6l6 bias adjustment. EL34 tubes don’t draw quite as much power as 6L6 tubes.

6l6 bias adjustment Put the tube type selector to 6L6 from the EL34 setting. 00 and available in our online store and can be used with 6V6 - EL34 - That bias pot basically sets the biasing of both the 6L6 power valves in your amp, so you only really need to set it for one valve because if your amp is using a set of two matched 6L6 valves, the second valve should bias up close to the other, ideally when biasing the power valves in an amp, you need three pieces of test equipment, a Sine/Squarewave Signal HI all, just curious what the proper voltage should be at the bias check point in a fender blues deluxe amp? I read on another website that the 6L6's int eh hot rod deluxe should be around 68mV. Nov 21, 2008 24,476 8,317 5,411 69 Lake Havasu City, Az USA. , JJ's), or does it just require biasing via a different means? thank you. The resistors R sp and R ss are 1. Putting it simply, with 2 tubes, one ‘pushes’ the speaker cone and the other While the Triple XXX's bias pot allows for adjustment, my understanding is over the years of reading about this subject from various sources (including Peavey specifically) is that the range of adjustment with the bias switch in 6L6 mode is rather limited and probably is responsible for not getting the tubes up to "proper" bias. chu2. They've been on here and said there is an adjustment pot for the bias inside. More Information. What range of adjustment does the 56K give you on a 6L6? On the D'lite there is a 45v bias tap, it has a 470R on the board then I use the dual 25K pot and 15K tail. For biasing your tube amp lets assume for the sake of clarity that you have a perfectly matched quad of OT's. These are still fixed/non-adjustable bias amps as there is no internal bias trimmer The stock power tubes on a Blues Deluxe and Hot Rod Deluxe amps are the 6L6's. Then back off of the bias to green. New posts Latest activity. However, you should get the amp serviced by an Amp Tech & get him to change the valves for you at the same time. Active Automatic bias adjustment function means changing tube types is easy; no manual biasing required. I don't know what number I'm getting. so, i dont see why the bias pot mod wouldnt work just fine. The tubes have great tone and nicely capture the “6L6 sound”, characterized by sparkling highs, warm mids, and controlled bass response. 150 mA for 6CA4 "The KT77's are calculated the same as EL34's, so using your bias probe you can run these in your JSX between 36 and 42ma per tube, based on you preference for headroom" when you turn the Dias adjustment counter clockwiese on the JSX it gets hotter (number goes up0 so it runs a little hotter . Each of the 220K resistors should go to some adjustable bias pot. Bias to approximately 70% (and no more than 70%) of that = 16. Once I took it off standby and inster. The best way is to observe the live current. When switched in, the effective shunt resistance of the parallel resistors is 9kΩ. the point of this negative voltage is to regulate the flow of electrons through the tube. hey guys, so the 6505+ has a bias pot, and probe spots on the back, though i have read that the ports on the back are useless, can someone explain to me why this is, or why peavey would put them on there if they are useless,and if there is a way, how can i use my multi meter to check with them. I have a couple of questions relating to biasing the amp and getting the best out of it. " Top. The 15K and 56K resistors in the original bias circuit form a voltage divider to reduce the voltage coming out of the bias rectifier diode. The power dissipation of the amplifier as a whole can also be measured using the Before I turned down the adjustment, I was measuring 0. Let me give you an example . You may wish to have certain fixed bias amps (such as Mesa Ideally you should bias using a scope to look for crossover distortion, not by dissipation. If there is no pot it is just a matter of changing the bias resistor to bring it into specs. SUBJECT: Bias Info/Procedure for JCM2000 series Marshall amplifiers THEORY OF OPERATION: We are setting the bias using the current method instead of cross over distortion method. Colossal Posts: 5111 Joined: Sat Oct 20, 2007 9:04 pm The instructions with the bias tester and most of the information I found on the Internet said that the bias settings for 6L6 tubes in a Fender Twin Reverb should be between 30mA and 35mA. I get that it's basically supposed to sound like a 6L6 but bias like an This amp has a non-adjustable fixed bias supply, no adjustment is possible (without modding) nor required. If it is not, it needs to be adjusted to within specs. Cole Bias pot out of range on this one so I had to adjust a fixed resistor. If not, you will need to troubleshoot the bias circuit to determine why the voltage is out of range. 035V to 0. Think of it as a rough adjustment before you do the fine bias adjustment. 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. for some reason pevey sends out all there amps ice cold,i tried all of the above mentioned tubes in a xxx and always went back to the 6l6's,with the bias adjustment set to el34 Amazon. Does this sound right to you amp gurus? I have mine at about 66-67mV. i know the II/+ models have a bias adjustment but the window of adjustment is very very small. kt66, 6f6, 6v6, 5881 I used the cathode bias setting, works perfectly that way. 65% is a nice compromise. jerem. 075 and that's exceedingly high. 5mA bias or 2mA as a resultant, then it is the best bias. 5 mA at 434V with 15K bias resistor. The Bias PCB assembly is as follows: PR1: Bias mini-pot adjustment for pin1 which is one side of the push pull amp (two output tubes in a 100watt amp and one output "With slight modifications (auxiliary 6. Because of one 6L6 power tube that was red plateing I installed an old set of tubes to see if the problem was only the tube. I ordered the tubes. Once that Maximum static plate watts dissapation wattage, 6L6GC = 23 watts. Just curious if anyone has some insight on the JJ 6CA7 and how it relates to their 6L6GC and regular EL34 offerings. Read the voltage drop across the resistor (in millivolts Frenzel did not put any bias adjustment on it, it is basically fixed or self biasing, take your pick. It is a cathode biased amp so I read but calls for bias adjustment for tube switches and tune ups. This is the amp with the Phaser FX, the Automixer, solid state preamp and a 2x 6L6 Tube power amp section. It's a bias adjustment, since the owner of this amp wanted to be able to use 6V6 or 6L6 tubes. Don't forget the joker. That's cool. I'm using 6L6s so the whole range of the knob is supposedly safe to use. For these reasons, bias calculation is important to optimize the maintenance and performance of tube amplifiers. I get no break up from the clean channel And remember if you are biasing 6L6 tubes in the 6l6 mode on the amp and the bias will not go above 35mA even with the trim pot turned all the way up, flip to the EL34 mode and bias it up from them. Also do this when the amp has been powered Anyway, upon installing the 6L6 and switching the tube selector switch to the 6L6/Kt88 side, I could not bias the tubes hot enough. The primary modification necessary is to properly bias the amp. Here is EL34 mode. Hope this hel I set the bias adjustment low to see what readings I'd get. Biasing was easy and well within the range of the standard bias adjustment circuit in the test amp. How to Bias a Fender Bassman (AB165) The 6L6 amplifier I chose is a 1969 Fender Bassman head (AB165). To use different tubes would require a modification to add test points and an adjustment potentiometer to the bias supply circuit, which is bias adjustment will allow you to try different tube instead of stuck to Mesa, or you have to hunt for tube in the same current draw range as Mesa. Does having the bias balance pot eliminate the need to re-bias the amp when changing the 4 6L6 power tubes, if a matched set is used (e. I have had a Bias Adjustment kit fitted to my Fender Blues Deluxe and I am now running JJ 6V6 power tubes. May 14, 2012, 8:40 PM For 8-pin power tubes such as 6L6, 5881, 6550, 6V6, EL34, etc. This The formula for biasing is the plate dissipation of the tube (12 watts for standard EL84’s or 9 watts for the JJ EL844’s) divided by the plate voltage (let’s use 350 for this example) times . Feb 6, 2018 30 13 4,551. 9 & very similar at the tube pins . 1 watts. com: Vacuum Tube Tester Bias Probe 8pin Tube Tester Current Probe Vacuum Tube Amplifier Amptata V8 DIY Octal Vacuum Tube Checker Quad Meter for EL34 KT88 6L6 6CA7 6V6 5881 6550 KT66 KT100 KT120 mA Version : Musical Instruments I need some help, I have a twin reverb 65 RI and I replaced the sovtek blue groove tubes. Some circuits have internal trim pots, while others may have adjustment controls on the rear panel. Or, you could ask the vendor for something with a higher or lower current rating. Other than that, it is real easy and I would suggest doing it. Bias should always be adjusted to each particular set of tubes anyway IMO. Sonicbluestrat. 6V6s and a bias adjustment trimpot would be cheaper than the Yellow Jackets, and would leave it sounding more Fenderish, I expect. PLEASE PLEASE PLEASE!!!! READ THE TEXT BELOW: We say that you can go as high as 90mv at the test points for the JJ 6L6GC's. Likewise, the Peavey head should sound best with 6L6's, but with a fixed bias, you should be able to swap them yourself. Running hotter than that will stress your output transformer. I'll reset the bias mine eventually. Even if the bias and voltages could be sorted out, the 6L6 will suffer in the distortion and in the power output No. About 59% plate dissipation. For 8-pin power tubes such as 6L6, 6V6, 5881, 6550, EL34, KT88, KT66, KT100, etc. It seemed to get stiff and harshgoing counter clockwise and got warmer and softer/buttery feeling going clockwise. These are both great 6L6 tubes. Here is s video to simplify the process. I'm retubing my '57 Twin reissue for the first time. When they're toast it'll probably get converted to 6L6 but no need to do that prematurely. I also know tube god Aspen Pittman says even when retubing with the same make tubes an amp needs to be biased correctly. Choose from KT88, EL34, 6L6GC. T-Bone-BBQ Member Posts: 32 These two (or three- 6L6 and 5881 tube are basically the same tube) tube types can be designed to work in the same amp, but the bias range is very different, so the switch places the bias adjustment control range for the tube type selected. Should the knob be turned all the way to the right (clockwise) or to the left (counter-clockwise)? According to the site 6L6's and EL34's should be Biased at around 65mA, is this correct? Home. 5 watts. if the windings require -1. The original 6L6 tube featured a metal envelope. Join Date: Feb 2010; The 6L6GC type can substitute for any of the types below, but may require a bias adjustment or modification to operate at peak performance. These can be substituted with appropriate bias adjustment. 2, and one sovtek rated 9. As long as I use a 6L6GC or equivalent (5881, for example), there is no need (or possible way) to change the bias. " according to the manual. Correct bias adjustment is a routine step to ensure regular maintenance and performance of the amplifier. Tube bias is also dependent on the high voltage (or B+) applied to the plate of the output tube, which can vary within tolerances of the transformers as well as in the AC line voltage fed to the amp. At 35 mA on 367 volt plates It is fixed bias but doesn't appear to have adjustable bias. My plate voltage is 455 V. For 6L6 mode, resistor R1 is switched out to create a shunt resistance of 22kΩ. Top. For fixed bias amps without a bias adjust pot, the If you want to increase the bias adjustment range, you'd need a higher resistance pot). pot. In fact, that covers 90% of the amps out there. How you adjust the fixed-bias voltage supplied to the control grid often varies from amp to amp and from brand to brand. 6mA - adjusted by me upon installation via the provided bias adj. New posts Search forums. Oct 16, 2017 What is meant by 'bias adjustment range is too low'? Can you not set the current draw as low--cool biasing/low plate Bias is generally set for between 60 and 70% of maximum plate dissipation. Bias adjustment Herbert. Last edited: Sep 28, 2018. Set bias balance. And AFAIK many of those who had the The calculator will give you the bias current for each tube. Cheers, Dave O. Read this link. 3volt filament transformer, rewiring of pin 1 on the socket, and a bias adjustment), a Fender Deluxe amp could use EL34, or 6550A. " Enzo wrote: This article presents line power measurements for the vintage reissue Fender '65 Twin Reverb guitar amplifier model AB 763. Setting the bias on your amp is like setting the idle on your car. Post Cancel. For amps with cathode bias resistors you can simply measure their voltage drop and use the Tube Dissipation Using Cathode Resistor The previous owner replaced the 6L6 power tubes with EL34s, I understand there is a switch and adjustment for either type of tube. Reeltarded Posts: 10054 Joined: Sat Feb 14 the biggest improvement of any of the peavey high gainers is to bias it to with in range,thou you wont be able to do it with the factory bias adjustment,. Left Adjustable Fixed Bias amplifiers (adjustment potentiometer to set the "fixed" bias point) Cathode Bias amplifiers (also commonly referred to as self-biasing) Often it is a switch that can select bias for EL34 tubes, or 6L6 tubes depending which is installed. Perhaps this is a factor in the extremely low noise and lack of microphonics that this vacuum tube exhibits. The 6L6 is an 8-pin "octal" beam power tetrode tube found in the power amp section of many guitar amps, mostly in sets of two or four matched valves. The Weber calculator has you entering the plate voltage and it then tells you what the bias should be for the 6L6GC. The bias trimpots don't last forever and they need to be periodically replaced, best at the same time of a The bias should get to at least -20V and maybe -25V at the most negative position. Puff O Smoke A 6L6 is a 30W tube, so to bias it using the 1 ohm resistor you need that reading from the cathode(pin 8 ) to find out how many watts the tube is dissipating at idle(no signal). Replace the ground wire Ok, new matched Ruby 6L6 tubes just installed. The built-in sockets are for the octal 6L6/EL34/6550 type tubes, but adapters are available to test other types, such as the EL84, 7591, 2A3, etc. theres no problem with installing a bias pot. Operates in Ultra-linear, Triode or Pentode modes. The two 8uF caps are filter caps to smooth out the pulsing DC from the bias diode. e. Cathode bias amps are by nature self-biasing, but there are fixed bias amps that are self-biasing. I. They are small enough that they have no effect on circuit operation. This website uses cookies to ensure that you get the best experience. 85 percent but you will wear out the tubes quicker! 12 watts divided by Re: Pin 1 and 8 - 6L6/6CA7/EL34 Cathode Bias Post by TUBEDUDE » Fri Mar 26, 2021 5:48 pm pdf64 wrote: ↑ Fri Mar 26, 2021 1:31 pm My understanding is that similarly to g1 and g2, as the voltage of g3 s is moved positive and negative (with respect to the cathode) from a notional operating point norm, anode current will increase and decrease. I know there is an "Internal bias pot added, allowing easy substitution of various 6L6 types. The existing 10K and 56K resistors form a divider that puts ~85% of the rectified DC voltage at the junction of the 220K bias feed resistors. The cheapest solution. Supporting Member. 3, 13, 11. are common guitar amplifier power tubes, and are all easily biased with this method. 6L6 Power tubes are rated at 30 Watts so you need to bias for 19. Maybe I'm getting confused or not understanding what bias really is. The bias adjustment is the 10K pot near the bottom of the schematic. Oct 7, 2013 #13 Oct 7, 2013 #13 Google "Bias King" for easy measurement. Just remember the plate load impedance of a 6L6 is lower Biasing the amp isn't hard to do, the hard part is pulling the chassis out of the head cabinet. This webpage offers two ways to calculate tube bias and plate dissipation. The Sovtek 5881 that became the standard 6L6 during the 90's is pretty much a regular 6L6. My 6505+ combo is this way. Click on the pics and they will shrink to size. Now without adjusting the bias control any further at this point, testing the various NEW out of the box matched set of JJ tubes I got 23. Members; Think it was rated at 60w which is alot for those tubes. I also wired the extra heater winding to two of the output tube sockets, since the 6L6 has twice the heater current. Note too that as things warm up, the optimal bias point can shift somewhat. It is recommended that this adjustment be done by a technician, but if you are handy with a Volt Meter you can check the voltage yourself. I just want to verify that it was done correctly. That's the starting point, best to use your ears after that for fine tuning. i think if you just run 6l6 tubes all the time. With only a bias adjustment, the amp could use KT66, EL37, &L&GC, 7581A or 5881. Beyond that the old V4-B was the same way, you had to swap resistors. 32 × 10 - 1) = 20000 × (2. My 5E3P Proluxe (fixed bias 6L6) power transformer is rated at 330-0-330 volts but actually puts out 341-0-341 under idle load and an input wall voltage of 122. This is something that must be done no matter which tubes you are changing. 15 volts. Keep working with the 6L6 tubes pulled and lift one leg of the mystery diode, then check for (+) voltage on the grid pin that shouldn't be there. Can anyone shed some light on what the solution to this problem is. 5, a Sovtek 6L6WXT rated 8. Right knob not responding at all, doesn't matter which tube is being checked. I've just purchased a new set of el34's to put into my Colonial, as I wasn't a fan of how the 6l6's in it sounded. There are lots of OP amps at the preamp pcb and there is no bias pot. • 6L6: 25–35 mA The BAL/BIAS ADJUSTMENT that GADGET linked in the previous post works well with 800-C, 500-C, 400, and SANSUI 1000A as is. Replace the ground wire on each power tube (pin 8) with this 1 ohm precision resistor. Current visitors. The bias adjustment should be near the power rubes about 3" from the edge of the chassis. In an amplifier with bias adjustment the substitution can be done if cathode current is measured The online schematic I found was inaccurate and showed cathode biasing of the 6L6 output tubes, although it was clear that this unit had fixed bias as the others discussed in this thread. Hi guys. One of the best ways to set a bias is to measure the cathode current, and that depends on the tube type and it's plate Real bias adjustment should be done by a combination of measuring/visualizing THD (distorsion meter/scope) AND ear. The plate voltage of these amps is designed for the 6L6 tubes and this is the reason why the previous owner has used a pair of JJ's I don't have access to the schematic so I have no idea if the bias circuit has an adjustment pot or not. the el34 sound like crap in the mark v anyway. I was quite intrigued with the tone I was getting. Next, your amp will usually have either 2, 4 or 6 output tubes. You can put 6L6's in an amp that has 6V6's with a bias adjustment, no real increase in power though it will tighten up. The schematic shows 6L6GC power tubes. EL34 tubes don’t draw quite as much power as 6L6 tubes. That would be about 35 ma to 40 ma total current flow through each power tube. Click to expand-19. On another note, Bob at eurotubes will send you 6L6's that can be run in the EL34 mode and brings the bias up to where a normal 6L6 should be. SactoBass. Some amps let you change this value with a pot, so while it is fixed, it still might be adjustable. Since you likely have 6L6GC 30W in there, go 36 to 41mA roughly for about 60% to just under 70% dissipation. Ampeg sj 12 T 6L6 and EL34 Biasing 10-29-2012, 02:07 AM . Consult an amp tech whenever substituting tube types. the 2 inner matched pairs at roughly 27mA and the outer tubes at around 38. To determine the bias current you could pick any one of the OT's and remove the wire going to pin 8 and connect it to a 1ohm resistor, and then connect the other side of the resistor back to pin 8 of the tube socket. Not the other way though as a 6L6 draws a lot of current, exceeding what is safe for a 6V6. The final bias voltage of the 6L6 would be -20 to -30v but a leaking coupling cap could send that much (+)voltage down the bias circuit. I also found links to Fender Twin (looked like a Reissue) and Deluxe user manuals. A 6L6GC is rated at 30W plate dissipation so _could_ be operated as high as It's strange that a 6L6 amp has the same bias setting as an EL34 amp. The 5881 is a 6L6-type tube, so the 6L6GC will work just fine My strong suspicion is that the adjustment pot is preferable (I don't see anybody modding their amp to put in a balance pot). 5mA. The JJ-6V6S is more like a 5881/6L6WGB with a low heater So, since you have a bias probe, I'd suggest biasing 6L6's in a Deluxe Reverb in the low 30 ma range. (3-4K as opposed to 5-8K for a 6L6). Just an presumption. In the case of your amp, Peavey tends to set the bias very cold from the factory. the formula is: 30/Plate voltage=Max current draw, for class AB tubes should be biased between 60% and 70% of max static dissipation (at idle), SO, if you're using 6L6 but, you cant run el34 tubes on each end and 6l6 tubes in the middle. 3v. Use this as a guide for setting your bias. de/bias-einstellen-beim-hot-rod-deluxe/ Ich zeige Euch Schritt für Schritt wie Ihr den Bias an einem Hot Rod Deluxe / Deville selbst einste Quad of 6L6 is relatively pretty friendly. Due to superior performance, the more modern 6L6GC, which also features a glass Cookies & Privacy. They were biased at 14ma. Without any tools to read voltage, I just want to bias it as hotly as possible. Customer ” Joe” puts his four brand new matched Tung Sol 6L6GC-STR into his amp and he checks the bias on each of the valves. What you'll be shooting for is a range of adjustment that goes from LESS voltage to MORE voltage than is set by the Stock bias voltage was -51V from the factory for the 6L6 power tubes, but go by the bias current. First I did the JSX, it has a bias adjustment on top of the chassis. That is if the circuit is similsr to most. Members; chu2. When I do the calculations, using the measured 430 volts at pin 3, and 60% using 30watt tungsol reissue 6l6gc str's i get around 42ma. Another poster said the 6L6 family was good for 30. 5VDC at the test jack. I had new 6L6 tubes in it as well when I cranked it up. There is only one source of bias voltage, so the voltage at every tube's grid will be the same (it better be!). Posted May 14, 2006. Each 6L6 is pulling 42 mA at 423 volts. Jul 8, 2009 8,553 9,891 Okay. I popped in some 6L6's and flicked the bias adjustment to "6L6", the adjustable knob was set at noon and I experimented turning clockwise and counter clock wise. The 7868 is only rated at 19 watts. Peavey XXX amps have a "fixed bias" and a bias adjustment knob. Early models were biased at 20, he didn't really know why. I think it made some of them with a fixed, nonadjustable bias circuit, like it Setting the bias adjustment controls to these listed voltages in no way guarantees that your amp is properly biased. g. The solution to obtain standard margins of adjustment (for example 34mA per tube) is to reduce the value of the resistor that every bias potentiometer has connected to ground. But seriously consider getting yourself a bias probe to do the job properly. Stops the output valves over heating and is a permanent repair 6973, 7591, and 7868 Output Tubes and Valves; 300B 2A3 6L6 KT77 EL34 Matched; KT88 6550; ECC82 (12AU7) and ECC83 (12AX7) for Hi-fi; 6N1P, 6SC7, 6SL7, 6SN7, and E88CC Valves and Tubes If you open up the cathode to ground wire off of pin 8 of each 6l6 and install a 1ohm 1% 2 watt resistor, then you can replace r84 in the power supply which is a 12k resistor with a 12k pot, you can then adj the bias while hook up a meter set for low dc volts across each 1 ohm resistor. Without a true bias adjustment pot I can't lower the bias of the tubes. The EL34 tubes & the 6L6 tubes require a much different bias voltage on pin #5 of the output tubes. mark v you have to run all el34 or all 6l6 power tubes. Since 2006 we have offered a single octal bias probe for use with a Multimeter. The 6L6 tubes are listed in the WIKI as suitable replacements for the 5881 with the following caveat: "These are not direct replacements for the stock Melody 5881 power tubes, as the bias voltage must be adjusted to compensate for the different tube specifications. He sets the first valve up for bias adjust TP6 The head Fender EVH engineer told me that biasing it to 30mA is fine, current models are at 30. We set 6L6GC power tubes to (-)55V DC and EL-34 power tubes to (-)43V DC. Proper bias adjustment of tube amplifiers can be performed accurately using a bias tool which directly measures the cathode current of each output 6L6 power tube. Had 33. These probes are $25. " How about that? The 6L6GC type can substitute for any of the types below, but may require a bias adjustment or modification to operate at peak performance. How to Measure Bias. Even better than the original IBAM (Individual Bias Adjustment Mod board) I've been using for the last 3-4 years. 7 will just get you out of crossover distortion and you can go as high as . Things to Consider When Calculating Bias. I bias my 6L6's fairly cool at 36ma @ 440Vp. 040V range and see how it sounds. Dec 13, 2009 #12 Kap'n New member. Looking at the tube itself, it's just a beam tetrode with the beam-forming plates externalized to pin 1 instead of connected internally to the cathode. Tom Don't let that smoke out! Top. You can run the bias max in either direction and you'll still be within the acceptable range. 2. 470v on the plates, allows for only about 60mA (6L6GC) or even 40mA (6L6). Link to the Tube Set I Used: https://www. Before just throwing parts at the problem, I'd swap positions of the 6L6GCs and see if the problem stays with the tubes, or stays at the same tube socket after swapping them around. There is no adjustment potentiometer included. Personally, I like it clean and loud, so I would bias each tube at 100 mAmp if I could, but the plate dissipation, with ca. Giving the bias adjustment ‘a little extra’ (just for ‘insurance’) is not a good idea, unless you carefully measure your plate current, and are assured of not exceeding the maximum plate dissipation. The margins of adjustment they have are very low and you may not get a good setting with a standard medium grade 6L6. How to do a quick bias adjustment. 6L6GC = 8 6V6GT = 10 For example, if you’re using a 6L6GC with a screen voltage of 400V, then aim for a raw negative voltage of: For example, if the bias adjustment circuit amounts to 20k to ground, and the mu of the power valves is 10, then we need a series dropping resistor of: Rseries = 20000 × (0. Likes 1; Comment. Turned it all the way down for 6L6s, which draw more current at the same bias voltage. Soren, my ST-70 has the original power transformer in it. I ran mine at 39ma and it sounded great. The bias adjustment will only allow me to vary between approx 10ma on the low end to about 36 max on the high end. Joined Jan 2, 2002 Messages 1,678. They found that selecting tubes for that "Sweet spot" and branding them "Mesa Boogie" eliminated the components for bias adjustment. It has a built In all but a few cases, 6L6 and EL34 tubes are not interchangeable without modification. The problem however is that the bias range for 6L6 may not be great enough to get some 6L6's into a bias setting where they sound their best. 6L6 Type Vacuum Tube Shootout November 1, 2016; That Time McIntosh Saved the Grateful Dead Per the Onix SP3 WIKI, the bias voltage for the 5881 tubes is 1. These are still fixed/non-adjustable bias amps If you find that your bias adjustment does not run hot or cold enough bias circuit of the amp must be modified to change the range of bias voltage it can produce. May want to save the link. The plate voltage for your amp is 375V based on the schematic, if you're doing cathode current biasing for a 6L6GC (specs at 30W dissipation) with 375V on the plates at 70% you're right at 56mA. Like barely got to the 1/2 way mark before I could no longer make an adjustment. A fixed bias just means it doesn't adjust itself. An auto-bias The increase in bias voltage pushes the tube toward class B operation where no current flows at the zero-reference line of the signal. now, different tubes, even of the same brand, will require a different Typical output tubes are EL34, EL84, 6V6, and 6L6GC. This device reads cathode current, but not plate voltage. in the Stereo 70 assembly manual on Page 10, said that the 5881 and KT-66 can be used in the Stereo 70 with a bias adjustment. Different plate dissapation simply require a different bias adjestment if the 6L6GC has 30 watt PD and the 5881 has 25 watt PD. We recommend to use an ll plastic screwdriver to make the adjustment. Bias can be preset like many other amps have in the past. Hello all! Recently bought this amp, it has yellow led inside I suspect it's old model. I have a bias adjustment for . A pair of replacement 6l6 GT for my tweaker, I had one to tp andone to ground, The term "fixed bias" is simply the way that the circuit "fixes" or "sets" the bias level, and many fixed bias amps will have an adjustment pot, and some only have a non adjustable resistor to set the bias level. It was a single ended cathode biased amp, a heavily modded Fender silverface Champ, and after I installed a fixed bias pot, fixed/cathode bias switch, and bias test jack from Area 51 audio I set the fixed bias pot for el34, then when I wanted to run 6l6. This method will do in a pinch. 5 watts for 65%. Assuming those test points are measuring across a 1 ohm resistor and you have typical 6L6 type plate voltages, you appear to be running the tubes way hot! Lower the bias adjustment until you get into the 0. which is what I want to do is just put some new 6l6's in This is a 6L6 - based amp (self-biasing design, Chinese-made - like so many so-called "high-end" audio nowadays), and one of the ouput tubes quit on me. For EL34 tubes, the factory setting is ~42. As I adjust the bias control, the plate voltage seems to be i know the original 5150/6505 doesnt have a bias adjustment on it, and people mod that for having one. Members. Hello, I have the same amp and there is no bias adjustment on that model. But then why did Ampeg include the user bias adjustment feature on the PF-50T that has a pair of 6L6's, but not the V4B that has two pairs of 6L6's? Reactions: BOOG. There must be a bias pot and some negative voltage supply. That will Adjust the bias pots accordingly to obtain a 50mA reading on each of the milliamp meters My question is, since the 6L6GC is a 30 watt tube, as opposed to the EL34 25watt tube, should it be biased higher? My B+ as measured at pin 3 of the power tube is 468vdc. Installed 10K resistor again. The output stage uses Grid biasing which is apparently factory set fairly cold so that any matched pair of 6L6 valves should work fine. If you want to bias the amp properly, you will need either a bias probe, When you adjust the bias on your amp you are adjusting the level of current available to the power tubes. Then an adjustment will be required. However, when trying to check and adjust bias according to manual (2 bias adjustment knobs) something looks really strange. Assuming your bias pot is wired as a variable resistor in series with the resistor to ground, then if you want a bigger range on your bias adjustment to handle both 6V6 bias range and 6L6 bias range, then put in a smaller fixed resistor and a bigger value pot. It is a set value. consisting of a (lower value) resistor in series with a potentiometer. Since both push-pull and parallel tubes are used here, the output tubes must be replaced by a matched quartet. 04 volts and tube balance on back of amp for o volts. 5. What's new. 2 For 8-pin power tubes such as 6L6, 6V6, 5881, 6550, EL34, KT88, KT66, KT100, etc. 5U4 is rated for 225 mA, vs. A pair of 6L6s putting out 55w means that the amp must be operating in class AB, fixed bias (cathode bias, which doesn't require biasing when you change tubes, is unusual in 6L6 amps and would only put out around 30w). The bias setting range is not the big. This seems abnormally low for a 6L6. For 6L6 tubes, the factory setting is ~55VDC at the test jack. Things do run quite warm, and I removed the cover as a matter of course. ; makes it less negative), it might just happen that running 6L6GC with bias switch in EL34 position could let you adjust the bias to where it puts the tubes at 60-70% of max plate dissipation. Then switch the amp back into the play mode and the bias will jump up from about Bias adjust for each 6L6GC tube and even gain on 12AT7 I just finished modifying my Fender Twin 135RMS and it sounds better than the day I bought However, if I recall correctly, it did not go back to using a bias adjustment control in all of its fixed-bias amps. I sprayed contact cleaner on the tubes, inserted the tubes, adjusted the bias pot to as low as it goes and set the amp on standby for 5 minutes. We said to put the bias switch into the EL34 mode if the bias will not exceed 34mA with 6L6's, or if you just want to run a little higher up to about 40mA. . Like an idiot I bought Groove Tubes 6L6-GE in Medium (4-7). Then you will be able get a bigger range. Had it about 9 months and I can tell it needs a retube. B-string. In the real world they would be different with the 6L6's generally biased a little higher. lol JJ-6V6S (JJ-6V6GTS) can be substituted for 6L6 with fixed bias adjustment, which is unnecessary if the amp such as Epi BC30 (a Bassman clone) is cathode/self biased. Thanks, is for the 6L6WXT. In other words, I had maxed it http://ritteramps. The Mesa/Boogie Dual Rectifier has a grid bias supply that is switchable to support EL34 power pentodes or 6L6 beam power tetrodes. The Classic VT has a full solid state (op-amp based) preamp and then uses transistors for the phase splitter before the push pull 6L6 output stage. If you replace the 10K with a 5K in series with the element of a 10K bias pot, and take the bias voltage from the wiper of the pot, you will have a variable divider with a range of about 79% to 93% of the rectified DC voltage. Added push/pull gain boost and eL34/6L6 switching So id the bias current is to be set to 35mA you should measure 35mV on your meter. My tech fitted the Bias adjustment kit from Torres and I've biased it to read 64mV with 2 JJ 6L6 GCs. Hotrod Deluxe “How To” Bias Video. So an amp can be both, but it can be self-biasing and fixed bias. This mod also allows you enough adjustment range to bias 6V6, 6L6, EL34, KT66, KT77 and KT88 power tubes. (such as Mesa Boogie™) modified by a technician to provide for bias adjustment. This is a negative voltage. s Apparently some people have added a global bias adjustment mod to theirs, but mine has not got this. Gearhead; If you look at the 6L6GC in a class AB1 push-pull configuration running 450 volts on the plates (which is what you have now) the load resistance (plate to plate) is about 5. This is why fixed bias amplifiers - which AFAIK ODS #0121 technically operates perfectly and without any problems at an idle current setting of 125 mA for a pair of GE6550A as specified by Alexander Dumble on the Bias adapters will properly measure the cathode current on most all amplifiers, even if they do not offer a bias settings (certain Music Man amps are an exception). Tube type: ----Choose---- 6V6 EL84/6BQ5 5881 EL34 KT66 6L6GC KT88/6550 FJA Mods modified Peavey 5150 with 60 watt Metal Mod. Requires separate digital volt-ohmmeter capable of reading current. More Information There are many web sites that explain setting bias. Its deffinately worth it especially since you can go back to 6L6s with just a bias adjustment if you dont like it. 5881 and 6L6WGB types The Tung Sol is the most asked for tube here. The bias on a fixed bias amp should always be checked to see if it is within specs. Bias test probe 2 in tube 3: 27mA If i switch to positions 2 and 4 I get the same. I also understand that many prefer to measure plate current but since I don’t have a bias probe at the moment, I’d like to check the voltage at the test points. If you had standard 6L6 19W then he suggested a bias current of 26mA for 70% dissipation. Also, I would suggest either TAD 6L6GCs or SED =C= 6L6GCs in that beast. I adjusted the bias for a fairly rich 45mA, although I may back that off a bit. This is so you can throw just about any 6l6's in there without problems. That doesn't necessarily mean something will blow up if you don't get it biased but there is some risk: 1. Yes, the actual adjustment is the grid-cathode voltage (-). Most of the Peavey amps from the late seventys and up were all plug and play with a fixed Not general bias and balance between sides. Read the voltage drop across the resistor (in millivolts Adjustable Fixed Bias amplifiers (adjustment potentiometer to set the “fixed” bias point) Cathode Bias amplifiers (also commonly referrred to as self-biasing)Each of these types is unique. 1, 9, and on older tubes Fender Groove tube 17. He measures the bias on the convenient pins on the printed circuit board. The 5881 and 6L6 come from the same family of tubes. When I adjust bias for low power then check bias at high power I get different readings that are much higher. Those rare amps simply require a bias adjustment, however the majority will likely require some physical modifications. It was like 48mV from the store, Eurotubes Fender Deluxe - Deville "How To" Bias Video. An auto-bias should take the warm up in consideration which is around 15-30 minutes on a well ventilated amplifier. g1. I checked the bias with my meter, it was showing 25-26ma on both tubes. Vintage blackface Fender™ amps from the 1960's offer true bias I replaced a well-used 6-year-old set of tubes that were scratchy and noisy on this Fender Blues Deluxe Amplifier. For bias adjustment I used a 25W 1 k-ohm rheostat bypassed with a 200 uF capacitor which allowed me to set the bias at each operating point. The 25 range will be pretty cold for 6L6's. Hopefully the bias adjustment allows enough latitude, worst case might have to change out one of the series resistors. Forums. This mod allows wide range of adjustment to bias 6V6, 6L6, EL34, KT66, KT77 and KT88 power tubes. But this doesn’t mean that they don’t need a bias-setting A discussion on the Marshall JCM2000 Bias drift issues and a relatively low cost solution. The plain jane 6L6 seems to be 20. The front of amp has a high power 100watt and low power 25watt switch. It seems to have all the original tubes including the blue GT 6L6's. Using the data from my tube tester,the 6L6 uses -18 bias volts where the 6550 uses -23 with all other parameters being the same,I have to imagine the 6550's are going to be less than ideal using the same cathode resistor,if it is self I can replace a 10K resistor with a 15K in the bias circuit to give me more of a bias adjustment. Biasing a Peavey Windsor. I will tell you now I have had this along side The 6L6, 7581A, 6V6, 6550, EL34, 5881, KT66, KT88, KT90, KT100, etc. But if you don't have access to one, the 6L6WGB is rated for a maximum plate dissipation of 26W, so Voltage from bias test points provided by peavey: 58v Bias test probe 1 in tube 1(from the left) set to 38. Amp sounds great! I've placed an order for a bias plate current probe. of 125 mA for a pair of GE6550A as specified by Alexander Dumble on the back of #0121 on the right beside the two bias adjustment pots (see the attached picture). Decreasing the R30 resistance lowers the bias voltage, (i. You have to repeat this proceedure a few times as setting the bias current will have an affect on the plate voltage. 9 to -24. Often it is a switch that can select bias for EL34 tubes, or 6L6 tubes depending which is installed. 5 ohm sense resistors so that I can measure screen and plate currents at each operating point. (this is why it came with 6L6s in stock As has been mentioned, the bias adjustment is not available on MC amps without modification, and due to lower maximum current, the 6L6 will make less power in front of a MC75 output. I like to know I am working with something fairly reliable, not a big investment, worth it to me. when you adjust the bias, you are changing the value of that negative voltage. ONLY FOR AMPS WITH BIAS ADJUSTMENT CONTROL. like a mark iii or mark iv. They're set pretty cold as is. The 6L6WGB is listed at 26 watts. Pulled 15K bias resistor. With that I get 434 volts B+ with a solid state 1N4007 diode rectifier and 416v How To Bias EVH 5150 III Stealth 50 Watt & EVH 5150 III 100 Watt Stealth Head, 6L6 Power Tube Bias Adjustment How To Bias the EVH 5150 III Stealth Head 50 Hello. what needs to be done on the 5150II/6505+ in order to get that window to open up ?? a different This is the grid bias we’ve been zoning in on, and the vast majority of tube guitar amps boast this type of bias. Thread starter Ampaddict; Start date Nov 1, 2006; Ampaddict Well-known member. This is for the 100w. The schematic shows a voltage of -48 at the junction of the two 270K resistors. The various 6L6 types, 1614, 5881 and KT66 all have the same pinout but among them the KT66 has different biasing requirements. After cranking the bias adjustment fully clockwise (from inside the chassis), the best I could do is get 37ma of Cathode Current at a Plate Voltage of 436 volts. That is for 6L6 tubes so I'm not sure what the 7868 bias at. 6k ohms. The plate voltage on the 6L6 was about 490V and was biased a little bit on the cold side. I was changing the power cap in mine when it blew. I've gone through the amp and it looks good, sounds good including the trademark hiss, lol! I'm planning to mod it over the next weekend. Their recommended 6L6 bias setting was 20mA. otabcq nozp fin rzdeg ach kiza mqq ahteh wspmlru hws