6l6 bias adjustment the other to adjust the bias voltage The Studio Signature 35w amplifier is built to use 6L6GC power tubes. Dec 11, 2008 · " on the back there are two trim pots and three measuring points. Feb 4, 2009 · I Got round to pulling my GEC kt66 out of my no#2 Metro/Marstran 45 build, installed a set of Sylvainia 6L6 WGB tubes I scored from Tramp, Man does this amp sing with the 6l6 after a bit of bias adjusting and good warm up it really sounds cool, real inspiring to play,thats when I know I have hit that sweet spot in an amp, I would like to hear from any of you guys who run 6l6 in JTM45 amps and Jul 31, 2014 · Hello there, I recently purchased a Fender Princeton Reverb re-issue (the "68" silverface), and a few days after I took it home, on my first chance to crank up the volume (5 or 6), I started noticing a rattle / hiss on some low and medium frequencies. You could replace your output tubes between sets and go right back on stage. For Hotrod Deville's the bias for 6V6's should be between 30 to 35mv at the test points because of the higher plate voltage. Non Fixed Bias is Cathode Bias which self adjusts. You may wish to have certain fixed bias amps (such as Mesa Boogie™) modified by a technician to provide for bias adjustment. On the 5150 II, even if you crank it all the way up, the bias will still be cold. I believe the amp will run with two tubes pulled, don't ask me which ones though. Tubes have a maximum dissipation rating in watts for how much power (and therefore heat) they can deal with so we adjust the tube's bias voltage (voltage between the grid and cathode) to keep the bias current below the tube's max dissipation (power & heat) rating. Hope that helps!. Every fixed bias amp I built, I used a 1 ohm resistor to measure the voltage drop from the cathode to ground. Often it is a switch that can select bias for EL34 tubes, or 6L6 tubes depending which is installed. com May 5, 2013 · When you adjust the bias on your amp you are adjusting the level of current available to the power tubes. Jun 3, 2022 · The next improvement is to increase the bias for each of 6N1P and 6L6. One slightly hotter and one slightly colder. The guy i bought it from said he put EL34's in it, but i looked at the bias switch Nov 3, 2011 · 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 Generator, an Oscilloscope, and a Sep 6, 2017 · Although cathode bias amps are "self biasing" it does NOT mean the bias should not be checked, especially when changing brands of power tubes (and significantly different versions). 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. Jul 15, 2006 · By the way the Buster Amp would be considered fixed bias whether it has an adjusting pot or not. The other pair are the stock Ruby 6L6's. Turned it all the way down for 6L6s, which draw more current at the same bias voltage. In cathode biased amps we must change the cathode resistor value to change the cathode voltage to adjust the bias. 3v dedicated filament trans. I also wired the extra heater winding to two of the output tube sockets, since the 6L6 has twice the heater current. (6V6 tube max plate dissipation) / (Voltage on the tube) = (Max bias current trought the tube) => 14W / 438V = about 30mA PER TUBE. Adjust bias until crossover notch just disappears. This video is one we decided to do to show how easy it is to bias the Peavey JSX, XXX and 3120 amps using our new Eurotubes "Pro One" Bias Probe. If you're squeamish about cutting traces, and if there's room in the head cabinet, you could insert like a permanent bias probe jig (tube socket into tube plug, with 1 ohm resistor) in each, that would add about the height of a tube base to each power tube. I then inserted a set of JJ 6L6GCs. For now the amps are built using fixed value resistors, but I have some precision trim pots I can substitute for R13 to adjust the bias. 5, a Sovtek 6L6WXT rated 8. 090 volts is normal for 40 Watt amplifiers with (4) 6V6’s. Obviously there is a variable pot between the plates, but doesn't that just adjust the balance between the two tubes? Should they be biased individually first? Jul 26, 2001 · If you want to maximize the sound from your amp, you need to have it properly biased. And as I stated up front, the OP's amp HAS no adjustment. Feb 27, 2012 · - I'm targeting a bias current of 1. Mar 1, 2013 · Setting the bias, if you know how to do it, is not difficult and will allow you to save on costs. The problem is that if you increase that current draw, your plate voltage and B+ will go even lower. The JJ-6V6S is more like a 5881/6L6WGB with a low heater current (6V6) and can take very high HT (B+) voltage. Biasing was easy and well within the range of the standard bias adjustment circuit in the test amp. 5 mA for El34's. Nov 5, 2015 · Thanks guys. Congratulations again! You have just biased your amp! The following info is for octal power tubes. ‘Properly’ adjusted bias, as seen by the Tektronix 453 oscilloscope. mark v you have to run all el34 or all 6l6 power tubes. Feb 29, 2012 · bias adjust TP6 The head Fender EVH engineer told me that biasing it to 30mA is fine, current models are at 30. Mar 21, 2025 · EL34s went in and biased with some adjustment. Run it into a load bank. The best way is to observe the live current. May 21, 2010 · Only needs a bias adjustment. 5 watts of plate dissipationthat is not an 'incorrect' amount of plate dissipation. Fixed bias also minimizes warranty expenses (read: more profits). Jan 28, 2008 · Fixed bias with or without an adjustment pot. This is like setting the idle speed in a car. Again, a quad matched set. (this is why it came with 6L6s in stock form, to keep it from ever Nov 10, 2010 · You can't calculate plate dissipation without knowing the plate voltage. This is why it is important to buy matched tubes (so they all idle at the same 'speed'). OP- If you flip the switch ,set the bias knob at noon ,and pop in the another type of tube you will be OK. 060 volts is normal for 50 Watt amplifiers with (2) 6L6’s. - Allow amplifier to warm up for 20 minutes and adjust bias for both - tubes again. :) Jul 28, 2017 · but, you cant run el34 tubes on each end and 6l6 tubes in the middle. For 8-pin power tubes such as 6L6, 6V6, 5881, 6550, EL34, KT88, KT66, KT100, etc. Dec 7, 2015 · They are cathode biased so they don't come with a bias adjustment pot. The tubes have great tone and nicely capture the “6L6 sound”, characterized by sparkling highs, warm mids, and controlled bass response. Apr 11, 2014 · Put your meter to DC volts, plug it into the bias test points on the amp and set the thing for -42. Ill turn on the power and let them warm up, and then flip the switch for standby. Drive it with a 1 to 2 kHz signal to just below the point of clipping. Same power transformer, and same bias circuit, but the total circuit load has changed, so I had to adjust one of the bias resistors to get the bias range right. This bias adjustment will improve the sound quality but it has a drawback. I've installed them and am trying to bias them, but my multimeter doesnt seem to be reading them very well. Please post the correct schematic if you can. EL34 tubes are typically idled hotter tha 6L6 tubes. Cheers, Dave O. The primary modification necessary is to properly bias the amp. I is simply a 'cold' bias piont. With the bias cranked fully clockwise (from inside the chassis), I can get 44. Aug 31, 2009 · It's a bias adjustment, since the owner of this amp wanted to be able to use 6V6 or 6L6 tubes. Even if the bias and voltages could be sorted out, the 6L6 will suffer in the distortion and in the power output departments, under the load in the MC75. 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. Most amps seem to have a wide range of bias voltage available. New Month We are at $40 this month in Donations. Think of it as a rough adjustment before you do the fine bias adjustment. 1V to -0. 6% with a 30-watt 6L6 or 27% with a 24 watt tube. Jan 12, 2024 · There is absolutely nothing wrong with JJ 6L6 output tubes. I re-checked and tested the two bias settings: Adjusting the negative voltage to -50Vdc at R59/R60 (most of the way left of the bias pot) - I got 27. EL34's are 25W tubes, so (25/500) * 700 = 35. => (21mA x 2 =) 42mA is the normal bias value for this tubes with an HRD amp (if I bias Oct 5, 2001 · The amp has the bias adjustment trim pots located on the back near the EL34/6L6 selection switch. when you adjust the bias, you are changing the value of that negative voltage. Nothing really old or vintage about the sound. Those older batch Svetlanas made it sound much bigger. Jan 11, 2024 · How exactly can I fine tune the bias of the 6l6 output stages. It's the MQ13 version with two probes and one probe has the switch so I can flip between plate voltage and cathode current readings. But this doesn’t mean that they don’t need a bias-setting adjustment. To make the 5F6A's bias adjustable you simply replace the bias circuit's 56k resistor (connected to 15k resistor and ground) with a mini-50k linear pot (or trim pot) and 27k resistor. 055 to 0. Cathode Bias (Self Adjusting) Jul 11, 2020 · To set bias on the amp put your meter on DC V, one lead to output valve V2 g1, the other to OP valve V3. Dec 30, 2014 · The BAL/BIAS ADJUSTMENT that GADGET linked in the previous post works well with 800-C, 500-C, 400, and SANSUI 1000A as is. You can measure them individually but the voltage goes across them both so you should find the average value for both. the 2 inner matched pairs at roughly 27mA and the outer tubes at around 38. Jan 7, 2012 · 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. These are still fixed/non-adjustable bias amps as there is no internal bias trimmer pot to fine tune. Jan 16, 2010 · No but I have been through electroincs school and the JSX has jacks on the outside and a adjustment knob right inside the back by the selector switch to that allows a change from EL-34 to 6L6 tubes. The Svetlanas were recommended to me by NBS Electronics (Lord Valve). YES YOU READ RIGHT! 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 Jun 3, 2022 · The next improvement is to increase the bias for each of 6N1P and 6L6. A bias adjustment pot - this adjusts the bias of AL the output tubes. Then switch the amp back into the play mode and the bias will jump up from about 20mA to about 35mA and you will have plenty of adjustment to get even the coldest 6L6's biased properly. Adjust to maximum negative bias. 6VDC, and the idle Plate current is measured at 20. FWIW slight mismatches are often desirable to 6-stringers for warmer, harmonically-rich sound, but steelers usually like precisely matched tubes for better clean tone and headroom. Dec 4, 2018 · So, here's what my buddy at Eurotubes says: "it sounds like you would do better with some hotter EL34 types. And my B+ is about 405 Vdc , so the 6V6 would be easily accomodated. As Sandman31774 mentioned a probe with plate voltage option would be better Oct 24, 2006 · Which also suggests Fender didn't pull "-45v" of bias out of the air, nor bias purely by idle current or dissipation. 6L6 bias levels if fixed biased will put a 6CA7 very close to class b operation. Aug 5, 2008 · I have a 15K pot in place of my bias range resistor to make bias range adjustment easier and rebias quicker. All you need to bias one Nov 1, 2006 · According to the site 6L6's and EL34's should be Biased at around 65mA, is this correct? Home. 120 volts is normal for 100 Watt amplifiers with (4) 6L6’s. 5ma with a Plate voltage of 428 volts. To prevent a non bias situation, I used 1m fixed R's from wiper to each end of pot. Then an adjustment will be required. Jan 18, 2012 · 2. Use this as a guide for setting your bias. Wait about 5 to 10 minutes with the amp in the play mode and do a final bias adjustment after the tubes are hot. 9mV reading at suggested bias test point 30. That said, the JJ 6V6 tubes are a little different and more robust than other tubes of that designation. My questions Mar 19, 2015 · Voltage from bias test points provided by peavey: 58v Bias test probe 1 in tube 1(from the left) set to 38. For example, with a simple bias adjustment, a 6L6, KT-88, or an EL-34 can sit in for the Mercury's KT-66. Bias test probe 2 in tube 3: 27mA If i switch to positions 2 and 4 I get the same. The total circuit load affects this, too. Nov 29, 2005 · The JJ's are fine and Bob at eurotubes. One more thing: some fixed/non-adjustable bias amps have a "bias switch" to select for a specific tube type. Apr 8, 2016 · A 6L6/EL34 bias switch allows the use of EL34's with no bias adjustment. 500 volts for KT88's or 6550's. Being cathode-bias the current won't change much, a resistor will give a semi-steady B+. Flip the switch to the target bias, then turn the bias knob for the tube til you get a green light. So you need to go less negative on the bias voltage. 7 = 45 mA . when set to millivolts it tends to not read anything, but when on milliamps its at about . If you like the standard 6L6 this company makes you’ll like the sound of this flashy cousin. Nov 20, 2008 · Bias probes are made like this so that a person won't shock the s++t out of himself. Feb 17, 2006 · The 6CA7 has considerably higher transconductance than a 6L6, higher filament current, and requires a very different bias point for proper performance. This mod also allows you enough adjustment range to bias 6V6, 6L6, EL34, KT66, KT77 and KT88 power tubes. I am trying some differrent preamp tubes to see if I can cut back on the hiss a little since they are so cheap and Peavey does no matching or Mar 31, 2003 · Unless you do something fancy, the sense-adjust type autobias circuits will not work for AB operation. And I don't use my unused socket pins as terminals. 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 voltage for the most part, and the small grid current. Yes, sorry the 6V6 would normally be adjusted to yield lower numbers. also I think the Mesa tubes sound great and no worries. like a mark iii or mark iv. - 5% variations between each tubes bias readings is acceptable. Mar 27, 2013 · 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. Feb 4, 2014 · - For out put tube 2 (VB-2), connect (+) probe to TP-2 and adjust VR-2 - until meter reads 330 mV. Note that data sheets for the 6L6 indicate a somewhat higher idle current (88mA) under these general conditions. 0. Setting the bias on your amp is like setting the idle on your car. 1v say, you want to adjust it to 0v if possible, though that's not easy with a one turn pot. If you bias too cold it will sound weak, if you bias too hot it will get flat (lose dynamics). Oct 29, 2012 · The one limiting factor when going from 6L6 to EL34 is the bias voltage range. The bias trimpots don't last forever and they need to be periodically replaced, best at the same time of a recap. donation. On more amps than you'd think it is simply impossible to adjust the bias anyway, and not just in cathode bias amps like Vox. Normally we bias at 70% => (30mA x 70% =) 21mA BUT at the HRD bias test point I read the current that pass through BOTH tubes. Comments: The rear panel shows the speaker load as 2 ohms. 21. 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. 2-1. is still fixed bias. This mod allows wide range of adjustment to bias 6V6, 6L6, EL34, KT66, KT77 and KT88 power tubes. Anyway, I tweaked B1 and B2 independently, meaning when I tested V5/V6, I turned B1 all the way CCW. I built a probe that measures current and volts at the plate. Feb 7, 2013 · Yes, if you are using 1 ohm resistors to measure your current you would need to break ground on each tube. Oct 5, 2015 · Perhaps this is a factor in the extremely low noise and lack of microphonics that this vacuum tube exhibits. I got mine with the 5881's still in it. Regards, Gordon. However, if I adjust bias pot 2 (B2) it affects V5/V6, but it also has a bit of an affect on V7/V8. Nov 11, 2008 · I put the bias pots so I can adjust them from the top of the chassis. EL34 tubes don’t draw quite as much power as 6L6 tubes. Verify the setting is in a safe zone (depending on the tube's reputation no higher than 90% of max dissipation, although I usually go no higher than 85% except for short studio sessions). Adjust bias for tone 5. If you do have a bias pot in the circuit, then I am going to thinkthat you have not gotten the current draw hot enough to negatively affect the trem function. With the XXX, JSX and Bugera amps that are switchable from 6L6's to EL34's you may find that some sets of 6L6's will not bias up high enough and in this case you should adjust the bias down as low as it will go, then switch into the standby mode and set the bias selector to the EL34 mode. 5 VOLTS for EL34 or -55 for 6L6. Nov 17, 2022 · In all but a few cases, 6L6 and EL34 tubes are not interchangeable without modification. theres no problem with installing a bias pot. The 8-Watt Mercury's built-in attenuator can send 2 Watts, 1/2 Watt, or 1/10 Watt to the speaker, allowing access to the full spectrum of the Mercury's tones at bedroom volumes. Fender chose-45v of bias because the "average 6L6" will require a ~45v peak drive voltage to pull enough plate current through the chosen OT load to make the chosen power output. In fixed bias amps we adjust the grid voltage to set the bias. I have a HRD that I rebuilt as a Tweed Pro (5E5A), and to get to 69% max plate dissipation, my cathode voltage reads about 108mV. It will work fine in fixed bias and cathode bias designs and sounds just like the standard Tung Sol 6L6GC. Your 65ma? You could adjust them back down to 38ma or 28ma or 18ma, and the amp will play fine. So, switch on the amp, standby on, no signal, speaker attached. 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. Then set the bias select switch to the EL34 mode. If possible, work out which end of the pot gives low bias, so that you can adjust upwards from there (instead of adjusting downwards from glowing plates). Dec 4, 2011 · If it is a Latino built with the VTA board there is a bias adjustment for each tube and with EL34's or 6L6's the bias should be . Aug 26, 2006 · Adjust the bias trimmer ("pot") until you get a reading across the 1-ohm resistor(s) somewhere in the 30-40 mV range, for everything but 6V6s or EL-84s. Set bias at 70% 4. May 4, 2010 · I set the bias adjustment low to see what readings I'd get. 01. The Bias Easy™ 800 allows you to monitor the bias setting safely and easily. In a sense, they're sort of a halfway house between a 6L6 and a true 6V6. If you listen to the video I will explain that I'm showing two different methods of reading bias and showing the probes and meters so you can get a better visual on how the bias works. Tube type: ----Choose---- 6V6 EL84/6BQ5 5881 EL34 KT66 6L6GC KT88/6550 Aug 17, 2009 · 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. Ex: If the meter says 16mathat's about 8ma per tube (if it's a matched set). Bias adjustment Herbert. The power tube sockets are wired for EL34 Jan 10, 2003 · Sonar, I am not sure what your point is here. The most asked question we get is "do I need bias probes that are like the ones shown in this video to bias my amp. 1, 9, and on older tubes Fender Groove tube 17. May want to save the link. Mar 30, 2023 · Per the Onix SP3 WIKI, the bias voltage for the 5881 tubes is 1. (50 watt) so I can adjust each side independently. If you use a power transformer that does not include a 50 volt bias tap you can use this circuit to generate bias voltage from the power transformer high voltage tap. As long as I use a 6L6GC or equivalent (5881, for example), there is no need (or possible way) to change the bias. That would be about 35 ma to 40 ma total current flow through each power tube. that should be a safe decent setting to go by . Bias should be checked each time output tubes are replaced or a defective tube is suspected. If your set is not matched, it will adjust them all up or down, but not to the same place. 2mA for this round of testing. The noise kicks in only 5-10 min after May 23, 2015 · 3. Grove tubes are fine and you get the warranty, also they will work fine in mesa with no biasing(use GT 4-5 #). now, different tubes, even of the same brand, will require a different bias voltage in order to control the flow of electrons. Something is not correct, methinks. Jun 9, 2011 · Power: 2 x 6L6; Bias: Fixed with adjustment pot Rectifier: Solid State. The C's are much nicer than the Ruby's and better than the JJ's that I tried as well. The - grid bias voltage is a set voltage that by itself really doesn't determine anything. It really makes it convenient and easy to adjust. It's a quad tube 120 watt head, but it has a 60 watt mode that uses the 2 outside tubes, and each pair has it's own external bias adjustment. Not the other way though as a 6L6 draws a lot of current, exceeding what is safe for a 6V6. Click on photo for larger view: Revisit™ BiasEasy™ 200 Cathode Current Measuring Probe For 8-pin power tubes such as 6L6, 6V6, 5881, 6550, EL34, KT88, KT66, KT100, etc. if that's the only issue. Replaced R17 & R28 back to 47K played the amp for a few hours this afternoon went to recheck bias after I had been playing a while because I noticed that the gain was a bit less now then when i started. Note that these calculations are for only one tube. 5. I set bias at 38ma and now it was at 32ma the bias adjustment resistor VR2 will only allow me to adjust 27ma to 34ma so a total range of 7ma. Aug 6, 2022 · if the amp is rated at 100 watts I would consider the 6L6 as a 25 watt tube in my equation for bias if it's 120 then I would use 30 watts which is the actual max dissippation of a 6L6GC so . 5mA. Man the OD channel is like no other Fender Ive played! Thick meaty,singing are a few adjectives that come to mind. As you adjust the pot the reading will go + 0. Feb 1, 2005 · 1. I have not found a detailed factory procedure description on the factory bias test points on the back of the amp. 2, and one sovtek rated 9. This is for the 100w. More Information May 4, 2014 · I'm using the pair in my Egnater Vengeance head. lol In other words you are reading bias on 2 of the 6L6's not just 1so the real bias for 1 6L6 is about one half of what the meter shows. Is it just a matter of adjusting the bias? Or is there a current draw issue other than heater current? I'll get a 6. The two bias resistors in a typical bias circuit form a voltage divider to reduce the voltage coming out of the bias rectifier diode. 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. Click on the pics and they will shrink to size. Nov 10, 2024 · The bias section in the amp is: 50v bias tap -> 1k resistor -> Rectifier Diode -> 10k Bias divider; 2x 10uF Bias Filters and then a 50k bias pot with a 22k resistor in series. pot. 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. Ill have my Bias Adjust knob turned to all the way to the left. i think if you just run 6l6 tubes all the time. For example, if the amp wants an 8 ohm speaker with the 6V6 you can run a 6L6 with a 4 ohm speaker to optimize the output impedance match. You would adjust your bias knob until your meter reads 35mv, which because of the 1ohm resistor means the tube is drawing 35ma of plate current. This is because the bias pot is limited by a 12K resistor and does not have enough range. One or two electrolytic caps are filter caps to smooth out the pulsing DC from the bias diode. I'd like to try EL34s in this Pro Reverb build- preferably an easy switch between 6V6, 6L6 & EL34s. The calculator will give you the bias current for each tube. The Buster amp will function well without adjustable Bias providing the Bias is at an acceptable level/the tubes Mar 8, 2009 · the point of this negative voltage is to regulate the flow of electrons through the tube. 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 Nov 11, 2008 · I put the bias pots so I can adjust them from the top of the chassis. Sep 8, 2016 · 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. 030V WARNING! In fact, the internal assembly looks almost identical to the standard Tung Sol 6L6GC-STR. The power dissipation of the amplifier as a whole can also be measured using the amplifier input line power using an AC power meter such as the P4400 Kill A Watt device. Please consider making a donation. See full list on killerrig. Aug 9, 2019 · if the windings require -1. Note too that as things warm up, the optimal bias point can shift somewhat. Do not set above listed range! Jan 20, 2006 · 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. 035V to 0. While adjusting the bias control pot of your amp, the Bias Easy™ shows the actual current flowing through each of two power tubes. Before making any circuit changes test the voltage at each output tube socket (pin 5) and measure the range of bias voltage as you turn the adjustment. Remember to check this for both tubes, and if something's different, use a conservative setting. This design allows the use of many of the octal base output tubes including the 6V6, 6L6, 5881, 6550, KT66,KT77 & KT88. Jul 7, 2018 · According to my Groove Tubes bias kit manual, that means 20-50 mV is the range, with a nominal set point of 37. Apr 1, 2013 · No one thinks bias doesn;t matter, what we do think is that bias is not a critical adjustment. Never tried anything else. Jun 20, 2024 · Based on B+ of 380V and a screen supply voltage of roughly 300V, I predict you'll need about 22V to 24V bias, and that a 250Ω resistor will get close to this. The 5881 and 6L6 come from the same family of tubes. This website uses cookies to ensure that you get the best experience. We recommend to use an ll plastic screwdriver to make the adjustment. I think its often a point of confusion. I liked it so much I added another one. So, you use the probe with one tube, dial in the necessary voltage on the bias pot. The differences between the 5881 and 6L6GC are large enough that a bias change will be required. Later versions use a true balance adjustment that decreases the bias (less negative) alternately on one socket or the other depending on the direction of rotation. I would say that it would work here with minor modifications. Requires separate digital volt-ohmmeter capable of reading current. The Trainwreck Express, although fixed adjustable bias, is famous for working with EL34's and 6V6's, if the pairs are well chosen there's not even the need to change the bias. On most amps, changing the bias pot without a meter would be a very bad idea. It seemed to get stiff and harshgoing counter clockwise and got warmer and softer/buttery feeling going clockwise. There is also a test point under the chassis by this selection switch for checking the voltage. Fender Custom Vibrasonic; Oct 24, 2004 · Example: 6L6 max plate dissipation 30W bias current for each tube = 30/Plate voltage (with no signal applied). Oct 24, 2012 · First I did the JSX, it has a bias adjustment on top of the chassis. My 6505+ combo is this way. yes, wiring a variable in place of the 15k lower leg of teh voltage divider will work. Jun 18, 2008 · I just got a Peavey Triple XXX head, and it allows the option to adjust between biasing for 6l6 tubes or EL34's. Hook a scope across the output. Jun 16, 2004 · The Peavey 5150 has no adjustment for bias. If PRS recommends 30 mA for 6l6's, their recommendation for El34's must be around 25. The bias is measured at -42. The sound will have it's best beauty/agressivness ratio at a certain bias point. 115 to 0. Dec 31, 2014 · There are many web sites that explain setting bias. A reading of 0. I found some nice PEC pots that have a locking collar nut so once you set the bias where you want it, you just tighten the lock nut. I checked the bias with my meter, it was showing 25-26ma on both tubes. You have to either build in a means of sensing when there's signal and having the circuit automatically hold the last value (a uP type circuit is probably the best implementation), or do a variation of Chater's circuit for solid state amps (Audio Amateur, 2/88). One for the EL34 and one for the 6L6. the el34 sound like crap in the mark v anyway. Dec 22, 2007 · update: After it was pointed out a 6L6 is a 30 watt tube I bumped up the bias to 38ma or roughly 65% idle dissapation. The bias circuit uses the power transformer's 50v tap and features bias balance and bias adjustment for 6V6, 6L6 and EL34 tubes. And no, I won't change the bias to cathode bias. 4A since I will be using this amp to drive 4 ohm speakers, and because I have the heatsink capacity. It appears the Thrasher 50 is geared more for 6L6’s, which take a lot more negative bias voltage to come into the safe operating range than EL34’s, so if the bias voltage range doesn’t get to less than about 38 negative volts it will choke the cooler EL34’s down a bit too much. With a 100w amp, there is only one such 'idle adjustment' to control the setting for all four tubes. AS reswot notes, the PRinceton REverb is a fixed bias amp that in stock form has no adjustment pot in the bias circuit. So you need a range of something like -35 to -55 volts. May 18, 2021 · The output transformer impedance ratio is the biggest drawback. Put the tube type selector to 6L6 from the EL34 setting. So if you're biasing 6L6's and the bias will not get up above 34mA then adjust it down to about 20mA and put the amp in standby. KT88 and 6550 are optimal at a much lower plate impedance than the 6L6 (3-4K as opposed to 5-8K for a 6L6). That's all of the pertinent information I can think of at the moment. Feb 22, 2015 · 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. It also changes as they warm up so let them get to operating temp before you do a fine adjustment. Jan 11, 2014 · The bias setting range is not the big. When setting the bias pot to the coldest setting I'm getting about 25W with those tubes installed. Here is s video to simplify the process. 6mA - adjusted by me upon installation via the provided bias adj. Those rare amps simply require a bias adjustment, however the majority will likely require some physical modifications. Very, very few product amps use individual bias, you have one bias pot to bias all 4 tubes. (clintj) Yes I have a BiasPro tube bias probe that a just purchased from A-Sharp Fretworks. If you bias your amp too hot you may shorten the life of your tubes but bias it Feb 23, 2019 · I've placed an order for a bias plate current probe. Oct 16, 2008 · I want to verify the bias on my 120w Triple XXX. More Information. 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). It is possible that you might prefer the tone of the amp with the bias set at a voltage that is different than what the factory sets it at. Apr 26, 2012 · Yes, the bias is set for both tubes together, not individually. The amp as designed gets its tone from the preamp, the power amp is loud and strong, but the tubes run very cool. Take the 480V on a new 500V-600V cap, insert ~~700 Ohms, and feed your "400V" point, going to OT B+ lead and all those other R-C filters. Aug 17, 2015 · The bias is again at -55. Thread starter Ampaddict; Start date Nov 1 Jan 26, 2009 · Even thought the OP is a n00b, you guys are not quite right about this. Bias is calculated by ohms law and by wattage percentage of the tube in terms of max watts. Fully assembled and ready to use. Feb 15, 2006 · 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 back of #0121 on the right beside the two bias adjustment pots (see the attached picture). And then you swap the probe from tube to tube to check the bias off all four tubes. 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. 040V range and see how it sounds. The previous owner replaced the 6L6 power tubes with EL34s, I understand there is a switch and adjustment for either type of tube. 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. Oct 22, 2020 · I noticed that the bias pots behave strange in that if I adjust bias pot 1 (B1) it only affects V7/V8 voltages at pin 5. Turn the trim pot to adjust the plate current to the desired setting. Early models were biased at 20, he didn't really know why. 5mA bias or 2mA as a resultant, then it is the best bias. 8Vdc Feb 2, 2016 · The bias adjustment should be near the power rubes about 3" from the edge of the chassis. Biasing became something that could be done in literally 10-15 seconds. Top Nov 17, 2022 · In all but a few cases, 6L6 and EL34 tubes are not interchangeable without modification. Carefully adjusting the bias, volume and tone controls, I achieve the following waveform. com will make sure you get ones that work, in your mesa, without a bias adjustment. This seems abnormally low for a 6L6. so, i dont see why the bias pot mod wouldnt work just fine. The three measuring points: one is mass, two are the fall of voltage per tube via a measureing resistor. Adjusting bias test point 30 to 60mV (the recommended setting) was at most of the way to the right of the bias pot - then measured at R59/R60 I got -42. " The answer is no. That you have the current in milli amps helps but you have nothing to calculate it with. 085 to 0. Mar 3, 2010 · Yes, the actual adjustment is the grid-cathode voltage (-). Of course the same procedure can be used on the Bugera amps such as the 333, 1960, 6260 and 6262 amps. They are commonly substituted for 6L6s in many amps with no bias bias adjustment faciliy at all. They found that selecting tubes for that "Sweet spot" and branding them "Mesa Boogie" eliminated the components for bias adjustment. The bias of the amp was set way too high when I bought it used and caused red plating in a TAD 6L6 WGC as it goes, plugged the bias master in, replaced with a pair of NOS Jan Philips and set bias near to the top end of what they recommend for this amp and now it sounds fantastic and I have the price of mind that everything is as it should be. Any chance some one can point me in the correct location for a from the factory detailed explanation on how to use the Bias test points for the XXX Ultra series amps. But if you aren't experienced in dealing with high voltages, refer to a qualified tecnician. 2VDC, in close agreement with the Elenco We adjust the voltage difference between the cathode and control grid (bias voltage) to adjust the amount of electrons flowing from the cathode to the plate. For 6V6s, you'll want to start out at around 20 mA and work upward from there. Go by sound if you can. 400 volts, . This is something that must be done no matter which tubes you are changing. I'll reset the bias mine eventually. Jan 13, 2006 · Here is an example: Let's say your plate voltage is 500V, which I think is about right for the JSX. There is a switch on the chassis that selects 6L6 or EL34. " Jan 27, 2011 · With regard to that 6. Did you flip that to EL34? PV purposely limits the adjustment range of the bias controls so amp owners cannot overadjust them. Not doing this - and assuming it never needs to be done - can lead to mysteriously ragged distortion, cold clean tones or inordinately high headroom - especially if It seems to have all the original tubes including the blue GT 6L6's. You can run the bias max in either direction and you'll still be within the acceptable range. Each trim poti controlls the bias amperage per tube (exactly the bias voltage and by that the amperage). . Cathode biased has no pot and is basically self biasing. Feb 8, 2018 · We used a four-position switch for 6L6, a hotter 6L6, EL34, and hotter EL34. The output transformer impedance match can be optimized by using a lower impedance speaker than with a 6V6. Not for 7591 tubes. *Bias ranges are measured and set in volts *Do not set bias higher than recommended levels as it can lead to tube failure and possible damage to other components in the amplifier! Factory Bias Setting: . Since I will push them to run hotter, then the lifespan of those vacuum tubes can be decrease. An auto-bias should take the warm up in consideration which is around 15-30 minutes on a well ventilated amplifier. For example a Fender Bassman is fixed Bias but it has an adjusting pot to set Bias. Higher the bias will make all the vacuum tube running hotter. We set 6L6GC power tubes to (-)55V DC and EL-34 power tubes to (-)43V DC. This Dec 17, 2010 · Cathode biased amps can do that. Don't forget to adjust the speaker ohms to Cookies & Privacy. 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. The only way to adjust bias is to modify the bias supply. If not in this range, adjust by turning bias screw SLOWLY a small amount. I had a Bedrock Royale Reverb which could use EL34's, 6L6's or even 6V6's. Heavy-duty output and power transformers are used to give crystal clear tones. This has its pros and cons. 3, 13, 11. Even better than the original IBAM (Individual Bias Adjustment Mod board) I've been using for the last 3-4 years. If you want to really push them then you need to drop the value of the final bias resister down by about 20%. A sympathetic tech can show you how to perform the procedure and how to optimally adjust the bias. A bias setting between 30 to 45 is best for the JJ 6V6's. 25 divided by 387 x . 15 volts. 6l6's are 25 to 60, with a set point of 45 mA. 450 volts for KT66's and KT77's, and . The higher you set your bias, the hotter your tubes will run and the earlier your sound will break up. enfoxmdjampwlybywarkyaacifwtpasztivjrzdnffvkycy