The red lines are the actual operating points seen by the tube. Play the amp and sample the clean and overdrive tone. This paper offers a remedy by introducing a new software tool for easy. It monitors the total current flowing through the tube. It features a new guitar match technology, new amps, new pedal effects, racks, and advanced modelers. You can play your guitar through the amp each time you adjust the bias, and see how you like it.
As the name suggests, these amplifiers are essentially plugandplay and will not need to be tinkered with if tubes are being replaced. Tube guitar amplifier oscilloscope diagnosis 1102014 6 steps i follow when i assemble an amp i always start with the power supply. Ive never advocated using an oscope to bias an amp. Enjoy your music and if you blow a tube, just replace it with a matching one and youre ready to go. Amps with power tube cathode resistors greater than 10 ohms are usually cathode biased. If you have not been trained to work with high voltage then have an amp technician adjust the amp s bias. Obviously the specifics will vary from amp to amp and somewhat from tube to tube. Each output power tube needs a certain bias current to keep it operating at the point where the amount and type of distortion under normal conditions is well controlled.
An amplifier, electronic amplifier or informally amp is an electronic device that can increase. For audio work a bk precision 1472c or a newer 1477 can be found on ebay often for much less than 100. Fixed bias amps have a bias circuit that puts a negative voltage on the. How to build a tube amplifier rob robinettes car and tube. You now have 2 voltages that correlate to the upper and lower current draws.
Please give suggestions for a good quality bias probe, and why your choice if you. Next up is the power amp which includes the output transformer. I can understand adjusting for zero crossover notch if you are working with hifi or sound reinforcement gear, but not a guitar amp. Subscribe to test and measurement industry news, new oscilloscope product innovations and more. Odd block tube amp with cover removed measurements pushpull kt88 tube amplifier 12sl7 driver measurements were made using a hp 331a distortion analyzer, tenma low distortion signal generator 0. Amp bias tool recommendations telecaster guitar forum. The signal injection method of biasing is circumspect because of all the varyance. Semiconductors capacitors final assembly checkout setting the bias. Ive got a line on an hp54600a with a warranty from the seller. You are biasing for the best perceived tone within acceptable tube dissipation limits. You have to download the program, and have flash player, but it is definitely worth it. Now re bias to the upper range using the transformer shunt method. When finished, i again test all voltages without tubes installed. Tube amp theater how to test amplifiers via oscilloscope by dlab.
A comparison between a wellknown guitar tube amplifier and two. These are based on a completely reengineered dsp engine and are available in a new easytouse interface. I now own 6 tube amps currently, and will probably acquire more as time goes by. If your amp is cathode biased and the bias is measured above about 110% max dissipation you can install a larger value power tube cathode resistor to cool the bias. This is certainly not the all inclusive tutorial on biasing tube amps, but the basic information is correct and can be applied to most any tube amp. We will continue to offer these because in conjunction with a multimeter this is the most cost effective tool for measuring true plate current allowing you to accurately bias your amp. Then, turn the bias pot a small bit and play it to find out which direction one should turn the pot to increase or decrease the current draw. Individual tubes vary widely in the grid bias that sets the correct idle bias current. Oscilloscope for tube amppedal work id like to get into tube amp and effects pedal designing, and others have said their scopes were very helpful in the learning and troubleshooting process. How to hook up the output of a pmt to a pc for time dependent.
Dear enthusiast i was wondering how many of you guys actually purchase a reasonably good enough oscilloscope for biasing their tube amps. Thats just the nature of ab class pp signal amplification. Here are a couple of shots of a pc based oscilloscope that connects to the usb. Learn how to correct crossover when changing the tubes on a vacuum tube amplifier in this free video. Nov 04, 2015 tube amp theater how to test amplifiers via oscilloscope by dlab. Here we use the focusrite scarlett 2i2 interface as an example. I have had many requests for testing amplifiers using a dummy load and oscilloscope. I did professional audio servicing, part time, at home for a company. I initially opted for bias fx, which includes a ton of stomp boxes as well as a range. A generally accepted rule of thumb is to bias amp such that the tubes are idling at 70% of their rated dissipation the most accurate way to bias an amp is with a signal generator and an oscilloscope. I have performed bench testing on amplifiers both tube and solid state ever. Audio amplifier testing with oscilloscope all about circuits.
How to set up my audio interface with bias fx amp pedal. Previously i used an audio signal generator, a distortion analyzer, and an oscilloscope. Read about bias adjustment on the web and get a thorough understanding of it, and the different ways to measure it. All of these little imperfections that add up throughout the signal chain of a typical tube guitar amp make the magic that is the sound and soul of a good tube guitar amp. If youre going to signal inject then an oscilloscope is fine. I took these to a short length of wire to a cheap connector. Is it always necessary to bias tubes when replacing them. We set an amplifiers idle plate dissipation when we bias it. All you need is the amp, a guitar and a tool to turn the bias pot. This class ab 14 amplifier uses four cathodebiased el84 tubes in the output stage. Oscilloscopes free delivery possible on eligible purchases. Is it always necessary to bias tubes when replacing them with.
If you want to use them as standalone applications, please set up your audio device first. Tube amp theater how to test amplifiers via oscilloscope by d. Matched output tubes and proper bias are the ingredients to quality sound and longer tube life. Hi, i changed the el84 tube in my universal audio la610 mkii and i need to make sure its properly biased. After startup remove the light bulb limiter and measure and set the bias. Try it with your usual pedals to make sure it behaves. The bias will gradually fall back down in a couple minutes. Oscilloscope probing high voltage valve amps electronics. If you have a sixth meter, connect it to the negative bias voltage r7 as shown below. It uses pin eight as ground which will give the actual dc plate voltage your tube sees.
If the op227 is configured as a 20 db gain amplifier, then the bandwidth goes down to. How to signal trace fender champ guitar amp oscilloscope stage. Ive checked bias current on op stages by measuring the voltage accross the cathode bias resistors with a meter. Diy audio projects forum dso2150 usb pc based oscilloscope. Heres a modification for setting bias on your marshall amp. Tube amp theater how to test amplifiers via oscilloscope. Number one youre never going to be able to fully eliminated crossover distortion. The fender 5e3 deluxe is a cathode biased class ab amp. Is it always necessary to bias tubes when replacing them with identical tubes in a tube amp. I did my mp15 by measuring the output transformers primary resistances both sides of the center tap, turning on the amp and measuring the voltage drop on those windings, and then calculating current using that resistance with ohms law, i er then since the two tubes were. A bias probe is inserted between an output tube and the amplifier. Dec 03, 2012 nige has a point theres not a great deal to a valve amp, and youd only need a high volatge probe on the power supply or anodes.
There is a negative voltage going to the tube to set the biasing what should the negative voltage be mostly. Good vs bad oscilloscope waveforms to know if the power amp is biased right. Check out the setting tube bias page at sweetwater the worlds leading music technology and instrument retailer. There is a very interesting assignment for this lesson. Tube bias calculator rob robinettes car and tube amp. Tube guitar amplifier oscilloscope diagnosis overview. Tube guitar amplifier oscilloscope diagnosis 1102014 7 diagnostic tests platetogrid this is the single most useful and important test of them all. Remember, each time you adjust the bias control, youll have to take a new reading from both the 1ohm resistor and the plate pin three and multiply them to see what the tube is dissipating. Th output from a photo multiplier tube pmt is basically a current signal. The following is an excellent article on biasing using an oscilloscope, posted by tom crocker to alt. An idiots guide to oscilloscopes many internet tube gurus will. Unhook the meter, power up, and measure the bias voltage again. Bias fx, bias amp and bias pedal can be used as plugins or standalone software. This is the way i normally go thru an amp looking for loss.
Software effect processor software sampler software synthesizer. The method, as proposed by several tube amp gurus, requires the use of a signal generator, an oscilloscope, and a dummy load for the amplifier. Power the amp up, set the output tube bias first, then set the driver tube voltage. You should re bias the amplifier whenever you change the output power tubes. If youre going to bias an amp, then the speaker load and cathode resistor or shunt method is preferable. For the push in wire, i used a piece of solder braid. The fender 5f6a bassman and most marshall amps are fixed bias class ab amps. Launch the software youd like to use, click the settings icon and then audio settings. Power the amp up and set the driver plate voltages as shown. Adjust the bias control gradually until this notch just disappears. The fender 5f1 champ is a class a cathode biased amp. Depending on the design, amps sing when the output tubes are between 55% and 70% of max plate current.
Our bias tools are easy to use, and provides a fast and accurate way to get the best sound from of your amp. Dlab recommends how to repair tube amps best oscilloscope iwatsu ss5702 duration. It is the dso2150 which i purchased from china on ebay. This method requires that you have an oscilloscope, a signal generator and a noninductive power resistor as a dead load. Read up on oscilloscope usage, and biasing common vintage tube guitar amplifiers, by clicking here. The et pro one bias probe can also be used on cathode biased amps. If a semiconductor or vacuum tube is to accurately reproduce or amplify signals on its. After you have digest all the pertinent information, you can listen carefully to your amplifier, without using any effects pedals. I ve got bias fx2 and bias amp though i think they sound great in a live application as a vst in a home recording they just dont sound righti think mercuriall ssx11 is the best followed by fortin but they still fall short of a live sounding amp. Since 2006 we have offered a single octal bias probe for use with a multimeter. The green lines show the extremes seen by the tube at the signal minimum and maximums. I also have a mojo bias meter that still works fine after 5 years.
I need software because i cant make noise at home and now guitar amp. Were you hoping for a simple, succinct, one sentence answer. Marshall el34 tube guitar amp bias adjustment mod dlab. Jan 01, 2015 heres a modification for setting bias on your marshall amp. You can now play through the amp while tweaking the bias. When its finished i test all the voltages before proceeding.
Distortion analysis toolkita software tool for easy analysis of. Positive grid has released this week the second version of its bestknown bias fx guitar amp and effect software. Using alligator clips, hook the meter up to read the bias voltage. The bias adjustment is first set to the most negative voltage on the grids of the output tubes, and a 1khz or 2khz sine wave depending on which guru you follow is applied to the amplifier input. The old vacuum tube based wo33a or wo91b may look like a classic, but getting one to work properly can be a chore. For some reason marshall decided to make this effort difficult.
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