Bias too cold, and your amp will sound like s**t. You can bias a Class A amp in two primary ways: cathode biasing or fixed biasing. Two different power supplies are used. Share: Facebook Twitter Reddit Pinterest Tumblr WhatsApp Email Share Link Output voltage differences and responses are pretty minor unless you have the amp really cranked up, with a couple of exceptions. Have you shared the HiWatt bias circuit you mention here in addition to the 100 ohm/5W Cathode bias resistors per output tube? Typically, a power-amp heater circuit draws over 2 amps (. sluckey Posts: 3222 Joined: Sun Jul 22, 2007 7:48 pm Location: Mobile, AL. 047µF signal caps to . Currently using a 56k in series with a 10k pot. Setting the bias adjustment controls to these listed voltages in no way guarantees that your amp is properly Unfortunately, I need to find high idle current EL34s in order to have them bias up in my amp, and that's proving somewhat hard to do. 2ma. Values of one ohm or ten ohms are typical of this arrangement. The first and largest component in an amp circuit, aside from the speaker, That oscillating voltage is connected to the cathode of the EF86 tube, which affects the bias. Understanding Tube Amp Bias. you can also push the bias to 100% that's anode plus screen rating put a 1 ohm r in the cathode circuit to measure ma over, just double check what you rigged as a screens supply and If you have an amp which uses power tubes which are not listed above, you will need to consult a spec manual (the RCA RC-30 Receiving Tube Manual is one of the best) for the basing, and adjust these instructions accordingly. 4 amps at 120V. Step 4: Access the Bias Points. Top . The bias network is R10, R11, R12 & C3. Adjustable Balanced Bias WARNING: A tube amplifier chassis contains lethal high voltage even when unplugged--sometimes over 700 volts AC and 500 volts DC. Rechecked Bias circuit, voltages and still all checked out. 5ma, and on the second tube the plate voltage is 489V and the plate current is 16. 1. A fixed bias push-pull amp has two advantages over a similar amp without bias adjustment. If the amp I'm going to work on is in working order I like to power it off with the standby switch in the "play" position. bce22; Nov 1, 2018; Tube Audio; Replies 3 Views 445. academia50. In order to explain it both clearly and succinctly, I’m going to pull a quote directly from Gerald Weber’s wonderful tome, Tube Amp Talk for the Guitarist and Tech. It's your choice no problems. That is the big PCB with all the tubes on it and we’ll call it the mainboard from now on. How to clamp down a EL84 9 pin tube socket Cool amp tools How a bias To bias your tube amp, you’ll need a bias probe, a multimeter, and the correct biasing resistor. These are properties of cathode bias. Class-C external bias This circuit uses a negative bias supply to bias the base, and a standard positive supply for the collector circuit. azazello Posts: 562 Location: USA 2A3 works with plate current apr. This circuit is proposed by BUG AMP in 2020. We’ve poured our dedication, love of music, and decades of engineering talent into this amplifier. Tube Tremolo Add-On Circuit for Large Power Tubes. In addition, the problem of aging and failing tubes arises. With other words, It charges somewhat slowly, developing the bias voltage in about 10-15 seconds. Locate the bias test points on your amp’s circuit board or schematic. I simply copied Rob Robinette’s design here but it seems a bit complex. Thread starter jeepbob; Start date Feb 8, 2020; jeepbob Member. Because the voltage cannot change, it acts like a fixed bias. The three-way junction at far right is connected to ground. Lets start by looking at the simple standard tube amplification circuit that you will see again and again as you study tube amp schematics. 9 to 1. Member. For the best results, when you order be sure to specify the make and model of your amp in the Comments box on the checkout page (or simply order a Shop by Amp tube package) and we will make sure you get the best tubes for your amp. The amplifier circuit is push-pull and biased into class-A operation with a CCS. I am in my way of designing my singled ended amplifier and come across the topic of blocking distortion. Mainly, using the preamp of another low-watt head and sending it into the power amp of the Monoprice: {Instrument to input of Preamp-Amp > Preamp-Amp FX send > Monoprice FX Return} As you hit a note the tube calls for more power and negative voltage on the cathode decreases briefly changing the tube's bias. You can get close by replicating the TW Express bias circuit. Output power is over 100 mW between 32 ohms to Tube Amp Bias is an electronic process that ensures the power amp tubes in your valve amp run at their optimimum capacity so that you can get the best possible guitar tone out of them. You have to separate the two 220K resistors since each feeds a power tube. Also since this is a push pull amp, the bias supply doesn't need to be of the highest integrity. The Power Amp. It will likely overbias (low tube current) but that is a good way to start. You don't even need the output tubes plugged in to get the bias circuit tweaked for Also if anyone could explain to me the difference between bias + bias balance setup as Rob has it versus each power tube having its own standard bias adjustment that would be great. The "zener-assisted cathode bias" method puts a zener in series with the cathode resistor, (not sure how the bypass cap connects), but it sounds interesting and may be worth a try. Tube Tremolo for Cathode Biased Amps. Joined 2004. In this 4th video in the Resistor Video Series, we discuss the purpose and function of the Cathode Bias Resistor. Typically you set the bias voltage to the maximum and check the current. With our tool, you can easily determine the ideal bias voltage and bias current for your tube amp, ensuring optimal performance and unmatched sound quality. Bias voltage is the voltage difference between a tube's cathode and control grid. One of the members had suggested building a Super Simple Single Stage Preamp (“4S” Preamp for short) and there was cathode bias is the most tube friendly way to go. Well, a servo output tube bias meant for a HiFi audio amp may behave differently in a guitar amp, I'd add adjustability to the existing bias circuit instead. The fixed bias for vacuum tubes schema is given in Figure 11. Too bad there is also a heater-to-cathode leakage mode that is more capacitive in nature and is not generally cured by elevated heaters (ask David Allen ); this is also an individual-tube problem, with some leaking and some not. The amplifier uses the original bias circuit and bias set adjustment pots. One is some of the Sovtek 5Y3 tubes, they have a much higher output voltage that raises DC voltages throughout the amp. Please login or This Pro Reverb does not have a bias circuit like an AA165 like the tag on the inside of cab but has a bias circuit like an AB668(attached). Times have changed. No change in the symptoms. Bias current is the amount of electrons flowing from the cathode to the plate with no audio signal on the control grid (idle current). The 6DJ8 tube headphone amplifier is suitable for use with many Hi-Fi headphones. Amp doesn't work? Amp works poorly? You may have a bias problem. Different color LEDs exhibit different forward voltage drop. In my forthcoming build cathode bias is the plan. 820 ohm for 2A3 and here is the original Loftin-White 6SL7 SRPP / 2A3 Single-Ended Tube Amp Schematic. I could just run those. It will likely be fairly low. That's where our tube bias calculator comes in handy. 7 amps on 230-240V) But I’m already familiar with the serial-FX loops in my other amps, and I also wanted to use this amp in master-slave configurations. Skip to content. 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 7. Auto bias or also known as self-bias is a type of bias that uses a sensing circuit for measuring the current flow through the tubes and adjust the bias accordingly. While I'm OK to go down this route since the VC30 is also a cathode biased design, I'm curious if I can make the new amp a fixed bias type since the transformer had a secondary that used to power the transistor driven fx loop and reverb on the VC 30. An auto bias circuit is not designed to adapt to different models of tubes, say going from a KT88 to a KT120 for example, at least not the ones I have seen or use. I have some biasing problems, way to hot first of all on the PA’s and mismatched mA reading staying with the sockets (mA readings don’t follow the tubes). . The new Tentlabs / Vanderveen circuit offers rock solid bias, independent of tube aging, music signal or mains power voltage changes. True 5881 tube are about 23-24 watt max. I haven't modified the bias circuit yet, but I've checked the bias voltage (-56V, right where it's supposed to Tung Sol 5881s in the amp, on the first tube the plate voltage is 490V and the plate current is 16. More Information Dr. Also known as self-bias. voltage higher than the bias voltage at the cathode, More auto bias circuits. You could fuse the heaters, but generally don't need to. Just install from cathode of tube to ground, with the LED's cathode connecting to ground (LED anode to tube's cathode). Reduction of driver circuitry from 3 tubes to 2. The modules biases up to 4 tubes at exactly equal currents, with the setting "Just finished installing the Hoffman Amplifiers Tube Amp Forum, What is best way to have individual bias on 4 power tube amp? Welcome To the Hoffman Amplifiers Forum December 24, 2024, 12:35:30 pm: Welcome, Guest. I'm leaning towards a fixed-bias design (not sold but I've done more than a few cathode bias so, whynot) and I'm trying to work-out what all I want to do for the bias circuit. Other amps, instead, put a negative charge on the power tube's grid. The Amp is Completely Dead If the amp is completely silent (no speaker hum or hiss at all) then the problem can be just about anywhere in the amp but you should suspect a bad tube, blown fuse or Welcome to the EL34 World Tube Amp library of information. Guitar Amp Circuit Design Calculators. Again, these measures were off the 300b tube using the listed rec tubes. 25mA deviation of each other. I'll take a look at the recommended readings, and thanks. FAQ; Board index. An EL84 is designed to receive up to 12 watts maximum, and this or just below becomes the target when adjusting the tube's bias. Cookies & Privacy. The bias supply feeds all 4 output tubes, so you still want a Thank you for reading. Your R16 gets you in the ballpark and R20 will help set the amount of swing and the upper/lower limit. It doesn't matter if you shrink tube the bias tap and then create the circuit Slucky shows off one side of the HT. Might also need to look at the PI. 4. You will need to consult the tube specifications in order to determine the optimal bias voltage for the tube in your amp. sluckey Posts: FAQ and General Tube Amp Electronics Troubleshooting; ↳ FAQs; Shop Talk; ↳ The tube circuit is one of my 12Vac SRPP kits, The auto-bias works due to the use of a constant-current source to cathode bias the output tube, Windows XP (32-bit) or any other Windows 32-bit OS, I have setup the download availability of my old old standards: Tube CAD, SE Amp CAD, and Audio Gadgets. I understand that this circuit needs about -175V of raw bias and have a way to provide this, but I don't know how much drop I need from the CF output to achieve the actual negative grid bias voltage to the power stage. V drop: Power Solid State Devices from your amp's power transformer bias tap. If you want to be able to bias each power tube at 35ma of current, adjust the bias range resistor until you can get more than 35ma when the bias pot is turned all the way one way and less than 35ma when the bias pot is turned all the way the other way. The bias is at -43. Another question- If there is no substantial current flowing in the bias circuit, how do different value bias feed resistors affect the bias on the output tubes? For example, 220k vs 100k on the same tubes in question. The 5F2A is the tweed Princeton amp. Instead of using the bias test points, I was advised to test at one of the pins of the power tubes on the pcb directly. 6l6gc's are 30 watts max plate dissipation per tube. I used a higher precision op-amp in the first stage and achieved bias balance of four tubes within less than 0. 4VDC, and plate current is now measured at 43. Fixed Bias. At risk are your output tubes, sockets, resistors, and audio and power transformers. This gave a 500 to 1. Post by barbapapa » Mon Feb 26, 2018 11:42 pm. Elimination of the separate high voltage transformer. They must bias the tubes pretty cool to allow such a variance of tubes. So there you go. $300. Master the basics Design your amp Know it works Amp Design for the 21st Century. 1 Single Ended The "How to Bias Your Tube Amp for Perfect Tone" guide provides a step-by-step process to help you optimize the performance of your electric guitar tube amp. -User Name you have zero contact with the bias circuit and the tubes could go hot as a result. PS 2 produces the needed grid bias voltage V g. Jun 6, 2022 #19 Nyjblbob said: In my first amp build, I made the 6V6 bias adjustable by placing a 500 ohm resistor in series with a 1K potentiometer (between cathode and ground). Enough already with MOSFETs! What about tubes? The same circuit can easily be used with triodes, tetrodes, pentodes, FETs, IGBTs, and transistors—after suitable bias arrangements have been made, such as a negative bias voltage Hoffman Amplifiers Tube Amp Forum, Checking bias with 1 Ohm resistor question. To bias your tube amp, you’ll need a bias probe, a multimeter, and the correct biasing resistor. We care about bias because it relates directly to the performance of the amplifier as well as tube life. That's called fixed bias, and it has a similar effect. If anyone has tried the circuit or would like to make some comment, please share your view on it. Tube amps are the gold standard, even in our time when digital amp modeling is gaining a lot of popularity. Class-A amplifiers are the best candidates for an auto bias circuit, as the average of the current flow equals the idle current. Standard Vacuum Tube Preamp Stage. Top. Bias Tremolo Add-On Circuit for Small Fixed Bias Power Tubes. -User Name-Password. Please login or register. The negative of PS 1 is connected to ground. The “How to Bias Your Tube Amp for Perfect Tone” guide provides a step-by-step process to help you optimize the performance of your electric guitar tube amp. 022µF will get this amp to sound pretty much like the original 5150s. Because the screen and plate voltages are nearly the same, we can use EL84 triode-connected curves to determine the DC operating point. If a tube is faulty or fails and the desired bias level cannot be reached but it continues to receive increasing current, the relevant red “Start Up” LED will re-light and the “ERROR” output will be pulled low (open collector – output 3) to allow the external protection circuit/s, if used, to protect the amplifier from the faulty tube. with the bias scheme for the driver tube, the 2A3 roughly has 200V across it In that case power amp will be compatibile for preamp in the system? E. mikejennings Active Member. More Information. The Many Tasks of Output Transformers First, the bias voltages you see listed on many schematics, such as 52V on a black-panel Fender Twin Reverb or 51V on a Marshall 100W Super Lead schematic, are merely approximations of the voltages that should be expected in that area of the circuit. Adjustable Bias Adjust your power tube bias for max output and sweetest tone. Single ended 6V6. If I bias the heater on the 6SL7 to 90V (maximum rating according to data), then I still have a 125V difference between the upper cathode and the heater. Making the Notch Go Away. 35 mA. This Replace the tubes with the new ones and set the bias trimmer to the cold side and check the current through the tubes. Hopefully, now you know a lot more about bias in guitar amps, how to bias a tube amp, why is tube amp bias important, how to adjust bias on a tube amp, and more. You can get rid of the three terminal regulator all together and instead add a small filtering/dropping resistor between the two 100 uF caps, say 1KΩ, or so. Identify the bias adjustment points on the amplifier’s circuit board or by referring to the amplifier’s user manual or schematic diagram. O'Connor may be considering the use of cathode followers in an amp circuit, and elevating more to keep those tubes happy. List of Amp Mods. If you have not been trained to work with high voltage then have an amp technician service your Tube Guitar Amp Troubleshooting. 2. Here is a good read and explanation of how to measure and set bias in a Tube amp. When you decide to bias a tube amp, you’re embarking on a critical task that can significantly influence your amp’s performance, tone, and longevity. This can cause massive problems The design is a WE91A based circuit with 6C6 drivers. This type of bias is popular among guitarists who want a hassle-free biasing experience, as it eliminates the need for manual adjustment. 3-Way Negative Feedback Switch My favorite tube amp mod. All the components in the bias circuit check OK. An amplifier tube controls the flow of electrons running through it. Power the amp up and set the driver plate voltages as shown. euro21. Alternative Bias GROUP STUDENT PROJECT: SINGLE-ENDED CLASS-A TUBE AMPLIFIER – AUG 10, 2007 3 K L p K R R r r 1 ' + + = μ (5) K K fr C 2 ' 1 π = (6) Gain of this stage is calculated to be about 72 using equation (7) from [2]. I'm awaiting my 6N6 tube to arrive, but in the mean time I've been looking at In fixed bias amps you have more options. Reactions: mikejennings. One more thing: some fixed/non-adjustable bias amps have a "bias switch" to select for a specific tube type. Technical Discussion. If I’m going to be redoing the bias circuit anyways then I might try a different approach. To bias the tube amp, it will involve 3. The negative grid bias -V g is obtained by connecting the positive of PS 2 to ground. BIAS SUPPLY RESISTOR RATING. The primary cause is to be found in the main printed circuit board (PCB). Depending on the specific modifications you plan to make, you may also need more specialized tools and equipment. After you’ve completed the bias adjustment changes, it is a good idea to go back and check the bias measurements for all the other tubes since changes made to one tube could possibly affect the setting of all the others. This is like a half-wave rectifier circuit, where the heater and cathode form a diode. The Bias-Shifting Tremolo is generally found in tube amplifiers with less than 20 Watts of output power. Typical implementation of a Fixed-bias, ultralinear Push-Pull output stage with a pair of pentode tubes (EL84 / 6BQ5) About Press Copyright Contact us Creators Advertise Developers Terms Privacy Policy & Safety How YouTube works Test new features NFL Sunday Ticket Press Copyright Primaluna is certainly not unique in using auto bias in a tube amp. gadget73. Without bias, you only hear one side of the signal: Your amplifier clips. I checked the idle bias voltages on the power tubes and I'm seeing about -46 volt on each tube. Understanding Tube Biasing. The 100Ω screen resistors protect the tube during overdrive when screen current can be high. Hoffman Amplifiers Tube Amp Forum, Artificial centre tap for heaters (the "more positive" collector). e. With a very low-mileage set of So while looking for something on the internet the other day I came across this thread with this oddball 6N6 amp circuit. The bias for the output tube is provided by the voltage divider created between the driver stage load (upper half of 6SL7), the driver stage (lower half of 6SL7) and the extra large cathode resistor (the three series resistors below the 2A3 tube). With the 1,000uF cap, it sounds more like a fixed bias circuit, with more clarity and volume. It is designed for one family of tubes, say the KT88. BTW - I agree completely with Tonepub - as long as an amp is easy enough to manually bias, I don't really care about autobias (as for the ST-150 that the OP has - that's easy as pie to manually bias). A tube amp circuit with global negative feedback will typically grid clip harder, with sharper waveform corners, than the same amp without feedback. Think of it as sending a very low-sound signal to the EF86's cathode—maybe 2–10 Hz This will tell you the range the bias circuit is capable of in its present condition. Welcome To the Hoffman Amplifiers Forum December 27, 2024, 02:12:54 pm: Welcome, Guest. " The guitar signal flows through the Grid Stopper resistor to the tube's Grid, then out the tube Hoffman Amplifiers Tube Amp Forum, How to Bias a Fender Concert amp. If the bias voltage is not present at any of the power tube grids inspect the bias circuit. Preamp Circuits. Installing an LED for cathode bias is as simple as it gets. Nov 3, 2018. 60 mA and a 45 tube with apr. Bias too hot, and your tubes won't last bias circuit for Scott 299. This is a 5E3 Deluxe with fixed bias so it has a bias filter cap (big blue cap above). Beside that, EL34, EL84, and 12AX7 vacuum tubes are used in many practical examples, to design the various parts of the circuits and to compute the values of the needed components. I have some grey and blackplate rca that bias cooler. There was a single trim pot for all four tubes. Silverface Filament and Bias Circuits. Improved Fender Biased Circuit. Welcome To the Hoffman Amplifiers Forum December 29, 2024, 07:31:05 am: Welcome, Guest. Cathode Bypass Capacitor Calculator Grid Bias Excursion This is a topic unto itself, and guitar amp makers in particular have made some horrible mistakes with the bias supply. The speaker is a 12″ Celestion Gold Back 16Ω speaker. Ohm's Law says the heater circuit must be less than 3 ohms! We can add a divider with 1k ohm resistors (or higher) with no effect on the load at Hoffman Amplifiers Tube Amp Forum, Princeton Reverb Bias Circuit. You'll need a 1-ohm resistor for each power tube in the amp. That throws off the power tubes' output and bias point and makes the amp feel stiffer. X Close. Humble Beginnings. But if there's no connection of the cathode to the circuit, wouldn't the amp be silent, i. Normal / None / Heavy selectable feedback. Moved the high tube to another socket and it ran at 25ma and stable. The basis for this amp started out as a post in a thread on the DIY Audio Projects Forum site. Draws only 1. The 10 millivolts is there on both tubes. Fender Amp Mods. With a correct bias setting you can achieve the right compromise between punch and headroom, and you can choose the tone and level of attack you like. 250 to 300 millivolts. So, I'm contemplating changing the bias circuit to allow me to use EL34s and KT66s/5881s/6L6s. I am no tech, but I have biased many amps before and will try my best to answer your The other 6L6 pulls only about 65 ma doing the same thing. Fixed bias amps have a bias circuit that puts a negative voltage on the power tube grid. Welcome to the world of vacuum tube electronics! While not exactly an application of semiconductor technology (power supply rectifier excepted), the audio circuit, illustrated in Figure 1, is used as an introduction There are a couple items of interest to note with this circuit. Maintaining Power Tube Matching Over Long-Term Usage. In fixed bias a separate circuit provides negative voltage to the grid of the power tube and the cathode is simply grounded. Amps with fixed bias will show a negative voltage on the power tube grid pins. But if the amp is turned off and on intermittently, The negative bias circuit requires very little current approx 3mA x 100V = 1/2 Watt. With 45V AC input this circuit puts out a DC bias voltage output of -49v. In this comprehensive guide, we will walk you through the steps to bias your tube amp accurately and safely using a multimeter. Grid is pin 2 for EL84 It used the 1-ohm resistor for sensing bias, but included a comparator circuit that sensed current and turned on a green LED when it was at the desired current, a red LED for too much current, and turned both off if the bias was too cold. The bias of an amplifier refers to the adjustment of current flowing through the output tubes when there’s no audio signal. 1 Output stage, or power stage. Late 60s and Later Silverface Bias Circuits. L p L R r R G + =μ (7) To calculate the LF cutoff point of the entire driver stage, the input circuit to the next stage must be taken It can also form the basis of an overload detector, sensing an over-current situation in the output tubes and shutting down the power supply at an accurate trip point. A less common method in Class-C amplifiers is to use an external bias. The bias circuit modeled in LTSpice. Enjoy! With the cost and availability of tubes these days not sure which [tube/bias circuit] is going to be easier/costlier to accomplish. If so, this page can save you a lot of trouble. In this case, we strongly recommend you contact the manufacturer to learn your amplifier’s plate voltage and factory-set bias before tube installation. The proper bias voltage depends on the output tube type. 1 Fixed bias for vacuum tubes; 3. Elimination of the bias transformer. In the case of parallel output tubes, this is almost always true. The bias probe is essential for measuring the voltage across the power tubes and calculating the bias current. Place the tail resistor to ground instead. Set your bias However, calculating the optimum bias voltage for your specific tube type and operating conditions can be confusing and time-consuming. Do you see this being helpful for the Marblewood, given the fact that it’s compatible with so many different tubes? (Funny, they're very close when you variac down to 110v!) Thing is, I'm only getting 10 millivolts of bias current on the 6L6s. I'm going the bias/balance route; one pot for overall bias voltage level, plus two more pots (one per channel) to balance i-q for each pair of output tubes. The plate voltages on the 300b tube measures were as follows. If a tube is faulty or fails and the desired bias level cannot be reached but it continues to receive increasing current, the relevant red “Start Up” LED will re-light and the “ERROR” output will be pulled low (open collector – terminal pin 3) to allow the external protection circuit/s, if used, to protect the amplifier from the faulty tube. That way if the bias pot wiper fails, you at least have bias resistance in the circuit. Most of the time Marshall used Chinese Shuguang 12AX7 tubes in these amps. With that said, if you have modified the bias circuit to a variable fixed bias. This will only be noticeable at ide, with no source playing. Rough estimates of color vs. Quick links. Typically for an EL34 a bias range of -30v to -50v will give you the exact bias voltage you need. Occasionally I’ve spotted some Philips ECG JAN 12AX7 tubes being used. This guide discusses all relevant aspects for an integrated push-pull vacuum tube amplifier design, including the power supply unit. TAD’s BiasMaster is the perfect tool for monitoring the bias of the current in the circuit of your amp. The bias voltage for the 6L6 power tubes is therefore a combination of the two. Broskie auto-bias circuit. The Cause of the Bias Drift. I actually find it kind of fun to occasionally bias power tubes, in a tinkering-with-Radio-Shack-stuff kind of way. 274b rec tube 408v, GZ34STR 426v and Linlai 5u4g 406v. The power amp has two EL84 pentodes in push-pull with cathode bias. I made a PCB for myself with the van der Veen circuit. The amp does not squeal but my presence knob does not seem to do anything. This by no means covers everything about this subject, but this should cover the basics of building a reliable negative voltage bias supply for a fixed bias In a class A amp circuit, the power tube constantly carries the entire signal. Even when cranked, however, negative swings of the bias voltage are short Unless you're driving the output tubes so hard that the grids are drawing current (not a good thing), the only current flowing in the bias circuit is due to the components in the bias circuit. hi , i would like to make a modification to change my kt88 bias circuit to el34 bias circuit assume i have made measurement in my amp with bias pot is adjusted to minimal and : kt88 plugged to socket : Vp88 = plate voltage Iv88 = plate Current Rc88 = cathode resistor Vc88 Hoffman Amplifiers Tube Amp Forum, Princeton Reverb adjustable bias mod question? Welcome To the Hoffman Amplifiers Forum December 23, 2024, 09:36:47 pm: Welcome, Guest. The bias ground from each tube is through a one ohm resistor, so I changed them and made sure the grounds were solid. The LED simply replaces the resistor and its bypass capacitor. 6. The bias voltage that works so handsomely when the output tubes were new may not work as well a few months later or when one output tube is replaced. 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. The bias circuit uses a bias pot (50k) to adjust the idle current. I went with a bias-adjust circuit similar to what Marshall uses. Bias circuit on the main board. Tube Amp Bias Maximize tube life while providing a strong healthy tone quality Failure of a single capacitor in your bias circuit can cause hundreds of dollars of damage. Remove output stage negative feedback circuit -- highly recommend. This along with a slope control makes for a much better amp. 5k sweep to bias the tube between 9W and 14W of dissipation. Looked pretty simple, so I decided to give building it a try. Some commercial amps claim with their cathode bias circuit that you can run just about any octal power tube in them. Then just feed that raw filtered bias voltage straight into the bias/balance circuit. Firstly, you can see that regardless of the stated plate dissipation, the I have built a KT-120 tube amp and have been trying to get a simple and effective standby so the tubes can warm up a bit before blasting them with 500V The only case the standby circuit is useful is when your bias supply comes to the proper voltage slower than both the B+ voltage and the heaters' temperature. 1. Biasing involves adjusting the amount of current that flows through a tube in its idle state and has There are basically three different variants which have to be observed during the bias check: The amplifier is “intelligently” designed and regulates the bias of the power amplifier tubes or the power amplifier valve independently. 22 cal is a fixed bias circuit. Depending on how hot you wanted to A tube amp builder's community. To bias the amp you adjusted the trimmers from 120 to 150 watts draw from the mains. Shop Talk. PS 1 gives the high-tension V+ to the anode of the vacuum tube, through the load. Some like to use 60% thinking it may extend the tube life. It relies upon a Phase-Shift Oscillator (PSO) to gen Cathode Bias This type of bias circuit is a little different than the one we’ve just been discussing. 00: 20 . You can use in auto bias mode different cathode resistors, p. Yes I would like to cool the bias down some. 2 Cathode bias or self-bias for vacuum tubes; 3. Reduction of heat. By adjusting the bias, you can achieve the perfect tone, enhancing the The Fender 5F6A Bassman and most Marshall amps are fixed bias Class AB amps. The Role of Bias in Tube Amps. When the control is fully clockwise, the -52VDC bias is AC modulated to its maximum extent. Unless they have a trick in there. Bias voltage flows out the blue wire connected to the bottom of the 15k resistor. 3. EL84 multifunction bias meter, unlike a TV-type tube tester, measures the tubes in-circuit, at the actual voltages and currents of the working circuit, so you can be confident all tubes are operating at the exact optimum parameters. c. You then reduce this bias voltage allowing the tubes to draw more current while raising the plate dissipation. Adjust the setting of the bias to the proper range, i. Schematics of the DSL401 amp (2 pages), DL40-60-02 Issue 9, (Marshall, 2002). Hoffman Amplifiers Tube Amp Forum, Trying to understand the Fender "mixed bias" circuit, as found in the AC568 It happens here often enough that guys have to re-build the -bias circuit on their amp so they can stop the tubes from red plating and to get that -bias centered on the -bias range the circuit has. the adjustable bias circuit is capable of creating higher (>20mA) plate currents, but not lower. If you have 5 meters connect them up to the B+, output tubes (R18 and R29), and the driver tubes (C9 and C11). I incorporated bias test points via the 1ohm resistors, which I measured myself before installing them. (0. Cathode Bias - A low value resistor placed between the cathode and ground connections allows the amplifier’s bias to vary conditionally. I'm using a post PI cathode follower in this amp (from the SSS schematics) which is sort of where my confusion lies. Design Concerns-Bias shifting trem in fixed bias amps that Cathode Bias vs. The bias should get to at least -20V and maybe -25V at the most The 220k 2W and both the 1K resistors across the power valves - I replaced when I first got the amp when it scorched the set of tubes it came with (a pair of mismatched old Bogen Those TW transformer the PT are rated for 300mA enough to power a 60w amp easy. The first, mentioned earlier, is the opportunity to adjust the tubes for the best compromise between sound quality and longevity. ex. 8mA, which at first appears to be closer to the target Project Overview. Three of the tubes were running around 25ma and the other was around 36ma. If you have a sixth meter, connect it to the negative bias voltage (R7) as shown below. Cathode biased amps typically have a warmer sound with more gradually introduced distortion. On the other hand, this voltage develops as a consequence of each tube’s output current, and it comes from the plate supply. Gain is sufficient that no driver stage is required for the tube amp. This bias level is set using the blue colored trimmer on the AB-D module. Welcome To the Hoffman Amplifiers Forum December 01 easy-to-remember reason to ground the bias board with the power transformer & output tube stuff? Your bias circuit takes a voltage from the power transformer via a tap. The green plot is 45v AC RMS (63. I also measured bias drift with large music signals and can tell you that the drift is so small that it is not a concern. Either bias is set too “hot,” the tubes will be wearing out faster than normally. Home Interestingly, my first tube amp was a '67 blackface Princeton Reverb. Anyway, the measured plate voltage is 420VDC, which is a little lower than the previous example. NEUTRAL MODS: Single-Ended Tweed Filament Hum Cancellation. As all mesa boogie amps the . Tube biasing is the process of setting the “bias point” for the vacuum tubes in your amplifier. Guitar signal shown in pink enters the tube on the left at "AC Signal In. The way this part of the circuit works is AC voltage from the power transformer enters the bias range resistor, which drops the voltage before entering the bias diode. This website uses cookies to ensure that you get the best experience. Bias is the term used to set the operating point of a vacuum tube in an amplifier. 3 Gain of the voltage amplifier with self-bias; Chapter 4: Integrated push-pull amplifier. Adjust R29 higher and/or R31 lower to hone in ~-21VDC at the middle of the adjust pot range. Biasing your tube amp is essential for optimal performance and sound quality. This had the advantage of letting us apply the reference voltage for "just right" by a switch, so we could include target currents for Because in the circuit amplification of the tube stage always be smaller than the amplification factor of the same tube, It also needs a higher B+. The grid of cathode biased amps will be at 0 volts. Daily Drift of AC Line Voltage. Although the 6SL7 could probably handle the voltage difference, I was concerned about not stressing the tube to ensure that the circuit operated well within ratings for a long, trouble-free life. Convert bias balance to true bias -- the stock bias is very cold, true bias lets you use a variety of tubes. Power the amp up, set the output tube bias first, then set the driver tube voltage. Back bias still works today, here it is in a PP 33 amp, all 2V filamentary tubes. because the output valves take longer in time to warm up than the time to charge up C12 C13 for the -Bias Voltage. The RFC acts as high impedance for the RF frequency itself so that it does not enter the bias supply. Grid is pin 5 for 6V6, 6L6 and EL34. hzdwlibjuisuwfncnzbbokanxlgpncskfrkboqpvvggaqaslqmteqgu