6l6 bias voltage amp Oct 6, 2014 · Operating conditions can be determined by using those recomended by the tube manual or by using the plate curves to determine a linear region of operation for the tube. My test data from a KT88 with a B+ of 400v and a 470Ω cathode bias resistor showed a plate voltage of ≈350v, a cathode bias voltage of ≈36v, and a total cathode current of ≈76mA. Amps made for 6L6GC tubes could be anywhere from 450 to 500 volts. 6k primary. Turn it to the middle for now. The plate voltage is about 315 volts. There is about 0. The 6L6 bias point at 60% dissipation for an amp designed to use 450 volts is: 30/450 x . Jul 26, 2001 · Actually, that 70% number should be based upon the rated dissipation of 25w for a 6L6. The Granger Amp bias probe measures in milliamps by insertion into the cathode connection that conducts the current to the plate (anode) of the tube. But for the best load, you need to connect your 8 Ohms speaker in the 16 ohms output (6L6 in place of a 6V6) For the best bias, you need to change the cathode resistor. Most amps using this tube run between 430-450 volts DC. This is near the middle of the KT88 UL plate characteristics curves at this plate voltage. Dec 8, 2010 · I recently swapped the stock 6l6's in my HRDV for a pair of Groove Tubes 6l6ge's with a rating of "1". No-calculation examples: A higher voltage (450v) deep class AB amp might use a 4k primary (possibly smaller), while a lower voltage (350v) hotter class AB amp might use a 6. Next you will need to put in the two 6L6 bias resistor in place of the 10 ohms R9 and R23. The beefiest PT I have at this time is a Hammond 273BX (350-0-350, DC ma 175). 301 Moved Permanently. Dec 23, 2010 · These figures will vary with HT applied and bias used. Apr 28, 2018 · I agree. 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. More Information. Thomas voltage on the grid you can control how much current is passed from the cathode to the plate. 04 or 550 ohms, 2-3W. What I have read elsewhere is that the 100% maximum current limit of a tube is found by dividing the tube's power dissipation in Watts by the amp's plate voltage. The manual doesnt say if its both 6l6 or one,, based on the normal plate voltage of these guys you would think 30-35 a tube. Jun 29, 2021 · For class AB Fixed bias amps using 6V6, 6L6, EL84, EL34 power tubes and probably most other tubes, the negative fixed bias voltage should be close to 10% of the plate (to cathode) voltage. Im wondering if this is the reason for the "cold" Apr 11, 2009 · I'm not sure if 5881s were supposed to be "better" than 6L6 in some way, were "European versions" of 6L6, or something else. If Vp is increased, Ip must DEcrease so the tubes don't melt. The 5D3 amp cathode resistor should be around 270 ohms the 250 ohm in the original at or beyond the limilts of the tube. BTW: IMHO, not all "30 Watt" 6L6GCs are actially long-term safe at 30W. The OD side had so much clarity and command - no fizz but lotsa nice OD clarity - probably less intermodulation? Overall the amp was not compressed but very Cookies & Privacy. I have not found a detailed factory procedure description on the factory bias test points on the back of the amp. As long as you know (or can test) the plate voltage in your amp, and have a way to measure the idle current, you can determine where to bias it, using our calculator. And if a large enough bypass capacitor is used there will be almost fixed bias conditions for short enough signal bursts - a method I mostly use. c Pin 7 – Heater . Measure the voltage across the cathode resistor and divide by the value to get the cathode current. Jun 20, 2024 · The increase in bias voltage pushes the tube toward class B operation where no current flows at the zero-reference line of the signal. (Approximate numbers). Put the red probe on pin 5 and the black probe on pin 8. Mar 1, 2013 45 8 4,551 Vermont. Its the black 5150 iiiS. 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. What bias current do I want for my 6L6GC power tubes with the Twin Rever? The Weber bias calculator*, at 445 measured plate volts, suggested 47 mA but my amp wont let me go above 45 mA. Jan 26, 2024 · Here's the resistive loadline for 560VDC plate and 400VDC screen-voltage, with the 5K6Ω OPT and using fixed-bias. Just curious what opinions are in setting cathode bias. Too hot of a bias can give you a weak tremolo. A higher value cathode resistor will cool the bias, a lower value will warm the bias. Jan 11, 2024 · The desired sound of a Hi Fi Stereo amplifier is often very different than the desired sound of a guitar amplifier. Cathode biased amps can safely run at 100% plate dissipation at idle. Aug 31, 2012 · 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 6V6, which can handle the higher voltages of these amps. Sep 25, 2011 · Hello, I'd like to accurately check the bias on my 6L6 amp. If the amp was born with a pair of “hotter” tubes, the setting on the wheel will be far to the left. B+ is about 325 or so. Feb 5, 2012 · If your amp is an 6V6 SE one, don't worry, you can put 6L6 if we are talking B+, bias etc, the idle current will be lower. 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. I had to make a change in the voltage divider for the grid 2 voltage. 3 watts. But tubes were much more robust back then and fairly cheap so I guess most people didn't care. (This amp also had variable HT selection and could take a range of octal rectifier tubes). 6L6 tubes are frequently used in fixed bias configurations, demanding a precise bias voltage for optimal operation. Thus, it's not unreasonable for the amp to be setup this way. Bias is generally set for between 60 and 70% of maximum plate dissipation. Power Output Test. Lots of headroom and didn't clip unnecessarily. I've done biasing before. Feb 27, 2021 · I almost always use this formula, works for any amp, any tubes. Dec 22, 2007 · In a Cathode Bias amp, what value are using for the EL34, EL 84 power tubes cathode resistor? These usually have to be tweeked upward, most amp designers were using resistor values at (or in Ampeg case, above) design Maximums. The schematic shows the bias voltage as -51V, but the voltage before the modification was -60V for 6L6 and -44. Aug 22, 2008 · I just got 4 JJ 6L6GCs for my Twin Reverb. With my 4 6v6 amp each tube is about 30ma in cathode bias. c Pin 2 – Heater Pin 3 – Anode (Plate) Pin 4 – Grid 2 (Screen) Pin 5 – Grid 1 (control) Pin 6 – n. For fixed bias amps without a bias filter cap on the bias output side of the bias circuit you will need to add a Master Volume AC Ground cap so the amp will be silent at the minimum master volume setting. The 7868 is only rated at 19 watts. Feb 27, 2019 · Re: Svetlana 6L6 Bias BDRI It depends on the actual plate voltage of the specific amp. This is measured using the same test as I did to determine that under an AC load the left 6L6 is drawing considerably more bias current that does the right 6L6. Report. Jan 27, 2011 · I'm looking at my SFVR, trying to coax a little more clean headroom out of it. The - grid bias voltage is a set voltage that by itself really doesn't determine anything. My questions 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 29, 2012 · I know alot of people say these amps are biased "cold" from the factory but ive read a description of the amp now which includes this statement "Custom made transformers and special biasing help achieve Edwards fabled Variable AC effect". I adjusted the bias to 45mV and the heat and smell have Jul 17, 2017 · Oh, and how much power a particular amp can put out mostly depends on its power supply, whether this can as the name says supply that power. because of the similarities between 6L6 Nov 10, 2010 · You can't calculate plate dissipation without knowing the plate voltage. Some smaller amps with 6V6 power tubes also have adjustable Oct 4, 2008 · In fixed biased amps 70% is usually considered the max bias at idle. As mentioned, looks like the 5F6/6-A Bassman ran them around 425v Jul 18, 2011 · The tube with the higher bias amp draw also has an AC voltage on the grid of around 46 VAC. Before I got it someone put new JJ output tubes in it and fender pre amp tubes. Pin 1 – n. 6 = . 5 volts It seems right on the mark! The RCA manual calls for that voltage for a fixed bias class A amp. The bias setting of the amp is currently 20mA per tube. Mar 4, 2011 · The voltage amplifier/driver stage desing can affect as much. the amp is still running 420 plate after 15-20 min warm up at idle. An amp with 300 VDC on the plates compared to an amp with 400 VDC on the plates, using the same tubes, should have a different bias voltage, but how much Jun 21, 2021 · The 12AT7 does like about 200V plate-cathode, and perhaps 3mA to 4mA plate current, which should require a cathode voltage of about 2V to 2. Check that DC balance pot. This means that if you are using an amp with a higher plate voltage, you could damage the 5881 tubes. What exactly did you bias yours to that you were OK with?? Was it ma or mv???. The bias sweep on this amp can get either a quad of 6L6's or E34L's up to the proper bias level and if you really want to know what the bias is set at we would suggest you get a bias probe to check it with because the test points on the amp are measuring voltage and not plate current. It is important to note that the volume difference is actually quite minimal. Therefore, if in a UL 6L6 design -- operating with fixed bias and 500 volts B+ -- the plate voltage swings from (about) 500 vdc down to 80 vdc under full power conditions, then the screen grid voltage is also dropping from (about) 500 vdc, down to 319 vdc. I don't know what your B+ Plate voltages are, so I can only speculate, but any of the calculators out there will tell you if a bias point is good enough or not. Most amps with two or more 6L6 or EL34 power tubes fall into this category. Jun 16, 2004 · and I'd like to adjust the bias in my 5150 to get the plate current up to around 34-35mA Jan 2, 2002 · first thanks for the responses, i appreciate it since i don't mess with bias much given i rarely switch to different power tubes. Terry Jul 28, 2017 · With this modification, the bias voltage of my MV can now be adjusted from -65V to -28V. Amps without adjustable bias include Mesa-Boogie amps, some Peavey amps, and even some Fender amps (early versions of the Hot Rod Deluxe/Deville and Blues Deluxe/Deville, for example). I am wanting to go with either the JJ/Tesla 6L6GC's or the TAD 6L6GC's. Not sure why some manufacturers bias so cold, other than to extend tube life. 7)/400 = 52 mA per tube. 65% is a nice compromise. nginx/1. My question is, do I bias these to the same mv rating or will it be diff Dec 10, 2019 · The plate voltage as seen on pin 3 will change as the tube is adjusted by its bias to idle hotter or colder. Here is a schematic could someone familiar with negative bias please let me know which resistor and value change as a starting point to run the tubes Abit cooler. At least on paper. Oct 20, 2009 · Hey guys, I am about to change out the power tubes in my amp for the first time. 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. Apr 21, 2022 · EL34/6CA7 and 6L6GC will be fine with just the tube swap and rebiasing, and 4k primary impedance will be fine for both. This website uses cookies to ensure that you get the best experience. Mar 4, 2022 · Notice that 6V6 has almost double the bias voltage that EL84 uses, and 5881/6L6 has about 3x the bias voltage. I just built a cathode biased 6L6 PP amp. Many tubes are run over specs in guitar amps but it sounds like a real 6L6 has more stamina than a real 5881 regarding higher voltages. 18. The power output given for a standard 6L6 here (28 W) is slightly less then the 6L6GX (30 W) in the table above. 8M resistors on the Master Volume pot reduce the max pot resistance to 220k and add a failsafe path for bias voltage. Then by R=V/I, the Load Impedance must increase to properly absorb the power. A bit of bias drift in a 7591 bias circuit can lead to the output tubes "running away" with increasing grid leakage current resulting in the tube drawing ever more power. i have a computer/laser printer in the room which may be screwing with the the voltage a bit, but i turned them off. This means you will need 3x the signal level to cause the 6L6 to distort. Oct 11, 2008 · you should just get a bias probe man. A higher reading across the 1 ohm resistor will make the tube idle hotter and in turn you will see the plate voltage drop down because your loading the power supply heavier and yanking more current out of it. safe line. You need to measure your plate voltage. 4 to 1. But you could try a 470R screen resistor and a 470R cathode resistor. People tend to use the 6V6's lookin for a lower ouput power and earlier break up, but honestly I don't think that this is the best approach to get it. com: Nobsound 8-Pin Dual Bias Current Probes Tester Meter for EL34 KT88 6L6 6V6 6550 Vacuum Tube Amp Amplifier (2*Meter + 2*CT1-C, Cathode Current) : Industrial & Scientific Jul 24, 2013 · The early tweed amps didn't generally use screen resistors. Jan 12, 2024 · I'm working on A Fender DeVille 60 Watt amp with two 12's Eminence speakers. 6k or 8k output transformer, which by the way is perfect for a pair of 6V6's. Welcome To the Hoffman Amplifiers Forum May 19, 2025, 02:09:49 pm: Welcome, Guest Plate voltage is 495v Nov 30, 2021 · Hoffman Amplifiers Tube Amp Forum, Bias help 6l6 models. 9A filament current Jul 5, 2006 · Spec sheets say that El34 in class-ab1 push-pull operation can run with a plate voltage anywhere from 350-780 Volts (max being somewhere around 800). EL34 tubes are typically idled hotter tha 6L6 tubes. Many tube amps are over powered for the average users volume Feb 23, 2019 · I've placed an order for a bias plate current probe. The simple take-away from this is that due to the large bias voltage of the 6L6, more drive is needed and more drive from the phase inverter is mostly a function of the supply voltage available. Stealing PRR's old suggestion, grab a handful of 100Ω resistors and stick maybe 5 in series between the shared 6L6 cathodes & ground. This amperage is a part of the equation of simple ohms law where amperage x voltage = wattage (tube wattage in your case). Apr 3, 2008 · Anyone know how or why i would get two different bias voltage readings on a 6l6 push pull quad?? For example the one pair will will read bias sweep from -59 to -48 and the other end from -59 to -30. From what I know Mesa's bias while fixed is not adjustable without resistor changes, correct? Also gotta know plate/screen voltage and current bias voltage so that the tubes don't redplate. 04 or 40mA. 46 X=67. If your amplifier has bias tremolo you need to sample the tremolo after you adjust the bias. Aside from a few late-1950s amps, Fender's "6L6" amps have always expected tubes beefier than the original metal 6L6 or the lamer 6L6G variants. I've seen amps that have a switch to change the bias point for 6L6-to-EL34, but they mostly seem to let the plates stay the same (around 450V). Schematic and tube sheet in this amp explicitly shows this as a 5881 equipped design running 443VDC on the plates, mind you! (Mine shows 449 on the plates quiescent, at 121VAC). Tube rated max watts, times 0. How Mar 14, 2007 · No bias voltage means lots of current draw. I was quite intrigued with the tone I was getting. . I have run 42W 6550 at 39W for days rock steady, but several 6L6GC got drifty at 25W. I have one that reads plate voltage and bias current I got cheap off fleabay, very handy so you can read plate voltage and not have to guess where the 60-70% bias current range is. (when I was building amps, I found that most guitar players preferred the fixed-bias scheme, whereas the Harp players preferred cathode-bias). With a fixed bias amp, for 6L6 (30 watt), I like it around 60%, whereas for EL34 (25 watt), and 6V6 (14 watt), I like it around 70%. I hope this helped . 7V of forward voltage across the diode, creating 70V peak in front of RS. There's certainly parts of the chart that don't apply, and out of whack. And I love the amp. If you set the bias to about 22V and 40ma, you will need a 22/. With a 10% taper audio pot, this is pretty close to your 9-o'clock position moving to 3-o'clock. More Information Oct 25, 2019 · The bias setting for any amp depends on the actual voltage the amp runs at. are common guitar amplifier power tubes, and are all easily Oct 20, 2013 · I just installed a DPDT switch that takes a 50w tradional fixed bias 6L6 amp down to Cathode bias to 250R/220uf. Feb 5, 2023 · Some thoughts. com: Douk Audio Nobsound Vacuum Tube Amp Amplifier Plate Bias Current Probe Tester 8 Pin Socket for EL34, KT88, 6L6, 6V6, 5881, 6550, KT66, KT100, KT120, 7027 (CT1-P, Plate Current) : Electronics Cathode biased amps are less efficient than fixed bias amps. The KT88 have 220k and 6L6' bias circuits I remember use about 100k. Pin 5 voltage will tell you nothing. I was running SED 6L6GCs and after testing them, they turned out to be pretty much death after only 6 months of usage so I guess he was right that 42mA is to high Most amps with two or more 6L6 or EL34 power tubes fall into this category. It was certainly running higher voltages than originally intended, maybe by 40-50V due to higher mains voltages, and sitting right on 100% plate dissipation. Now you choose a power tube type that will work well in that amp, eg a 6L6 whether you have a 30W or a 75W amp. It could run the 6L6 with 250R or 470R cathode resistor with either a 5U4G or 5AR4 rectifier. I checked the bias at the bias test point and it was at 60mV, which I thought it was supposed to be. 5V (grid bias of -2V to -2. Our bias probe measures true bias current. Any self respecting gear whore needs one, lol. 6V6 in a 6L6 amp : no. There is plenty of tremolo signal generated by the LFO to take on the 6L6 bias. 5E3P Amplifier Build. I used an CTS 10K bias Pot, and adjusted the resistor values on the circuit so that I got a voltage range with the Bias pot from -44V to -58VDC I schematic of the Bias supply circuit with these mods is attached, new resistor values in RED. By varying the negative grid bias the technician can correctly set up your amp for maximum performance, thus ensuring that Nov 23, 2019 · It's not really how much power the amp puts out, it's how much bias voltage it has. As a result, my plate voltage on the 6L6s is about 338v. output with resulting lower distortion than cathode bias at intermediate outputs; but advantage of cathode bias is safety (self-regulation). Jan 1, 2014 · For example, my 1961 Concert with OEM 5881 as supplied by Fender, originally at 441 plate/-53 measured stock bias, and now happy as a clam and sounding great at -45 bias. He asked if he can put 6V6 tubes in instead. The other tube shows 36VAC. Now, leaving the bias set for el34s made my amp sound a lot better, more dynamic and punchy. Select your tube type and amp class (Class A, Class AB Low, Class AB, or Class AB High) and enter your Plate Voltage or B+. The Weber site assumes the power is 30W fo First, every 6L6 amp has more supply voltage feeding the phase inverter than any 6V6 amp, regardless of the bias level of the 6V6 amp. Example: a cathode bias Tweed Bassman type amp with two 6L6’s would be about 25 to 30 Watts. Jan 31, 2005 · I was trying my amp out with EL34's the other day, so I had the amps bias switch set over to "el-34," and of course when I put my 6L6's back in I forgot to change the bias back to 6L6. If the amp doesn't have 1 ohm cathode resistors in the amp to measure cathode current as a voltage or you don't have a bias probe, best to leave biasing your amp to a qualified tech. I received the amp dead. The schematic shows 6L6GC power tubes. May 26, 2018 · A 6L6 would only stress the 6V6 amp's OT primary / PT HT winding if it was idled hotter than would be appropriate for a 6V6; an amp's signal power output is mainly determined by the OT primary impedance and HT voltage. Please post the correct schematic if you can. It just needs to be delivered properly. Use a bias probe and a calculator, and do it right. You don't need to make big power output, and low-voltage is likely to mean small-bias-voltage, which is going to make the power section a little easier to drive (so simpler preamp works well). Dec 17, 2009 · The Renegade bias points are measuring both tubes, so this means that each tube would be biased somewhere between 30-35mV, if I followed this rule of thumb. 022 14X=9. Just pull the 6L6s out of the amp and measure the voltage on pin 5 of the empty sockets. Bias is calculated by ohms law and by wattage percentage of the tube in terms of max watts. The 6L6GC will take 500 on the plate and 450 on the screen. The attached plate curves are for a triode connected 6BQ5. I get the second amp, put JJ EL34's (not matched, i need to get a matched quad)and JJ12ax7's matched quad 803s series. 6L6 Power tubes are rated at 30 Watts so you need to bias for 19. Anyway, you won't find this info on a schematic, and what little info I've seen on the Marshall schematics indicates that they do not bias their amps, but rather set the grid voltage to a 'safe' number (translate: cold setting) so they can crank these things out in a cookie cutter and get them out the door. May 9, 2008 · I'm modifying my tube guitar amplifier with an adjustable Bias Pot instead of having fixed resistor values. Cathode bias amps must run HOT. Apr 18, 2014 · The Fender bias circuit is simpler than your Marshall circuit and it will provide enough bias voltage for 6L6 tubes. The 6L6 specs listed here are: Max Plate Voltage: 400 V Max Screen Voltage: 300 V Grid bias: -125 V Max Plate Current: 100 mA Max Screen Current: 12 mA Nov 4, 2005 · Now, when you use the bias calculator on the Weber-site, they do make a difference between 6L6, 6L6GC and 6L6WGB and for the same plate voltage, they all have different max settings. I biased a pp about 25 years ago and Aug 26, 2006 · HOW TO BIAS YOUR AMPLIFIER. Sep 20, 2020 · I have a question for all loadline experts here. I was only getting about 160 volts. Also, those 1 ohm resistors aren't cathode bias resistors; they're there as a way to measure the bias current through each tube (measuring the voltage drop across a 1 ohm resistor will obviously tell you the current flowing through it) Bias voltage in tube amps is called negative grid bias because they use a negative voltage to set the amount of amperage the tube draws. That you have the current in milli amps helps but you have nothing to calculate it with. Dec 11, 2020 · Hi, I got an old amp going recently and noticed the plates glowing slightly on the two 6l6gc. 5 watts for 65%. I used a 0. The 6L6, 7581A, 6V6, 6550, EL34, 5881, KT66, KT88, KT90, KT100, etc. The bias voltage requirements for these tubes differ, affecting the overall amp design and tone. This is known as the grid bias of your amp – the correct bias level is vital to the operation and tone of the amplifier. 85 percent but you will wear out the tubes quicker! 12 watts divided by Nov 10, 2024 · They are fine with the plate voltages in the amp and can handle 30W. I measured the amp's power output at 35. It still has the bias balance circuit, so I'm about to convert that to the BF fixed bias, and I'm only getting 340v coming off of pin 8 of the rectifier. You also need to know what your plate voltage is to know what the maximum safe plate current value can be. We cannot stress enough how inaccurate the test points are! Jul 7, 2018 · IME, ideal idle dissipation, for no crossover distortion, is a minimum of 50%, ranging up to 70% of the tube max rated Plate dissipation. The bias point for 6L6GC tubes depends on the plate voltage that the amplifier is designed to use. It depends on which Fender this amp is a clone of. Aug 30, 2012 · A friend has a Mesa 50 cal plus, 6L6 powered. Green is the baseline and blue is the max. Say for example the plate voltage is 415 volts. 8842) His full load amperage was 80mA. The 1. In most common guitar amp circuits, cathode biased amps will self regulate plate current, in many cases plate Jul 22, 2008 · EL 34s take a lower bias voltage than 6L6s. 7V)(9kΩ) / (9kΩ + 1. Jun 4, 2013 · In the process of building a push pull amp with 6L6 Valves . So, using the bias calc, and assuming you'll have about 450v plates, you'll want 33-40mA per tube. 6L6 amps have more bias voltage than 6V6 amps, so it's going to take more tremolo signal to get the same percentage bias wobble for 6L6 amps. FiveG. It is a Line 6 Spider Valve w/ Sovtek 5881's in it now bias'ed to 38mv. You will probably have to adjust the value of one of the fixed resistors in you bias circuit to get this low a bias voltage from you bias pot. Apr 7, 2006 · I have a customer who wants an amp with an output along the lines of a Bassman/Super -- basically a pair of fixed bias 6L6's. 71) = 49. Apr 8, 2009 · I built a 2||6V6 or 1 x 6L6 SE amp a couple of years back. The magnitude of the DC voltage at node A is therefore (70V)(0. I have read somewhere that to set the bias one can put a 1 ohm resistor from pin 8 to ground then measure the voltage across this resistor and that will be the direct bias reading. . Mar 1, 2013 · Have a tech open the amp and take a DC voltage reading of the bias voltage with the mains voltage set at 120 VAC. Welcome To the Hoffman Amplifiers Forum May 19, 2025, 02:09:49 pm: Welcome, Guest Plate voltage is 495v May 5, 2013 · Tube values can vary greatly from one amp to the next. Fender schematics with 6l6 show bias points 15 -20 Volts either side of 45 . I don't know if 470Ω will work with your circuit but I would try before switching to the 345VAC source. Also, measure the voltage difference between pin 5 and pin 8. If you have not been trained to work with high voltage then have an amp technician service your amp. Mar 30, 2023 · Per the Onix SP3 WIKI, the bias voltage for the 5881 tubes is 1. I wouldn't use that chart at all. Many Cathode biased amps run the tubes at close to 100% idle. So take it to some one that can safely measure the plate voltage of your specific amp. The voltage drop across the cathode resistor of the 6L6 is about 12. The hardest you can push the 6L6GC into Class A Push-Pull within your requirements and without a meltdown is probably 250V screen and 425V plate . From 360-400 to 480V, the load should be 1. Both pin 5's should have about the same voltage on them at idle when the amp is not playing music. I'll take a look at the recommended readings, and thanks. My question is- how Oct 24, 2006 · Another example of nonsensical, reverse logic thinking that is fostered by the 'set bias to idle at XX% of limiting plate dissipation' is that for smaller 6L6 amps with lower HT voltages, such as this Vibrolux Reverb, it dictates that the power tube bias be set to idle at higher cathode/plate currents than with the bigger, higher HT amps such Oct 24, 2006 · Another example of nonsensical, reverse logic thinking that is fostered by the 'set bias to idle at XX% of limiting plate dissipation' is that for smaller 6L6 amps with lower HT voltages, such as this Vibrolux Reverb, it dictates that the power tube bias be set to idle at higher cathode/plate currents than with the bigger, higher HT amps such Feb 15, 2021 · The more negative the voltage, the more effective the grid. The 6EM7 Vertical Amp; The 6L6 “Spalted Alder” Amp; The 6V6 Lacewood Amp Variation in bias voltage at a given plate voltage really doesn’t have a great This page is dated December 1946, an early source. Readings at idle are as follows: HV: +405V Plate voltage: +400V Screens: +395V (I use 470R screen resistors and a 1k dropping resistor) Drop across 270 Ohm Cathode resistor: 30V Current through the Nov 6, 2017 · Amazon. And know that even 6L6GC data sheets often reprinted conditions that were on the original 19w 6L6 data sheet. Feb 4, 2014 · Just a mention though, the max voltage ratings on a 6L6 or 6L6G are fairly different than max for a 6L6GC, so just make sure your amp won't smoke a 6L6. Since this grid is not a current carrier, unlike the screen grid, a moderately changing voltage should have no discernable effect. Cathode bias resistors can be determined by using Ohm's law when the bias voltage and plate current are known. Nov 7, 2015 · Pin 5 should have something like 2 to 4 volts on it. I am experimenting changing my fixed bias amp to cathode. 0 (Ubuntu) It is fixed bias but doesn't appear to have adjustable bias. The power tubes connect through a separate mini-circuit board that connects to the main board through two press-fit 13 pin connectors. The negative voltage is in the range of -35 to -75 depending on the tube used. Pin 8 – Cathode & beam-forming plates: Typical class-A amplifier operation; Anode voltage: 350 V: Anode current: 54 mA: Screen voltage Most amps with two or more 6L6 or EL34 power tubes fall into this category. set the May 15, 2006 · Argument for is that fixed bias will give a higher max. Nov 18, 2007 · A JTM45 if setup properly will most likely have a 6. So you need to go less negative on the bias voltage. The switch works perfect, but I'm actually getting the opposite effect, with the cathode bias actually sounding louder! Dec 28, 2018 · That means that as the plate signal is swinging low, so is the screen grid voltage. Apr 29, 2018 · Amazon. In fixed bias I run the 6L6's at 30ma per tube at about 480 plate voltage. is to purchase a very well matched Quad of 6L6 tubes (6L6GC is best for long life if the amplifier is driven hard into overload clipping). Plate is 483/480 GR. So the bias point (on the bias wheel) from one amp to the next will vary quite a bit to arrive at the same reading, say 60mv. After adjusting the bias, your plate voltage will change (because the current draw thru the power tubes will cause the power supply to sag). Many run in the 460-480 range. Feb 5, 2023 · Getting ready to bias one for my friend tonight. I am concerned about using the ammeter method in case the fuse Oct 24, 2004 · The amp is a pushpull of 6l6's ; the manual recommends to bias each tube at 35ma; I want to use 6L6 GC tubes instead; should I keep the same biasing or what is the optimized operating point per tube for a plate supply of approx 335V? Feb 10, 2012 · Another question- If there is no substantial current flowing in the bias circuit, how do different value bias feed resistors affect the bias on the output tubes? For example, 220k vs 100k on the same tubes in question. anyway, the formula is amps = volts / ohms. Feb 8, 2015 · Now if we apply this factor to the OP's amp his full load voltage would be 336(380X. The easiest way to make your Bass guitar amplifier work reliably and with long life . So you need a range of something like -35 to -55 volts. Everywhere I read it said the switch would cut the power by about 30%, since it's less efficient. Don't know why it would run hot though. So for 6L6, 30 watt tube, in an amp with 400 plate volts, would be; (30 x 0. 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. 15 volts. The same amp in fixed bias would be about 40 Watts. I like the warmer lead tone out of the 34's and the deeper bottom end out of the 6L6. The JJs are supposed to be able to handle "higher" voltages. EL34 tubes, however, tend to be more flexible, adaptable to both fixed and cathode bias setups, providing you with a wider tonal palette. And if the power tubes are a matched pair. Assuming that the amp is properly designed, unless the primary impedance is changed or the HT voltages raised, a 6L6 can't pass Apr 14, 2017 · to close this thread, I hereby attach the final schematic of the 6L6WXT+ stereo amplifier. They sound incredible, but after playing awhile the amp gets hot and smells funny. There are exceptions. Thanks 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 Feb 4, 2015 · If you have 425V B+ , need to keep the screen voltage under control at 250V in order to bias into Class A while respecting the plate dissipation. Setting a higher % of max dissipation will make The 6L6 Pinout, metal versions had the shell connected to pin 1. Put the tube type selector to 6L6 from the EL34 setting. Reported plate voltages vary quite a bit from around 400v to 455 volts. and the plate voltage is close to the same. 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. If you are going to do this mod, you must first check that the schematic and your MV are the same. The schematic shows a voltage of -48 at the junction of the two 270K resistors. 7 will just get you out of crossover distortion and you can go as high as . Mar 29, 2025 · It also turns out to be a reasonable point to operate the KT88 tubes. For 6L6, EL34, KT77, we won't be as high as the -45v or so needed in a higher-voltage class AB amp, but it would be likely to have a bias of as much as -25 to -30v. When setting the bias pot to the coldest setting I'm getting about 25W with those tubes installed. What i would like to know is, firstly is this Nov 30, 2021 · Hoffman Amplifiers Tube Amp Forum, Bias help 6l6 models. Where the 6l6 might take -50 volts to bias at 35ma the el 34 would only want maybe -35 volts. The difference in sound between EL34 and 6L6 is usually down to incorrect bias/load. (this is why it came with 6L6s in stock form, to keep it from ever By simply swapping out the rectifier and power tubes and flipping the bias switch I can go from 5E3 Deluxe (6V6 cathode bias) to 6G3 brown face Deluxe (6V6 fixed bias) to 5E5 Pro (6L6 cathode bias) to 5E5-A Pro (6L6 with high power fixed bias). Jul 5, 2006 · Recently I had a 1960's 2x 6L6 cathode biased amp in for some work. 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. Oct 22, 2020 · Aim between 50-60%. More Information > what to bias the 6L6's at "Bias" includes plate voltage. 7, divided by the plate voltage, equal bias mA, each tube. Reply. Sounded a little bit flat to me so I decided to leave the pre amp tubes alone for now and change the output tubes to better ones and see what happens. The amp is completely humm free and has a clear sound, good bass (without the 20Hz high pass filter after the input signal poti, the bass could be increased), goods mids and highs. I would never suggest running 6L6's at 600v and dont know many amps that require it. 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. If the more negative voltage varies, the grid ranges from effective to more effective. Yet, if you are using an amp made for 5881 tubes, then putting in the 6L6GC will work just fine. Tube Bias Online Calculator. This will bias the 12AT7 even farther away from grid current, and into a more linear area of its operation. 5% This would be a much safer bias point. Seriously though, they are killer to have. Feb 19, 2010 · Very correct. Jan 13, 2008 · Pin 5 has correct negative bias voltage -35 on all tubes. 65kΩ and 9kΩ resistances: (49. You can run the bias max in either direction and you'll still be within the acceptable range. You will need to put in two grid leak resistors of about 220K or so on the 6L6 grid. Most 5881 tubes available have a rating of 400 volts max plate voltage. Cathode biased amps run at max signal at idle, so with cathode biased amps usually 90% is acceptable for real class A, and 80% to 85% for class AB1. Cookies & Privacy. I dont wanna yank it out like the old ones lol. Some smaller amps with 6V6 power tubes also have adjustable Feb 19, 2010 · Very correct. , Bias voltage is Biasing your amp doesn’t have to be difficult. a home brew 6L6 amp which has provided two good 6L6s (tested on my valve tester), mains transformer Oct 29, 2012 · The one limiting factor when going from 6L6 to EL34 is the bias voltage range. Feb 13, 2020 · The 7591 grid bias voltage being a lot lower than the 6L6 means the bias voltage is more critical in any 7591 design. 65kΩ) = 42V The DC grid bias is therefore -42V. 7V The output voltage is attenuated by the voltage divider formed by the 1. How to test this one with the Divo VT1000 ? In the 6L6 range we can select 6L6, 5881, KT66 but not the 6L6GC which is in between the 5881 and KT66 in specifications: 6L6, 6L6G, 6L6GA, 6L6GB have the same ratings 19w plate dissipation, 375v plate vmax, 0. Some smaller amps with 6V6 power tubes also have adjustable Jan 11, 2014 · The bias setting range is not the big. 75V for EL34. I'm 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. Max plate voltage on the early types is 360v and max screen is 270v. for 400VDC on 6L6GCs in Class AB, RobRob's site says 45mA is 60% dissipation For 360VDC, Class AB, 50mA. You could consider EL34 rather than 6L6 for your high-power SE amp. 1v peak-to-peak 400Hz sine wave into the Normal Hi jack, plugged in a dummy load (4 ohm 50 watt resistor) to the speaker jack, connected my oscilloscope leads to each end of the dummy load resistor, turned up the volume until the top of the sine wave began to flatten out then backed off until Sep 22, 2019 · This will DC decouple the cathode follower driver which also set the 6L6 bias. I have measured with the tubes removed and they are then equal in measurement sweeping from -59 to -30. Feb 15, 2006 · The plate voltage on the 6L6 was about 490V and was biased a little bit on the cold side. I don't know what plate voltage their test amp was running, so I can't say whether or not the bias was set where I would want things to be. With cathode bias the plate is 463v and the bias is 45 ma per tube. At 410V, you won't go wrong idling 35mA-45mA. You will annihilate the tubes and possibly damage the rest of the amp. 5V). The grid reference resistor will likely be ~220k (though in some cases you could use 470k). 8 times higher, 7K to 9K. You will need a bit more negative voltage for 6L6, about 4-5V if both types are on-spec, and depending upon your B+ voltage. Nov 20, 2008 · Say you set the bias at 22ma and read 430 v 14X/430=. Aug 22, 2013 · Most modern valve amps are often built around the 6L6GC power tube. For use with octal tubes EL34, E34L, 6CA7, KT66, KT77, 6L6, 6V6, 5881, 6550, KT88, KT90. WARNING: A tube amplifier chassis contains lethal high voltage even when unplugged--sometimes over 700 volts AC and 500 volts DC. In fact my JTM45 is setup with an 8k OT and a modfied bias circuit with enough range to bias 6V6 to KT66's. So, recheck the plate voltage and refine the bias voltage. I'd wire pin 1 to the raw bias if EL34's were used. Mar 19, 2015 · I bought another JSX to put EL34's in versus the 6L6 tubes. hwuxij nltdy dljn fgfne xvyqgjia kkh dxk slass bvuqrmi rcfzg