6l6 bias adjustment Tom Don't let that smoke out! Top. Without a true bias adjustment pot I can't lower the bias of the tubes. Due to superior performance, the more modern 6L6GC, which also features a glass that they are the same type: 6L6 for the power tubes, 12AX7 and 12AU7 for the driver tubes, and 12AX7 for the preamp tubes. That doesn't necessarily mean something will blow up if you don't get it biased but there is some risk: 1. They are small enough that they have no effect on circuit operation. Feb 6, 2018 30 13 4,551. Added push/pull gain boost and eL34/6L6 switching 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. 6k ohms. The 6L6 tubes are listed in the WIKI as suitable replacements for the 5881 with the following caveat: 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. 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. Don't forget the joker. This is the amp with the Phaser FX, the Automixer, solid state preamp and a 2x 6L6 Tube power amp section. 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. If you have a class AB amplifier that uses fixed bias and requires the setting to be changed with every tube change, this chart can help you. Turned it all the way down for 6L6s, which draw more current at the same bias voltage. 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. The bias trimpots don't last forever and they need to be periodically replaced, best at the same time of a My strong suspicion is that the adjustment pot is preferable (I don't see anybody modding their amp to put in a balance pot). At 35 mA on 367 volt plates the point of this negative voltage is to regulate the flow of electrons through the tube. what about swapping 6l6 for el34s with no bias? always at least test the bias when changing output tubes in a fixed-bias amp (that includes ones with and without bias adjustment pots, so don't get the term "fixed biased" confused with "has no bias pot"). Reactions: Kevin Wolfe. That is for 6L6 tubes so I'm not sure what the 7868 bias at. I also found links to Fender Twin (looked like a Reissue) and Deluxe user manuals. Voltage from bias test points provided by peavey: 58v Bias test probe 1 in tube 1(from the left) set to 38. Join Date: Feb 2010; The Mesa/Boogie Dual Rectifier has a grid bias supply that is switchable to support EL34 power pentodes or 6L6 beam power tetrodes. We recommend to use an ll plastic screwdriver to make the adjustment. Set plate voltage at 450V. Our general rule When you set the biasing of power valves, you are setting the operating conditions so that when the amp isn't amplifying a signal, the power valves aren't conducting much Most practical and sufficiently accurate method is to measure the voltage across the cathode resistor. 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. One or two electrolytic caps are filter caps to smooth out the pulsing DC from the bias diode. If you want to bias the amp properly, you will need either a bias probe, Hook a scope across the output. This mod also allows you enough adjustment range to bias 6V6, 6L6, EL34, KT66, KT77 and KT88 power tubes. pot. 035V to 0. Early models were biased at 20, he didn't really know why. Amp sounds great! Bias pot out of range on this one so I had to adjust a fixed resistor. Click on the pics and they will shrink to size. 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. neikeel Senior Member Posts: 7231 Joined: Tue Dec 06, 2005 2:31 pm I have a 15K pot in place of my bias range resistor to make bias range adjustment easier and rebias quicker. 99. They're set pretty cold as is. Every amp should have an external bias. Tube bias is also dependent on the high voltage (or B+) applied to the plate of the output tube, which can vary First I did the JSX, it has a bias adjustment on top of the chassis. . See more How exactly can I fine tune the bias of the 6l6 output stages. It is fixed bias but doesn't appear to have adjustable bias. This one pot adjusts the bias current on both valves. Adjust the balance control for equal currents thru each 1 ohm resistor. Page 3 Technical specifications and appearance are subject to change without notice. Colossal Posts: 5111 Joined: Sat Oct 20, 2007 9:04 pm. The answer is a big NO, that would drop the bias back down! The switch is there because Ok, new matched Ruby 6L6 tubes just installed. 6L6 Power tubes are rated at 30 Watts so you need to bias for 19. My strong suspicion is that the adjustment pot is preferable (I don't see anybody modding their amp to put in a balance pot). Love it!! Reactions: GuitarGuy08, errrrrl, stephen sawall and 3 others. 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. This mod allows wide range of adjustment to bias 6V6, 6L6, EL34, KT66, KT77 and KT88 power tubes. Lots of headroom and didn't clip unnecessarily. Enzo Member Posts: 9535 Joined: Sat Sep 13, 2008 3:24 am Location: Lansing, Michigan. 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. Oct 23, 2008 #3 a non adjusting bias can overheat the tubes (or not heat enough and f*** up the tone) and Proper bias adjustment is critical to the performance and long life of the amplifier. Thread starter Ampaddict; Start date Nov 1, 2006; I think they are cheaper than buying from Mesa online. I'm not sure that i understand well. My plate voltage is 455 V. 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. In case of fixed bias the resistor can be anything between 1 to 10 ohms. 5 watts for 65%. It's a bias adjustment, since the owner of this amp wanted to be able to use 6V6 or 6L6 tubes. If not, you will need to troubleshoot the bias circuit to determine why the voltage is out of range. Hello, I have the same amp and there is no bias adjustment on that model. Running hotter than that will stress your output transformer. The For 8-pin power tubes such as 6L6, 6V6, 5881, 6550, EL34, KT88, KT66, KT100, etc. The previous owner replaced the 6L6 power tubes with EL34s, I understand there is a switch and adjustment for either type of tube. Search. Often it is a switch that can select but, you cant run el34 tubes on each end and 6l6 tubes in the middle. This will idle tubes at 19 watts static plate dissipation per tube (about 40 mA per tube), which is about 60% of design center max value. Biasing was easy and well within the range of the standard bias adjustment circuit in the test amp. Like barely got to the 1/2 way mark before I could no longer make an adjustment. Not general bias and balance between sides. you can simply plug in a new tube of the same type (or a direct substitute) without needing a bias For 8-pin power tubes such as 6L6, 5881, 6550, 6V6, EL34, etc. It seems to have all the original tubes including the blue GT 6L6's. For 6L6 tubes, the factory setting is ~55VDC at the test jack. 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. 5 ohm sense resistors so that I can measure screen and plate currents at each operating point. Cathode bias amps are by nature self-biasing, but there are fixed bias amps that are self-biasing. basically locations you need to place the probes and then there is an Pin3: Same as pin1 but for the other side of the push/pull amp. I was quite intrigued with the tone I was getting. How you adjust the fixed-bias voltage supplied to the control grid often varies from amp to amp and from brand to brand. 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. This DOES NOT mean that 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 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. Biasing a Peavey Windsor. The bias adjustment should be near the power rubes about 3" from the edge of the chassis. Using Ohm’s Law and measuring across a precision 1 resistor, we know that 1mV = 1mA, we want to establish a good starting bias point for the matched output tubes. The schematic shows 6L6GC power tubes. It's set at 3 It's a good idea to at least check the bias on any fixed bias amp when changing power tubes, you may or may not need to change/adjust the bias. Joined Dec 12, 2005 Messages 107 Reaction score 0 Location New Orleans. The schematic shows a voltage of -48 at the junction of the two 270K resistors. From: posted 01 May 2004 03:48 PM profile . And my B+ is about 405 Vdc , so the 6V6 would be 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. 150 mA for 6CA4, so you could bias the tubes 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 I want to verify the bias on my 120w Triple XXX. Often it is a switch that can select bias for EL34 tubes, or 6L6 tubes depending which is installed. 3volt filament transformer, rewiring of pin 1 on the socket, and a bias adjustment), a Fender Deluxe amp could use EL34, or 6550A. Sonicbluestrat. 3, 13, 11. That is if the circuit is similsr to most. Drive it with a 1 to 2 kHz signal to just below the point of clipping. 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. Read this link. Hook a scope across the output. 5. But seriously consider getting yourself a bias probe to do the job properly. The tubes have great tone and nicely capture the “6L6 sound”, characterized by sparkling highs, warm mids, and controlled bass response. Choose from KT88, EL34, 6L6GC. Quad of 6L6 is relatively pretty friendly. 1, 9, and on older tubes Fender Groove tube 17. 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. This is the grid bias we’ve been zoning in on, and the vast majority of tube guitar amps boast this type of bias. So, since you have a bias probe, I'd suggest biasing 6L6's in a Deluxe Reverb in the low 30 ma range. There is only one source of bias voltage, so the voltage at every tube's grid will be the same (it better be!). When switched in, the effective shunt resistance of the parallel resistors is 9kΩ. Remember there is more than one way to set idle current, either mod the circuit to suit the tubes, or change the tubes to suit the circuit. Congratulations again! You have just biased your amp! Plate Dissipation: 25 watts for 6L6’s, EL34’s, EL34II’s, E34L’s, 6CA7’s or KT77’s. g1. 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. 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. Adjust the bias to between 20 to 25mA which is a conservative setting for most all amps while you note the DC plate voltage and use the formula above to calculate the bias you want. HotBluePlates-So am I correct that with this circuit you don't really ever adjust the bias which is set at -50v like you might do with a Princeton or Deluxe Reverb, but you just adjust the current on the 6L6's with the 10k pot so that they're reading about the same? And you could use either a Eurotubes probe or the 1 ohm resistors setup on Pins 8 of the 6L6's to get this The 6L6, 7581A, 6V6, 6550, EL34, 5881, KT66, KT88, KT90, KT100, etc. 65% is a nice compromise. The nuked tube has been popped for about 15 minutes now as it's cooling lol. 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 . The Weber calculator has you entering the plate voltage and it then tells you what the bias should be for the 6L6GC. There is no bias adjustment on these. POWER INPUT: 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). All readings are mA. Using the bias probe you can set the bias on each valve by adjusting the pot ( its just a variable resistor. 040V range and see how it sounds. More Information. http://ritteramps. This can be used as a reference for future maintenance or adjustment. " 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; Guitar Amp Valves tubes Menu Toggle. The bias setting range is not the big. I checked the bias with my meter, it was showing 25-26ma on both tubes. Self bias, or cathode bias is a term referring to bias set by voltage developed across a cathode resistor, and does not apply to the MC circuit. Can anyone shed some light on what the solution to this problem is. 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. The 6L6WGB is listed at 26 watts. Or, you could ask the vendor for something with a The bias setting range is not the big. Set bias balance. Maybe I'm getting confused or not understanding what bias really is. now, different tubes, even of the same brand, will require a different Comes with 6L6GC power tubes, but also supports wide range of power tubes, such as KT88, KT90, KT66, EL34, etc. jerem. but you could set up directly 6L6 in a plexi with just adjusting the bias? Top. Assuming we are talking about a push-pull output stage, the fact is that after doing this, you might need to slightly adjust the bias for one No one thinks bias doesn;t matter, what we do think is that bias is not a critical adjustment. 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. mark v you have to run all el34 or all 6l6 power tubes. This method will do in a pinch. Perhaps this is a factor in the extremely low noise and lack of microphonics that this tube exhibits. Requires separate digital volt-ohmmeter capable of reading current. A fixed bias just means it doesn't adjust itself. 5 mA at 434V with 15K bias resistor. Adjust to maximum negative bias. I like to know I am working with something fairly reliable, not a big investment, worth it to me. As I adjust the bias control, the plate voltage seems to be Anyway, upon installing the 6L6 and switching the tube selector switch to the 6L6/Kt88 side, I could not bias the tubes hot enough. Just an presumption. cheers, Douglas Use a matched quad of 6L6-GC tubes configured in push pull, class AB1. New posts Search forums. Re: HELP!!! One of my 6L6 tubes 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. Supporting Member. In other words, I had maxed it 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 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. Put the tube type selector to 6L6 from the EL34 setting. Does this sound right to you amp gurus? I have mine at about 66-67mV. Thing is though, I have on hand probably a half-dozen serviceable 6L6 type tubes. Per the Onix SP3 WIKI, the bias voltage for the 5881 tubes is 1. Without any tools to read voltage, I just want to bias it as hotly as possible. 2. (3-4K as opposed to 5-8K for a 6L6). Operates in Ultra-linear, Triode or Pentode modes. For EL34 tubes, the factory setting is According to the site 6L6's and EL34's should be Biased at around 65mA, is this correct? Home. bias adjust TP6 The head Fender EVH engineer told me that biasing it to 30mA is fine, current models are at 30. Here is EL34 mode. 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 How to Bias a Fender Bassman (AB165) The 6L6 amplifier I chose is a 1969 Fender Bassman head (AB165). Things do run quite warm, and I removed the cover as a matter of course. Cathode Bias (Self Adjusting) The 6L6 is used officially in the TV-7 Field and Depot Maintenance manual to adjust range C, a standard NOS 6L6 has 6000 µ℧ of transconductance (6 mA/V, = the official transconductance value at design and manufacturing parameters, MUT COND and adjust the BIAS control counterclockwise until the meter pointer indicates 120. There is absolutely nothing wrong with JJ 6L6 output tubes. Another poster said the 6L6 family was good for 30. Run it into a load bank. 1/2 WATT PRECISION METAL-FILM RESISTOR FOR INSTALLATION ON POWER TUBES FOR BIAS MEASUREMENT. 5881 and 6L6WGB types The Tung Sol is the most asked for tube here. In order to 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. I set the bias adjustment low to see what readings I'd get. Hope this hel One more thing: some fixed/non-adjustable bias amps have a "bias switch" to select for a specific tube type. 5 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 i know the original 5150/6505 doesnt have a bias adjustment on it, and people mod that for having one. These are still fixed/non-adjustable bias amps as there is no internal bias trimmer pot to fine tune. We set 6L6GC power tubes to (-)55V DC and EL-34 power tubes to (-)43V DC. What's new. PROCEDURE: 1. Current visitors. 2, and one sovtek rated 9. ADJUST BIAS (POLARIZAÇÃO DE AJUSTE) E BIAS TEST POINTS (PONTOS DE TESTE Page 2 6L6 valves (Rectifier, SLO) or the classic British grind of EL34s (JCM) by changing the output valves, flipping a switch and adjusting the bias knob. Members. If you bias an amp too hot the power tubes can "red plate. I. Reply. Those rare amps simply require a bias adjustment, however the majority will likely require some physical modifications. You have to repeat this proceedure a few times as setting the bias current will have an affect on the plate voltage. [This message was edited by Garth Highsmith on 09 January 2006 at 08:50 PM. Hotrod Deluxe “How To” Bias Video. That's cool. For the 4 (BIAS ADJUSTMENT POT) fully CCW (MINIMUM SETTING) 3) Turn master control fully CCW (MINIMUM SETTING) 4) Turn power switch to on then wait for 2 minutes. The 5881 and 6L6 come from the same family of tubes. It has a built 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. Top. Adjust bias until crossover notch just disappears. I ordered mine built from Bob and he included an instruction sheet Apparently some people have added a global bias adjustment mod to theirs, but mine has not got this. It was like 48mV from the store, For 8-pin power tubes such as 6L6, 6V6, 5881, 6550, EL34, KT88, KT66, KT100, etc. 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. Ive heard that its not completely necessary, but i just thought id make sure since its only an extra $20 and its my first time replacing the tubes on it. Their recommended 6L6 bias setting was 20mA. 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. when you adjust the bias, you are changing the value of that negative voltage. About 59% plate dissipation. May 14, 2012, 8:40 PM Looked in the back and the second 6L6 tube was NUKE RED!!! Quickly I turned things off and let it cool. New posts Latest activity. It works for Neil Young. Sep 23, 2018 #5 Sep 23, 2018 #5 B on T, Thank you for the suggestions and for the excellent article! I've run the amp at wall 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. EL34 tubes don’t draw quite as much power as 6L6 tubes. The plain jane 6L6 seems to be 20. Obviously there is a variable pot between the plates, but doesn't that just adjust the balance between the two When you adjust the bias on your amp you are adjusting the level of current available to the power tubes. 6l6 40-watt 1x12 combo (4 pages) EVH 5150III EL34 - 2x12 Combo Amplifier Manual (article) REAR PANEL FEATURES H. " How about that? The EL34 tubes & the 6L6 tubes require a much different bias voltage on pin #5 of the output tubes. Bias can be preset like many other amps have in the past. The 6L6GC type can substitute for any of the types below, but may require a bias adjustment or modification to operate at peak performance. Cheers, Dave O. Each 6L6 is pulling 42 mA at 423 volts. Likewise, the Peavey head should sound best with 6L6's, but with a fixed bias, you should be able to swap them The 6L6GC type can substitute for any of the types below, but may require a bias adjustment or modification to operate at peak performance. This is so you can throw just about any 6l6's in there without problems. That will 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. 450 volts for KT66's and KT77's, and . (such as Mesa Boogie™) modified by a technician to provide for bias adjustment. Hope that helps! Stephen . I also wired the extra heater winding to two of the output tube sockets, since the 6L6 has twice the heater current. It seemed to get stiff and harshgoing counter clockwise and got warmer and softer/buttery feeling going clockwise. It is a set value. They found that selecting tubes for that "Sweet spot" and branding them "Mesa Boogie" eliminated the components for bias adjustment. 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. For 6L6 mode, resistor R1 is switched out to create a shunt resistance of 22kΩ. The 25 range will be pretty cold for 6L6's. I'm using 6L6s so the whole range of the knob is supposedly safe to use. Tube type: ----Choose---- 6V6 EL84/6BQ5 5881 EL34 KT66 6L6GC KT88/6550 The bias should get to at least -20V and maybe -25V at the most negative position. I would take it to a tech or get a bias probe to be sure, an adjustment of the bias may enhance the quality of your tone over the factory cool settings. So an amp can be both, but it can be self-biasing and fixed bias. Your 65ma? You could adjust them back down to 38ma or 28ma or 18ma, and the amp will play fine. This is a negative voltage. And as I stated up front, the OP's amp HAS no adjustment. The 5881 is a 6L6-type tube, so the 6L6GC will work just fine 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. May want to save the link. BUGERA has also developed a line of valves that are specifically matched with these amps to give optimum balance and compatibility. ] Mark Herrick Solder a 1 ohm resistor in the cathode circuit of each 6L6 tube. Meeotch Well-known member. That's 38 Watts of Plate Dissipation, which would fry 6L6's, or EL34's, but I ran the amp at 70 Watts rms at 165 Setting the bias adjustment controls to these listed voltages in no way guarantees that your amp is properly biased. 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. Some circuits have internal trim pots, while others may have adjustment controls on the rear panel. 6V6s and a bias adjustment trimpot would be cheaper than the Yellow Jackets, and would leave it sounding more Fenderish, I expect. Think of it as a rough adjustment before you do the fine bias adjustment. The plate voltage on the 6L6 was about 490V and was biased a little bit on the cold side. Proper Record Keeping: When adjusting the bias, it is important to record the settings made. g. Post Cancel. Set FUNCTION SWITCH The calculator will give you the bias current for each tube. I bias my 6L6's fairly cool at 36ma @ 440Vp. Use this as a guide for setting your bias. The measurements below show that this The BAL/BIAS ADJUSTMENT that GADGET linked in the previous post works well with 800-C, 500-C, 400, and SANSUI 1000A as is. Hopefully the bias adjustment allows enough latitude, worst case might have to change out one of the series resistors. Pulled 15K bias resistor. If you don't want to bring it to a tech, get a bias meter. ONLY FOR AMPS Hello. If you have an amp which uses power tubes which are not listed above, you will need to consult a spec manual (the RCA RC-30 Receiving Tube Manual is one of the best) for the basing, and adjust these 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. Since this takes just a pair, those These jacks are used read the Bias Voltage, which can be tweaked using the Bias Adjust control located next to the power tubes. " Wait about 5 to 10 minutes with the amp in the play mode and do a final bias adjustment after the tubes are hot. what needs to be done on the 5150II/6505+ in order to get that window to open up ?? a different 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. First I did the JSX, it has a bias adjustment on top of the chassis. Even better than the original IBAM (Individual Bias Adjustment Mod board) I've been using for the last 3-4 years. This is something that must be done no matter which tubes you are changing. The resistors R sp and R ss are 1. Vintage blackface Fender™ amps from the 1960's offer true bias 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. Jack heres a question. In an amplifier with bias adjustment the substitution can be done if cathode current is measured In all but a few cases, 6L6 and EL34 tubes are not interchangeable without modification. The differences between the 5881 and 6L6GC are large enough that a bias change will be required. 15 volts. By: Search Bias adjustment Herbert. This is why fixed bias amplifiers - which don't experience the bias change relative to output level - can generally be set to idle at lower current, which promotes longer tube life. (this is why it came with 6L6s in stock Cookies & Privacy. More Information "With slight modifications (auxiliary 6. i think if 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. In the case of your amp, Peavey tends to set the bias very cold from the factory. This seems abnormally low for a 6L6. What's new Search. I'll reset the bias mine eventually. i know the II/+ models have a bias adjustment but the window of adjustment is very very small. Then back off of the bias to green. We often get asked if the bias switch should then be moved back to the 6L6 position after the bias is set. With only a bias adjustment, the amp could use KT66, EL37, &L&GC, 7581A or 5881. Should the knob be turned all the way to the right (clockwise) or to the left (counter-clockwise)? 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. That would be about 35 ma to 40 ma total current flow through each power tube. Cathode Bias (Self Adjusting) It currently has JJ 6L6GC power tubes which i will be replacing with more of the same. slayer44 Well-known member. Likes 1; Comment. Jul 8, 2009 8,553 9,891 Okay. the 2 inner matched pairs at roughly 27mA and the outer tubes at around 38. 6l6 40-watt 1x12 combo (4 pages) EVH 5150III EL34 - 2x12 Combo Amplifier Manual (article) EVH 5150III 50S 6L6 - 50-Watt Head Amplifier Manual (article) Conecte a uma tomada ater- M. Bias test probe 2 in tube 3: 27mA If i switch to positions 2 and 4 I get the same. These precautions are important considerations when performing bias calculation and ensure safe and efficient operation of the amplifier. FJA Mods modified Peavey 5150 with 60 watt Metal Mod. yeti Guest. The OD side had so much clarity and command - no fizz but lotsa nice OD clarity - probably less intermodulation? of 125 mA for a pair of GE6550A as specified by Alexander 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. This device reads cathode current. Also do this when the amp has been powered The increase in bias voltage pushes the tube toward class B operation where no current flows at the zero-reference line of the signal. 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. But this doesn’t mean that they don’t need a bias-setting adjustment. so, i dont see why the bias pot mod wouldnt work just fine. Considering the variability of output tubes, the amps should be modified to allow adjustment of the bias for all the reasons mentioned in the previous posts. 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 They may flicker red when driven hard. Installed 10K resistor again. The margins of adjustment they have are very low and you may not get a good setting with a standard medium grade 6L6. I will tell you now I have had this along side another 2 channel Hello, I'd like to accurately check the bias on my 6L6 amp. How to Bias a Fender Bassman (AB165) The 6L6 amplifier I chose is a 1969 Fender Bassman head (AB165). 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 sorting so it is worth it to How to Bias a Fender Bassman (AB165) The 6L6 amplifier I chose is a 1969 Fender Bassman head (AB165). 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. $16 per 12ax/at7 and $42 for matched set of 6L6's. 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. Some amps let you change this value with a pot, so while it is fixed, it still might be adjustable. Setting the bias on your amp is like setting the idle on your car. Active Automatic bias adjustment function means changing tube types is easy; no manual biasing required. Thanks, is for the 6L6WXT. Since both push-pull and parallel tubes are used here, the output tubes must be replaced by a matched quartet. are common guitar amplifier power tubes, and are all easily biased with this method. 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. How to do a quick bias adjustment. Most of the Peavey amps from the late seventys and up were all plug and play with a fixed The 6L6GC is commonly associated with classic American rock ’n’ roll and the sound of vintage Fender amps. I'm using Gold Lion (re-issue) KT66's in mine and they sound good. This is for a 6L6GC tube typewith some common amp voltages and safe dissipation settings. You can put 6L6's in an amp that has 6V6's with a bias adjustment, no real increase in power My EVH 5150-III 6L6 50watt was showing "41" in the multimeter screen which shocked me at first, since folks were mentioning 55-60 as a range. 500 volts for KT88's or 6550's. 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 For 8-pin power tubes such as 6L6, 5881, 6550, 6V6, EL34, etc. No. Use fixed bias, with -42V bias delivered to grids. Not the other way though as a 6L6 draws a lot of current, exceeding what is safe for a 6V6. 6l6 60-watt 2x12 combo (21 pages) Amplifier Evh 5150 ICONIC Series Owner's Manual. 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 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. Had 33. Reeltarded Posts: 10050 Joined: Sat Feb 14 Bias is generally set for between 60 and 70% of maximum plate dissipation. Consult an amp tech whenever substituting tube types. A newly developed "Active automatic bias adjustment" function automatically adjusts the bias for the optimal condition for 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. 400 volts, . This is for the 100w. $64. The bias adjustment is the 10K pot near the bottom of the schematic. The two bias resistors in a typical bias circuit form a voltage divider to reduce the voltage coming out of the bias rectifier diode. For biasing your tube amp lets assume for the sake of clarity that you have a perfectly matched quad of OT's. PR2: Bias mini-pot adjustment for pin3, the other side of the push/pull amp. These can be substituted with appropriate bias adjustment. Note: This adapter will NOT test 7591 tubes. 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. The original 6L6 tube featured a metal envelope. , JJ's), or does it just require biasing via a different means? thank you. In cathode biased amps we must change the cathode resistor value to change the cathode voltage to adjust the bias. Members; 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). Search titles only. Nov 9, 2012 #5 Y. Then an adjustment will be required. Is it more accurate to read the current between pin 8 and ground with an ammeter or to use measure the voltage across 1 ohm 1% shunt resistor between pin 8 and ground. Eurotubes Fender Deluxe - Deville "How To" Bias Video. 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 fixed bias amps without a bias adjust pot, the Or, you could record your plate, screen & negative grid voltages (6L6 pins 3, 4 & 5 respectively) and see if you can find a vendor that has some tubes that bias up at around 35mA. like a mark iii or mark iv. 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. I don't know what number I'm getting. 5U4 is rated for 225 mA, vs. I get that it's basically supposed to sound like a 6L6 but bias like an The various 6L6 types, 1614, 5881 and KT66 all have the same pinout but among them the KT66 has different biasing requirements. SactoBass. ONLY FOR AMPS 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,. You can run the bias max in either direction and you'll still be within the acceptable range. Like an idiot I bought Groove Tubes 6L6-GE in Medium (4-7). Make sure amplifier is connected to a load with the proper impedance with 586 Volts on the Plates. I adjusted the bias for a fairly rich 45mA, although I may back that off a bit. Even if the bias and voltages could be sorted out, the 6L6 will suffer in the distortion and in the power output 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. This website uses cookies to ensure that you get the best experience. Forums. I can replace a 10K resistor with a 15K in the bias circuit to give me more of a bias adjustment. 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 Just curious if anyone has some insight on the JJ 6CA7 and how it relates to their 6L6GC and regular EL34 offerings. This amp has a non-adjustable fixed bias supply, no adjustment is possible (without modding) nor required. 5, a Sovtek 6L6WXT rated 8. 5mA. The 15K and 56K resistors in the original bias circuit form a voltage divider to reduce the voltage coming out of the bias I bias a 1987 Fender Twin Reverb amplifier. Soren, my ST-70 has the original power transformer in it. Vintage blackface Fender™ amps from the 1960's offer true bias There is absolutely nothing wrong with JJ 6L6 output tubes. Connect output stage in pentode mode, with 400V on screens. Amazon. There are lots of OP amps at the preamp pcb and there is no bias pot. In fixed bias amps we adjust the grid voltage to set the bias. Adjust bias until crossover notch just Per the Onix SP3 WIKI, the bias voltage for the 5881 tubes is 1. I've got a JSX and a new 5153 that I recently biased by ear. 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. This device reads cathode current, but not plate voltage. The primary modification necessary is to properly bias the amp. Log in Register. Topic: Silverface Bassman Bias Adjustment? Garth Highsmith Member . S. I like the JJ tubes as well One more thing: some fixed/non-adjustable bias amps have a “bias switch” to select for a specific tube type. Peavey XXX amps have a "fixed bias" and a bias adjustment knob. 6mA - adjusted by me upon installation via the provided bias adj. qas rsm gfm cgeh xdkdt wgf thz obkp kuq hhzrsw