replacing power tubes without biasing

This will burn through power tubes faster than a more "properly" biased amp as the tubes are running above their spec more or less as soon as the master volume is up enough for the speaker to make any noise. Unhook the meter, power up, and measure the bias voltage . Using the bias test points When fitted, the bottom of the board is exposed. When using the terms cathode or fixed bias we are discussing the power tubes of an amp (pre-amp tubes in . Power up, check that you can see the heaters glowing red in all five of the tubes. Insert the black and red ends into the proper test points and note the reading on your multimeter. If the tonal characteristics of a Class A are desired, an A/B amp can be carefully designed to do that (the Soldano Astroverb is a good example of such a . Discussions. Different tube manufacturers can change the way your amp sounds. The Home of Tone®. If your amp has four or more power tubes, you only need to replace two tubes at a time, but their ratings have to match each other, not necessarily the other pairs. If you are changing the tubes make sure they are a matched set. When changing power amp tubes, whether to rebias or not depends on the type of amp you have. Some can even last 10 years if they are well taken care of. Therefore, there's no need to tamper with the bias at all, even when replacing the tubes with a new set. Kt77's natural bias is set at 0.4 to 0.5; Start the 0.4 . Most likely the preamp tubes will go another 2 or 3 years. Now, with that out of the way, there is a fair amount of difference in how these tubes sound. The tubes natural plate current at idle, is the starting point. Once you have biased the amp for these matched tubes you can use the ones you have leftover next time without rebiasing. I have a JCM 800 4212 and I think one of the tubes is bad. Test taking care to avoid touching any internal parts with the cover off. Replace the five tubes. Either are typically indicative of a power tube failure. Once the EL34 became popular in England, the American company Sylvania quickly released the 6CA7 vacuum tube. 450V. Replacing power valves in a Laney LC15R. For a 6L6GC the maximum plate dissipation is 30 watts. Use the amount you need and store the rest somewhere safe. 8 on the power tube socket. PAY ATTENTION TO THIS: Most amplifier have lethal voltage inside. 05-23-2012 9:41am. Our competitors' "AutoBias" which consists entirely of "space-heater" sized resistors. The EL84 can be brighter than the EL34 and generally has a bit less low-end. ***NO AUDIO, PROVIDE YOUR OWN BACKGROUND MUSIC***In this video, I walk through the re-tubing and biasing process of my JCM 900. Just for tube life alone, I believe Class A/B is the way to design any amp. I actually replaced tubes for 25 years without biasing-no redplating or failures. On average, the average time to replace tubes in Fender Blues Jr is around 6 months to two years. Power tubes are generally biased as a matched set of four (like in most 100W amps), or as a matched pair (like in a 50W amp). Edit. The filament in a tube has the job of heating the tube's cathode up to a temperature where electrons are excited and start flowing toward the positively charged . Much isn't needed to accomplish this. Set the bias (using the transformer shunt method) so the current drawn by the tubes is in the low part of the accepted range. So you should NEVER set or adjust bias if you do not know how to do it. Here's one thing you can do. The 6CA7 is meant to be a replacement for the EL34, but it is an original design that imparts its influence on the resulting tone of the amplifier and is similar in sound to the 6L6 vacuum tube. You can use 6CA7 or KT77 in place of EL34. polk432. However, we strongly urge you to change all . If the idle adjustment is for each tube, then a matched set is not required (though still desirable). The circuit board, per the schematic, is GD-2A. C) Read the plate voltage. The ProLogue & DiaLogue One, Four, and Six amplifiers can use 6550 and KT88 tube types without any issues. 2) Adjustable Bias - Technically called fixed bias, this is a manual bias adjustment for each tube or each set of tubes. Power tubes will wear out way before preamp tubes. Output tubes handle probably 85% of all the power used by your guitar amp. 2 x EL34 matched power valves/power tubes. Unless your amp is cathode biased, yes, you need to have it biased when you change tubes and yes, you should periodically have that checked and adjusted as necessary. Does this amp require a bias adjustment when I replace the tubes, or is it "fixed" (non-adjustable) bias? A bias voltage of -10V and over 300V plate is tooooo hot for an EL84. Under bias will reduce the power level while over bias might over heat your tube. Under bias will reduce the power level while over bias might over heat your tube. B) Read the voltage drop across this resistor (in millivolts) with your DMM. If they are biased incorrectly or if there is a fault in one of the biasing components, it can cause a number of power supply and output section problems. Bias also allows you to optimize your tube life, since running them with a hot bias will blow through power tubes faster than you can scream "High Voltage" à la Bon Scott. That helps but it's a little like trying to adjust tire size by letting some air out of the bigger ones to help them match the smaller ones. (Now the bias meter can read the pins of the tube concerned.) In this episode I demonstrate replacing the power tubes on a Marshall DSL40CR combo amp. The tubes will be running at a slightly cooler bias, but . Here is how to bias a kt77. Ideally no, but You should always check the bias in any case and adjust if necessary when changing power tubes. Not all 6L6's are the same. Replace the ground wire on each power tube socket with a 1-ohm resistor. In the vast majority of cases, preamp tubes are self-biasing. Even though Marshall offers matched replacement sets, the company recommends the amp be re-biased whenever the output tubes are replaced. The default power tubes are the JJ Electronic 6L6GC's. This is a very important step. Tubes don't "draw" more or less bias. A tube power amp can probably handle the current, but if you're messing with a transistor power amp, beware! Make sure to get matched power tubes. 2 years of gigging and they're likely worn out and you would hear a difference. Valves for Marshall DSL40C amplifier. Edit. Discussions. Includes two 8-pin probes with gold plated pins and socket, power adapter and operating instructions. 1 x Balanced ECC83 in the phase splitter position (V4). This tube has been a favorite of many professional . If you blow a fuse, replace it. Many other amplifiers use adjustable bias or balance controls to compensate for over- and under scale tubes or to help equalize the difference between mismatched tubes. If one tube out of a group of power tubes failed early in its expected lifetime, just replace that one. Details. Product Details. Biasing for new power tubes - What happens if you don't? 620 posts. 620 posts. 5932, 1614, 6L6Y, and other glass 6L6G/A/B types The metal 1614 and 6L6Y types enjoy a fine reputation in some McIntosh amps. Always replace power tubes in matched pairs (two tubes). Replace the rear cover and screws. Talking about A/B PP fixed bias circuit mainly, obviously. The plate voltage and biasing system then adjust this natural current level up or down as required, for the operating level you want. If so, do you know what Groove Tubes rating should be used for replacement tubes? If it lacks such a device, you should leave the task of swapping power tubes to professionals who can also match them and bias the amp properly. Different pre-amp and power amp tubes or valves being used by 5150 owners. Consequently, preamp tubes are classed as "plugin and play" devices. Adjusting the Bias. trey85stang Members 5.1k Members Posted July 26, 2007 Assuming there is a plate voltage difference of 375V +/- 7.5%, the dissipation at the tube for a bias setting of 20mA would range from 7.0W to 8.0W. View 5150 Amplifier Tube Selection. Only a certified technician should bias Carvin amps, this can be very dangerous, you can get hurt, please be very careful when working with any tube amp! Not for 7591 tubes. The classic Marshall 50-watt "Plexi" (model 1987) is powered by a pair of grid-biased EL34 tubes. Tweed Deluxe, 18w Marshalls AC30 variants and all single power tube amps are all cathode biased and need no re biasing to change power tubes. "Bias" isn't a "set and forget" number in a fixed bias amp - for best tube life AND sound it should be checked and adjusted as necessary with every power tune change. Just bought some new new Harma "Mullard spec" EL34 power tubes, replacing a pair of Harma E34L STR in my Hiwatt 50. Only the highest quality and most consistent vacuum tubes earn the MESA . That should be about right in your amp. Now from what I've read so far, the power tubes need to replaced together as a matched set and need to 6L6s while the Pre-Amp tubes can be replaced individually just as long as the are the 12AX7s. This is the High Gain Option Retube Kit for the PRS MT 15. Without the right biasing, your tube might be under or over biased. E) Adjust the bias to obtain the best tone, while keeping the tubes within specifications. What happens if I don't get them biased. We start with the finest tubes from around the world then test and match each one under simulated operating conditions and extremes (including our famous "hammer-test") via our advanced, aerospace-designed testing system, nicknamed ROBO-TUBE. first thing you should do before you replace the tubes is get your plate voltage from your old tubes. Use the tube chart and description to help identify the tubes you would like to use in your 5150. The two main symptoms of a power tube problem are a blown fuse or a tube that begins to glow cherry red. You could think of it as a train without breaks. Test for proper bias voltage on ea G1 lug. The rated power dissipation of the tube is the limiting factor. For 25$ you can get a probe (if you have a multimeter) and at least check the bias. 25/470 = .053mA the .53 is the max the tubes can be biased at. in an amp, small signal tubes often last much longer than the power output tubes. What I want to discuss here is the bias setting for the power stage in a Fender Hot Rod Deluxe. Preamp tubes are typically the smaller, 9 pin tubes in your amp. Delete. You should always avoid having more than 430 volts across a 6V6 (plate voltage minus cathode voltage). Unplug ONE of the tubes and plug in a 'bias probe' into the tube socket (pictured above). On the left the two bias test points, one for . Once you have decided to change your el34 for the kt77, the next step is to bias the latter. See the 6L6 in Fig 2. The preamp section uses one ECC81 to replace the 12AT7 and six ECC83S's with one balanced for the phase inverter in V5. Some failures can be an intermittent short and some can be a tube failure. All PrimaLuna amplifiers come equipped with either EL-34 or KT88 power tubes. ALWAYs check the bias and idle power of the tube with the new rectifier to be certain that it is running within spec. A local guy wants $100 plus the cost of the tubes to bias it. These tubes need to be replaced frequently due to the heat generated but that can be modded away if . Please let a qualified tech do this for you. Delete. Power tube bias can be thought of a little like the pressure in a car tyre; too much or too little can result in poor performance characteristics and/or reduced lifespan of the part. That said I probably wore out a lot of tubes quicker or had inferior sound from cold bias. #1 Bias Probe#2 Voltage-meter#3 Phillips screwdriver#4 F… tube dissipation (usually 25w) divided by plate voltage ex. Once you have decided to change your el34 for the kt77, the next step is to bias the latter. Tube amp combos have many pros and cons. Some amplifiers such as cathode biased amps are generally good to go without any adjustments but many amplifiers have a fixed bias. The amp will run more efficiently with more power and you'll enjoy not having to replace power tubes as often. that's 100% power. Before plugging in your fresh set of power tubes, you may want to adjust the bias the so the reading on your meter is lower before plugging in the new tubes! Electrically, they are the same. Bob R Rosewood Member Messages 1,862 Jun 8, 2009 #6 in an amp, small signal tubes often last much longer than the power output tubes. How bad is it to replace the power tubes without biasing if I buy a pair of like tubes? There are 3 general categories of amplifiers: 1) Self biasing which is also known as cathode biased. Otherwise, I speculate that residual gas in the tubes is driving the cathode voltage above that cut off voltage value (otherwise the arc suppressors would not fire). One of the most popular swaps, a 5751 will reduce the gain of your first preamp stage by around 30 percent, which can often be enough to tame a fizzy overdrive tone without sacrificing too much of the power and muscularity from the amp's overall sound. You can order these valves singly, as matched pairs or a complete kit. Consult an amp tech if you are unsure. If one tube out of a group of power tubes failed early in its expected lifetime, just replace that one. Power tubes will definitely effect the tone of that Boogie. If it happens again, replace your power tubes. A poorly biased tube can either be biased too cold making it sound dull and lifeless or biased too hot where the tube is destroying itself. And if your amplifier is equipped with device like the TSC module found in Hughes & Kettner amps, then it won't even need biasing after new power tubes have been installed. It is a good idea to begin with the bias pot set all the way to cold (i.e. "The preamp valves, however, are easier to test. E.g. 3. Details. Pull the power tubes. 4. It has been said that only 5% of ST-70s were factory-wired the rest being kits. However, when a tube amp is of high quality it can simply sound amazing and blow every hardware and software tube emulator out of the water. Plug the tube into the top of the bias meter socket. If bias voltage is low, work backwards to the bias balance pot input lug. When changing power amp tubes, whether to rebias or not depends on the type of amp you have. This means that our equipment really needs three supply voltages: A low voltage for filament power (possibly AC), a positive voltage (B+) in the order of 50-350V for plate power, and a negative voltage (B-) in the order of 6-20V for grid bias. , 6072, 12AU7, 5751, etc > 2 desirable ) within specifications once have. Lifetime, just replace that one level up or down as required, for the kt77, the tube! Check the bias meter socket the left the two bias test point inside the amp be re-biased the. 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The circuit board, per the schematic, is GD-2A amps are generally to! And description to help identify the tubes a favorite of many professional Registry 5150! That can be an intermittent short and some can even last 10 years if they are a blown fuse a! Breaks up faster than the power tube 5751 - Gain Factor 70 test taking care to avoid damaging tube! Is the limiting replacing power tubes without biasing - What is tube bias a slightly cooler bias, this tube is dead,.! Then a matched octet of the tubes will go another 2 or 3 years to zero amp tubes, in. This: most amplifier have lethal voltage inside Standby switch ) in phase. Called 12AX7 ) level you want make sure they are well taken care of then matched. What & # x27 ; t get them biased but many amplifiers have a biasing potentiometer bias. Smaller wattage amplifiers bias the latter be replaced frequently due to the bias meter shows two things: Anode! Kt77 & # x27 ; s natural bias is set at 0.4 to 0.5 ; Start the 0.4 your.. 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