6l6 bias voltage amp. 2019-08-10 12:23 pm #11 2019-08-10 12: .
6l6 bias voltage amp There's also high-power and low-power settings that affect the bias voltage, so make sure that What should the bias be set at for a 6L6 amp? What should the pin 8 resistance be? I am running them in a st70 clone with adjustable pin 8 Home. 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 The primary modification necessary is to properly bias the amp. So, today I took my multimeter to my practice space and The 6L6 plate and screen is at 315v and 6L6 is cathode biased at 77ma There is 20v across my 270ohm cathode resistor. Member. " but the worst for the sound. Search. Cathode bias also looses power in the cathode resistor. on cold bias amps like 5150 and mesa boogie 50/50 I recommend replacing the output There are differences in anode dissipation, and the KT66 requires slightly different bias. to run as fast as you could from the Chinese 6L6-style tubes, and that the Russian 6L6-style tubes were less than stellar performers—but a lot has changed. Upon reading from the net i am confused how to bias each tube using a single thru the bias pot. If just using tubes with a separate G3, wiring them separately will give more options. Welcome To the Hoffman Amplifiers Forum December 28, 2024, 10:30:43 pm: Welcome, Guest. 3, 36. The KS 6L6 bias voltage level is determined by a fixed resistor divider, and it can be made adjustable by replacing the 10k resistor in the divider that With this modification, the bias voltage of my MV can now be adjusted from -65V to -28V. Members; lol. You could replace your output tubes between sets and go right back on stage. 425V B+ 6L6GC 4k center tapped ot Push pull class A Fixed bias TDSL says this for cathode bias: B+ 360 Vg2 270 Ia 50 Ig2 9. I am running the 6L6 at about 75% bias. Bias voltage is like a control valve that regulates the current through the power tubes. I have balanced the two 6L6GC tubes using the bias resistor that was in the amp. With respect, 10thtx. An example: a triode connected EL34 produces some 5 % of THD. { P = E x I } From this batch, tested at 400 volts, a Tony the reason I gave him the 70% formula is because 6L6GC tubes are 30 watt tubes, but I usually figure them at 25 watts for a buffer. You would need to input your power tube’s details and the voltage you 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). It can be done, but modern 6V6's won't withstand the higher current/voltage of a 6L6 circuit. 1 mA, 2. Hi, I got an old amp going recently and noticed the plates glowing slightly on the two 6l6gc. When you want to use a 6V6, you just move the speaker jack down one impedance setting (i. With a beam tetrode design, both types of tubes So in a cathode biased amp, the bias voltage varies up and down (that’s the ‘variable’ bit) automatically to keep the correct bias. 58 and 3. My recommended bias measurement method is the "Output Transformer Resistance Method. At the bias voltage is a negative voltage that is applied to the control grid inside the tube. Tube values can vary greatly from one amp to the next. View lesson. boxes for your drive tone and just run the amp clean at lower volume. "You measure the In the process of building a push pull amp with 6L6 Valves . 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. So the biasing voltage in an AB amp Here's the resistive loadline for 560VDC plate and 400VDC screen-voltage, with the 5K6Ω OPT and using fixed-bias. 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. How The 6L6GC is a type of beam power tube employing a beam-confining electrode at cathode potential in place of an actual suppressor grid. Have I missed something or misunderstood? Stevem: I thought that as long as the bias voltage was adjusted to set the ecommended quiescent current through the 6L6GC as above then the amp would be happy. to bits! A little knowledge is a dangerous thing. But you could try a 470R screen resistor and a 470R cathode resistor. Posted November 4, 2005. For amps with cathode bias resistors you can simply measure their voltage drop and use the Tube Dissipation Using Cathode Resistor Voltage Drop calculator. Variation in bias voltage at a given plate voltage really doesn’t have a great effect on peak RMS power output. Maintaining Power Tube Matching Over Long-Term Usage. There is a great YouTube video on how to bias Hot Rod style amps, by the guys at the "TubeDepot": "How To Bias a Deville Amplifier" It is the same procedure for We used a four-position switch for 6L6, a hotter 6L6, EL34, and hotter EL34. Safeguard your amplifier and tubes by measuring bias current at the low-voltage side, reducing the risk of damage and ensuring a safe testing environment. I would like to make two monoblocks with a 6SJ7 voltage amp, 6SN7 LTP and a quad of 6L6GC in push pull, run on approx. so is the screen grid voltage. The anodes have different load impedances and the control grids require a different bias. Requires separate digital volt-ohmmeter capable of reading current. For a 6L6GC the maximum plate dissipation is 30 watts. in Class AB1 fixed bias amps? The data sheets talk in terms of -25 to -40V for various tubes, but I suspect that the working techs here know better. He asked if he can put 6V6 tubes in instead. 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 If your amp is fixed bias and taps its bias voltage from the power transformer's high voltage secondary you must keep the bias tap BEFORE any diodes because the high voltage bias circuit must be fed AC voltage. Many guitar amp designs have used higher than recommended values without issues, however. EL34/6CA7 and 6L6GC will be fine with just the tube swap and rebiasing, and 4k primary impedance will be fine for both. Joined 2015. Reported plate voltages vary quite a bit from around 400v to 455 volts. Since my amp is an unadjustable Cathode Biasis this an ok switch? Will I {censored} something up? Doug did mention that 5881 is a type of 6L6GC Tube, but the all the "plate voltage" jargon confuses me If you are going to be biasing a lot, spend the money and get a bias probe tool from Amp-Wares. In contrast, in fixed bias amplifiers, once set 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. using a high-voltage setting of To the original question, the issue might be that the bias circuit doesn't have enough swing to get the bias voltage you need. Sounds fair to me but if you are using EL34 you need to upgrade the screen resistors and check the Power Transformer. The Weber site assumes the power is 30W fo Class-A amplifier, class-B amplifier, class-AB amplifier, (audio amplifiers) Height: 4. The Egnater Tweaker 40 is a unique 6L6 amp insomuch as it has the ability to sound like an EL34, too. Joined 2003. 70% X 25 watts = 17. 5 mA b+ voltage measures 416V DC or so volts at rectifier pin 8. Use this tool at your own Risk. This amperage is a part of the equation of simple ohms law What is the range of grid bias voltages you see for 6L6, EL34, 6V6, etc. It’s kind of like a thermostat. Take into account this amp ships with 6L6GC 30w tubes and uses 100ohm screen resistors. Neg voltage is fed into PIN 5 of any octal based I'm repairing an old Peavey Classic VT (212). End result: The JJ 6l6 gc's I should turn my bias dial pot UP to reach 82Mv. I know that measuring grid bias voltage is not a good way to bias, and regularly advise people not to do it that way. After playing a bit the sound cuts out and I'm measuring -120ish(!) volts on pin 1 of the 6L6's and the plates drop to about 285V. disco. Cathode-to-plate voltage was 414v and the amp was sitting at 57mA at idle. But if you don't have access to one, the 6L6WGB is rated for a maximum plate dissipation of 26W, so The bias on the amplifier is set at the factory with whatever tubes are in it on the date it was born. 49k Plate voltage measured 415V at 37. ignoring bias voltage (which subtracts from plate-to-cathode volts), 300v x 60mA = 18 watts ---> safe for even first-version metal 6L6s. Each tube sample was installed in the same custom built, fixed bias single-ended guitar amplifier (see schematic) with a Fender black face style preamp circuit. Amp is a recently aquired Ampeg Reverberocket reissue 50watt 2x12 combo. The larger the value of that R, it stops dcv Here are some ballpark areas of where the plate voltage is both bias modes with the amp's overall mode shown. BTW-One thing that had me concerned on the bias with this amp was the negative voltage on The bias circuit for the King Snake is not unlike some other Mesa Mark-series amps. (this is why it came with 6L6s in stock I installed new B+ caps, bias caps, and cathode caps. Well the EL34 data sheet has a bias voltage of The voltage amplifier/driver stage desing can affect as much. I've been on 110 lately, but the lower the voltage, the less the tubes seem to red plate - that could be coincidence as I haven't really tracked it that closely. Just a thought! Design Features. mullered. Control grid bias voltage Peak AF grid-to-grid voltage Zero-signal plate current Maximum-signal plate current JJ6V6S is not a 'real' 6V6, its more like a 6L6, and it is easily a 14W tube (some folks say 16W)! When you overdrive a cathode biased amp the cathode voltage raises, the effect is that it pushes the valve towards class B operation, i. 16 Ohms goes to 8, 8 goes to 4) to double the primary impedance. 2019-08-10 12:23 pm #11 2019-08-10 12: Hardly ever go wrong with 35-40 mA for 6l6's. Now use the plot to find the grid voltage that Bias voltage dividers? -AB Biasing. I was running SED 6L6GCs and after testing them, they turned out to be pretty I'd like to be able to swap in 6L6/6CA7/EL34. Stock, that amp has the dual-pot bias adjustment: one pot adjusts the actual bias voltage, the other one adjusts the balance. For 6V6 tone for higher voltage amps, the JJ's are the only way to go. instead of the 115/117 volts the amp were designed for. S/NC KEY 7 AC STR T 12 1 3/8 in. I will Very correct. (400-20 = 380 as above). It also has 2 12AX7 mic drivers. 15 volts. They tend to vary a lot on the Devilles that is on one amplifier I have gone around the 70mV mark, on others I have gone a lot closer to 78mV or so. I am getting voltage on only 1 output tube on the connected pins 1/8. acerv12340 New Member 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. The Fender bias circuit is simpler than your Marshall circuit and it will provide enough bias voltage for 6L6 tubes. safe line. What is the plate current I should get from matched pairs? I am thinking of buying a tube bias amp meter, and learning how to do the biasing myself. Learn how to test an air pressure switch, a sensitive I recently picked up a Harmon Kardon CA-35 PA amp. I mentioned that I use the Weber Bias Rite and that I ran my 6L6s around 42mA (according to Ideally you should bias using a scope to look for crossover distortion, not by dissipation. General discussion area for tube amps. Remember to double check your plate voltage after you bias it as the voltage will dip a bit when biasing higher. This won't work with a single 4 Ohm speaker using a 6V6 (unless you happen to have a 2 Ohm tap) but this is the quick After much research I went straight to the bias section. This looked very cool. The amp is cathode biased. Pins 3/4 read 458-463 volts on all tubes. 6mA - adjusted by me upon installation via the provided bias adj. Hoffman Amplifiers Tube Amp Forum, Checking bias with 1 Ohm resistor question Now Plate,you are right when you say 35ma is a good place to bias a 6L6 FIXED BIAS Fender with about 450v on the plates,but with this amp that does not necessarily apply. That seems low to me. I want to check the plate voltage so I can figure out the optimal bias setting for my amp. Soldano uses 2k2 on its 6L6 based amps and this rolls off a little the higher frequencies compared to Fender. 5 Divided by 420Vdc = 41. 5V (grid bias of -2V to -2. Most 5881 tubes available have a rating of 400 volts max plate voltage. Assuming that the amp is properly designed, unless the primary impedance is changed or the HT voltages raised, a 6L6 can't pass I built a modded Dynaco Mk III amp pair, with 6L6GC output tubes. At the tester recommended bias voltage of -48V: 6L6GC-STRs both 16. Its got that hollow but sweet high end and darker The bias voltage requirements for these tubes differ, affecting the overall amp design and tone. Flip the switch to the target bias, then turn the bias knob for the tube til you get a green light. The author cames no responsibility for any information provided. If you can measure the plate voltage and plate current, then you can calculate the plate dissipation. This is one reason why its good to bias fixed bias amps Instructions to build a Single-Ended (SE) 6L6 / 5881 Vacuum Tube Amplifier. Too much bias current and it’s turned down automatically. Pin 6 is getting 460v only on the # 2 tube, must the only place with a tie-in point, weird. EL34 tubes are typically idled hotter tha 6L6 tubes. I've seen these listed as 30 watt tubes by thetubestore. 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. Search titles only By: Search Advanced search Search titles only Since its fixed bias and slightly higher rail voltage you may achieve some benefit by increasing bias per tube to say 40mA. D. These should The differences between the 5881 and 6L6GC are large enough that a bias change will be required. Discover how to address gender bias with the gender bias calculator and bias amp calculator. Understand that the bias is a consequence of the neg voltage, and that the neg voltage itself is not the bias. Ideally, you'd like to have a lower screen supply voltage so the bias The cathode of any octal based amp (EL-34's, 6L6's, 6550's, 6V6's, KT-88's, 7027's) is on PIN 8. A common trick is to design the amp to take either a 6L6 and a EL34. 220k vs 100k on the same tubes in question. Moderators: pompeiisneaks, Colossal. 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. He looks great in the They convert 6L6 based amps to run EL84's. And if the power tubes are a matched pair. 5 250ohm shared impedance: 9,000 output power: 24w You will burn up your tubes with all but the coldest grades of EL34's or KT77's. Amps ; 6L6 tubes and bias 6L6 tubes and bias. You can change the plate-to-cathode voltage, and even the screen-to-cathode voltage 5V, and it will have very little effect on the idle current. 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. I'll take a look at the recommended readings, and thanks. Start to bias 6l6 tubes, or any other types, by plugging in the bias probes and the new power tubes. As it is I'm getting my bias reading from my Amp Head bias probe tool, and if it reads 58mA there is nothing I can do about that. A missing 5AR4 rectifier. ) should be -35V! That's not anywhere near -52V. it is true that using the bias voltage to bias an amp is not the best method ; However, I think it is a good place to start for a newbie - It involves measurements away from the power tube socket, helps to Cathode bias resistor dissipation is justv on 1 Watt, use a 5W "Rats Coffin" style resistor. In Cathode bias we have an effective voltage of 400-20 or 380 v. I added a 5U4 and brought it up Plate voltage: 510v Voltage from bias test points provided by peavey: 58v Bias test probe 1 in tube 1(from the left) set to 38. The resistor in there now 15. That's ok for 6L6. Welcome To the Hoffman Amplifiers Forum December 24, 2024, 08:59:58 pm Might as well cathode bias, and the voltage across the tube will be reduced by the amount of bias voltage dropped across the cathode resistor. I've done biasing before. That's tipically due to the kind of power tubes you have. For 8-pin power tubes such as 6L6, 6V6, 5881, 6550, EL34, KT88, KT66, KT100, etc. Tube Dissipation Using Cathode Resistor Voltage Drop. They really aren't 6V6's but more of some smaller 6L6 hybrid The Switchable Bias feature in Mesa amps allows the user to choose between 6L6 or EL-34 power tubes with just the flip of a switch (and once the corresponding tubes are installed). I have 560R screen resistors and I measured the voltage drop across them yesterday. 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. Measure the voltage across the cathode resistor and divide by the value to get the cathode current. Take note of the voltage displayed in the multimeter. I. anyway, the formula is amps = volts / ohms. Using a 6L6GC in an amplifier with ECC83 driver and direct coupled cathode follower with 10dB negative feedback I managed 1% THD at 1W output. The bias setting for any amp depends on the actual voltage the amp runs at. Just 1 volt difference in the bias voltage can make a difference of 10 or even more ma, of anode current. Reactions: assimaki. It depends on your own amplifier and I would suggest you trust your ears. Changing the fixed resistor on the bias pot to 10K gave me -35V in the middle of rotation. My wall voltage is 116v I never measured how much the voltages are dipping at maximum wattage but I will check it when I get the amp back from the studio. M. 8mA for my efforts. The grid cannot operate at zero volts. It could run the 6L6 with 250R or 470R cathode resistor with either a 5U4G or 5AR4 rectifier. 9 posts • Page Post by jimipage » Sat Nov 08, 2008 11:17 pm. The cathode bias for the 6L6s sits at 23 volts, no signal in Hoffman Amplifiers Tube Amp Forum, Bias help 6l6 models. The early tweed amps didn't generally use screen resistors. 200 Ohms is probably OK for an EL34 (higher gm so doesn't need as much negative bias voltage for the same idle current) but if using 6L6 then Rk=200 Ohms is asking for "cooked" tubes which will wear out really fast. I'm a bit surprised that it starts to clip at just about 18 watts rms. 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. xtian How to Bias a Fender Bassman (AB165) The 6L6 amplifier I chose is a 1969 Fender Bassman head (AB165). It depends on which Fender this amp is a clone of. 66mA per tube for 70 % bias keep adjusting till you get 70% at what ever plate voltage you end up with. the grid voltage can creep up in value, which makes the bias voltage a bit The rated power dissipation of the tube is the limiting factor. The beefiest PT I have at this time is a Hammond . Read the bias voltage, no tubes installed, where the 1K resistors attach to the tube sockets. (This amp also had variable HT selection and could take a range of octal rectifier tubes). Hoffman Amplifiers Tube Amp Forum, Suggestions for a 6L6 Cathode biased amp. Like the early Canadian amps that wired G3 to Biasing your amp doesn’t have to be difficult. My friend bought a "Dared" 6L6 PP amp. It was certainly running higher voltages than originally intended, maybe by 40-50V due to higher mains voltages, and As before, the plate voltage seems to be the major driver in maximum power output. At least not for long periods of time. I biased a pp about 25 years ago and Peavey JSX biasing 6l6GC. Yes, but the bias voltage (cathode to grid) is going to heavily dominate the readings, compared to the plate-to-cathode voltage. 6SN7-6L6 valve amp. Enter plate voltage: Select output tube: EL84: 6v6: 6L6: 7027: KT66/EL34: 6L6GC: kt77: KT88: 6550: KT120: This information is for reference only. NEUTRAL MODS: Single-Ended Tweed Filament Hum Cancellation. The other three 6L6 tubes are conducting, this one tube does not get hot at all. 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. That's the total current through the outputs and becomes a good conservative number to use since it also has the screen current included. Schematic attached. 4, 34. I fitted a new pair of G-T 6L6 valves, straight out of the box the bias was spot on, both the screen grid resistors then had an equal 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. Back to using 30W as the base for a 6L6. (Approximate numbers). Here is s video to simplify the process. My initial readings before i tweaked the bias pot were: Plate voltage 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. , will be chosen to handle the 6l6's. At 330 volts drop across the tube, plate current would be roughly 18W/330V = 54 mA. With that, and the info from the tube type, a tech can do the math and figure out the appropriate bias range. Please The 50K pot with the 22K tail R form a voltage divider. a EL34 and got 43. Making the Notch Go Away. The amp is a pushpull of 6l6's ; the manual recommends to bias each tube at 35ma Search. The tube with the higher bias amp draw also has an AC voltage on the grid of around 46 VAC. This version is a 40-watt combo with a 1×12 Learn how to calculate bias with our online calculator. Members; LordRiffenstein. Seriously though, they are killer to have. And the Ruby's currently using are good to go where at now Any signal higher than the bias voltage will be clipped. The bias range for the Hot Rod Deville can go between 70mV-80mV. 3. As an example for a 6L6 in a single ended amp setting the current at 65ma, the current setting resistor would be 19. Based on my type of amp. for 400VDC on 6L6GCs in Class AB, RobRob's site says 45mA is 60% dissipation For 360VDC, Class AB, 50mA. It uses two 6L6GC's for power tubes, fyi. Change the grid-to-cathode voltage (bias) by 5V- and it's a HUGE difference. The best way is to observe the live current. cooling off the bias/reducing the current. 4 posts • Page 1 of 1. The rest of us probably do not possess the engineering credentials or tenacity of Bill Orr. That being said- after unplugging the unit, discharging all the caps with a 50 ohm resistor, and removing the tubes: 1) replace any old selenium diode rectifiers with a silicon diode 1n4007 etc 2) replace any burst or leaking electrolytic capacitors (replace all low voltage ones, high voltage This suggests that for a plate voltage of 446 volts and a 6L6GC tube a current of 47ma will yield a 70% bias. Pin 5 has correct negative bias It will be built as a 6L6 amp, so the speakers/trannies, etc. Fender Amp Mods. Fixed bias is the best for the sound, but the worst for convenience. Max plate voltage on the early types is 360v and max screen is 270v. . Home; Contents What did I end up with? A bias voltage of -41VDC, a Plate voltage of 419, and an idle Plate current of 44. So take it to some one that can safely measure the plate voltage of I've been trying to chase down an issue with my 102 build that happens after the amp is played through for 5-10 minutes. 40 transconductance KT66s 36. SE 6L6GC Amp - dialing in the 12AT7 driver Bias. So fixed bias amps are biased about 70% max so when the amp is cranked the maximum plate dissipation isn't exceeded. Ok, so I am trying to bias my amp. Thread starter cg45acp; Start date 2019-06-14 1:48 am; Status Not open for further replies. Hello i am a newbie on tube amps and wanted to learn how to bias my old amp because the output is much much lower compared to my valveking 112 which uses same output tubes 6L6. Then by R=V/I, the Load Impedance must increase to properly absorb I had a discussion with my amptech about biassing 6L6 tubes. The amp had 22uf/300R in the cathode bias and was putting 456v on the plates. The Old Fender amps have 1k5 grid stoppers on power amp. EL34 needs higher value until 5k6 for grid stoppers. If this isn’t done, then the life of the tubes might be lowered and the performance won’t be optimal. tying the suppressor grid to a negative voltage (bias voltage) in a fixed bias amp. 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. If your amp is cathode biased or fixed biased with a separate power transformer bias supply wire then this mod will work without any The amp runs at 480 or so volts so 35ma would be 70%. The JJs are supposed to be able to handle "higher" voltages. Extreme Single-Channel Blackface Then take the 6L6 plate voltage and subtract the cathode voltage for your plate dissipation formula. The tubes are 6L6GC(HP) when I slowly increase the voltage from the 43mv and I start getting around 51 or 53 mv it starts to hum this is with the amp volume all the way down. Don't, for example, put 6l6's of any kind into a Quad II, unless you change the cathode bias resistor. 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. Okay, depending on the tube variability the voltage on the cathode would be the value measured x 1. Most 6L6GC tubes are 25/30 watts, but the S, ? Logged If it ain't broke Hi Friends, I have a good xformer with a 390+390v 300mA to supply TWO 6L6's in PP. The conversion requires some knowledge of the B+ voltage, and of the grid bias voltage of the original circuit (Ask, it can be answered). This will bias the 12AT7 even farther away from grid current, and into a more linear area of its operation. I've greatly enjoyed the cathode bias 6L6 amps I've had and currently have. Amps ; 6L6 bias range? 6L6 bias range? By The4thlast1 October 11, 2008 in Amps. Since 2006 we have offered a single octal bias probe for use with a Multimeter. Not so much a big issue with old production tubes but not The increase in bias voltage pushes the tube toward class B operation where no current flows at the zero-reference line of the signal. 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. The bias of your amp acts in much the same way as the throttle on your car. Daily Drift of AC Line Voltage. These aren't exactly correct for every amp because each amp is different and power tubes have different natural plate current draws. Tone Lizard Tales from the Tone Lounge. So there's a big enough difference in both current draw and transconductance to confirm the oft-heard claim that a true KT66 is like a 6L6GC on steroids. 6L6 fixed bias/PT question. HOW TO BIAS YOUR AMPLIFIER. You could easily bring the plate voltage of the preamp tubes back in line by To calculate the ideal bias range for any amplifier a tech will first measure the amp’s plate voltage. Use the cathode voltage and resistance to get the cathode current. Fender schematics with 6l6 show bias points 15 -20 Volts either side of 45 . Forums. This is a friends amp. How to Measure Bias. Blackface-style 6L6 Pentode Ratios. Is there a bias voltage and is it adjustable? If so are each 4 tubes independant for biasing? Normally there's a small resistor (10-15 ohm) on the cathode of the tube, you measure Re: Svetlana 6L6 Bias BDRI It depends on the actual plate voltage of the specific amp. The intersection of these perspectives puts the screen voltage in the vicinity of 300V with a 200Ω "self-bias resistor," which is included as an example design in an RCA 6L6 data sheet. The power P, is equal to the voltage E multiplied by the current I. This webpage offers two ways to calculate tube bias and plate dissipation. Since its fixed bias and slightly higher rail voltage you may achieve some benefit by > what to bias the 6L6's at "Bias" includes plate voltage. After having finished my two bi-amp monoblocks with two quads of 6V6s on each monoblock, i think about a new project. e. The 6L6GC-STR has ruggedized construction making it especially suitable for use in musical instrument amplifiers. A formula > what to bias the 6L6's at "Bias" includes plate voltage. Heater voltage will be high too, since 6V6 use less than half the heater current of 6L6. 25 in (108 mm) Bias voltage: −18 V: Anode resistance: 5 kOhms: Typical class-AB amplifier operation The voltage and power ratings of the 6L6 series were gradually pushed upwards by such features as thicker plates, grids of larger diameter wire, grid Hoffman Amplifiers Tube Amp Forum, 6l6 bias pot value question. Or use any one of the good overdrive/dist. How to calculate the bias ? I plug these numbers into Weber's bias calculator (the voltage/resistor calculator method) and I get 10 watts per tube and 26ma plate current per tube . DIY Audio Projects; DIY Audio Forum; DIY Audio Photo Gallery; so I can later experiment with an additional voltage gain stage or driving the amp with a double triode tube (like 12AX7 or 5751). Maybe nothing will be needed!! Logged wsscott. You also need to know what your plate voltage is to know what the maximum safe plate current value can be. Testing an Air Pressure Switch. 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. As to how to improve the sound, those guitar amplifier fans that read both the Tubes / Valves threads, and Instruments and Amps There is only one source of bias voltage, so the voltage at every tube's grid will be the same (it better be!). Search for a biasing tube amplifier calculator online. the point of this negative voltage is to regulate the flow of electrons through the tube. Amps made for 6L6GC tubes could be anywhere from 450 to 500 volts. The 6L6GC will take 500 on the plate and 450 on the screen. EL34 tubes, however, tend to be more flexible, adaptable to both fixed and cathode bias setups, providing you with a wider tonal palette. DISCLAIMER: LORD VALVE (W. The cathode voltage (grid bias) will rise until an equilibrium point is reached - ie the tube current is the value supported by the value of the bias, ie cathode Tube amp Bias Calculator. 2 ohms. 6L6 tubes are frequently used in fixed bias configurations, demanding a precise bias voltage for optimal operation. What Exactly Is Biasing in Electronics Anyway? According to “the Googles” (© Henry Rollins), bias is, and I quote: “In electronics, bias usually refers to a fixed DC (direct current) voltage or current applied to a terminal of an electric component such as a diode, transistor, or vacuum tube in a circuit in which AC (alternating current) signals are also present in order to A PP 6L6 amp using a floating paraphase phase splitter, a 6SN7. the cathode voltage is the grid bias voltage. Don’t touch anything inside with your bare hands even if the amp is switched off. If Vp is increased, Ip must DEcrease so the tubes don't melt. Just pull the 6L6s out of the amp and measure the voltage on pin 5 of the empty sockets. 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. A lesson with D-Lab Amps. They are known for their large plate voltages and substantial power handling. This is a 30watt mono 6L6GC PP amp with 2 5814-A (12AU7) driver/tone control tubes. If you carry a high bias voltage, depending on the plate voltage, you will hit the tube cutoff voltage (-) before you get the (+) signal up to the bias voltage. When biasing cathode tubes, and knowing the current through the 1 ohm r, calculate the power using Plate - cathode voltage for V. 1 which is about where it was set when I measured it at the bias point at R21 when I started this biasing quest. The difference in sound between EL34 and 6L6 is usually down to incorrect bias/load. By LordRiffenstein November 4, 2005 in Amps. Fender 6L6 Silverface - MOD B. (when I was building amps, I found that most guitar players preferred the fixed-bias scheme, whereas the Harp players preferred cathode-bias). I don't know what your B+ Plate voltages are, so I can only speculate, but any of the calculators out Found limited data on 6L6GC in class A PP BUT for: The tube bias current should be 36mA per tube. Class A Fixed Bias 70% Cool = DC milliamps 80% Average = 90% Max Safe Dissipation = Class AB *Merlin Blencowe believes up to 85% is OK for Class AB Fixed Bias Amps. The Measured Voltage Gain Test Setup. The entries at the top of the table demonstrate that It looks like the goal is -35mA as a good bias for 6L6GC's. Then by R=V/I, the Load Impedance must increase to properly absorb Gah, be damn careful, there might be upwards of 400VDC inside that amp. Cathode bias amps must run HOT. 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 A 100W PT could obviously handle the current requirement for 4x6V6 (~40W), but the output voltage will be high for 6V6, and higher still due to the reduced current draw wrt 6L6. Playing 2 CQ. High voltage capacitors retain their charge, I built a 2||6V6 or 1 x 6L6 SE amp a couple of years back. WHITTAKER DBA NBS ELECTRONICS) WILL TAKE NO RESPONSIBILITY FOR ANY INJURIES OR DAMAGE SUFFERED BY ANYONE AS A RESULT OF THE MISINTERPRETATION OR MISAPPLICATION OF THE INFORMATION CONTAINED IN 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. The Voltage will change everytime you change the current setting so it will take around 3 times and be very careful!!!!! In our Vibrolux example, we know from the schematic, the high tension voltage is 437 volts and we can guess due to losses in the output transformer , we will lose around 25 volts so in this amp, the 6L6’s are running at approximately 400 volts. Power output is about 10 Watts in class-A operation. I remember on my JTM45-ish clone, I had to do a little work to the bias section in order to accommodate both KT66 and 6L6. Start new topic; Recommended Posts. Does this sound right to 6SN7-6L6 valve amp . The amp has the bias balance circuit in it. 70% of that would be 21W. About 430v B+, fixed-bias using the 60 watt-rated Hammond 1650P output transformer (6600 ohm primary). This is measured using the same test as I did to determine that under an AC load the left 6L6 is drawing The later schematic suggests the bias voltage (I understand we don't care about the voltage that much, but it's a good baseline I would think. Where the 6l6 might take -50 volts to bias at 35ma the el 34 would only want maybe -35 volts. The output stage uses Grid biasing which is apparently factory set fairly cold so that any matched pair of 6L6 valves should work fine. So too little bias current and it’s turned up automatically. That amp with the 6l6's is like a cross between having that feel and using steve vai's legacy. This is why fixed bias amplifiers - which The bias setting for any amp depends on the actual voltage the amp runs at. I just got 4 JJ 6L6GCs for my Twin Reverb. 56, 3. Welcome To the Hoffman Amplifiers Forum December 10, 2024, 01:27:52 am: Welcome, Guest. Sep 17, 2007 #3 ; 11 Gauge Doctor of Teleocity. What is the methodology of "Auto-Biasing. I still have some tubes left, 6SJ7, 6SN7 and two matched quads of 6L6GC. Biasing became something that could be done in literally 10-15 seconds. I like 6L6GC at 70% and EL34's at 60%. We will continue to offer these because in conjunction with a Multimeter this is the most cost effective tool for measuring true plate current Max Screen Voltage: 300 V Grid bias: -125 V Max Plate Current: 100 mA Max Screen Current: 12 mA Max Grid Current: 5 mA with 100K resistor manages to get a 6L6 RF amplifier to work on 6 Meters, probably just to prove he could. But tubes were much more robust back then and fairly cheap so I guess most people didn't care. 6V6s and 6L6s you are going to need a 25K pot and a 10K resistor, presuming everything else is the same (transformer bias tap voltage etc. 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. The schematic shows 6L6GC power tubes. Your biasing now shows @ 80mA and I believe that is very close to the OPT transformer max current rating. Joined Mar 21, 2003 Posts 13,683 Location Near BWI Int'l. 6L6GC and 6L6WGB and for the same plate voltage, they all have different max settings. The schematic shows the bias voltage as -51V, but the voltage before the modification was -60V for 6L6 and -44. The 7868 is only rated at 19 watts. ). 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. Tone Lizard / Workbench / Bassman Bias. It changes bias voltage following by the sound envelope. This can be done Some thoughts. Recently I had a 1960's 2x 6L6 cathode biased amp in for some work. Please login or 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 In fixed biased amps usually the opposite happens and plate current rises with output. DigitalJunkie. or at least experienced in dealing with high voltage appliances, this type of modification is best left to a professional. If you want to bias the amp properly, you will need either a bias probe, Some amps have you set the grid voltage and what that ultimately lands at current-wise is going to depend on the tube. Amazon. 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+. Joined 2006. 400V on plate and little less on screen. 75V for EL34. Photograph 8 Hoffman Amplifiers Tube Amp Forum, High Voltage 6L6 SE. Pin 5 voltage will tell you nothing. Top. He says it's Class A AND Auto-biasing. The one limiting factor when going from 6L6 to EL34 is the bias voltage range. I have measured with the tubes removed and they are then equal in measurement sweeping from -59 to -30. So in this particular amp a variation of only 3 volts at the factory test points made an incredible 20mA difference in bias!! Carefully remove the old, worn out tubes from the amplifier. This forum is for talking about all kinds of Peavey guitar amplifiers. The Classic VT has a full solid state (op-amp based) preamp and then uses transistors for the phase splitter before the push pull 6L6 output stage. 5 mA and 3. With the bias set to 45 volts at the factory test points the bias went way down to a very acceptable 37mA. S/T (mode) - 355 to 375V(Plate voltage) - 12 mA(bias current) - 6L6 mode S/SD - 390 to 400 -18 There are many web sites that explain setting bias. regardless of any plate voltage variation, unless your amp is way out of spec. in audio-frequency power amplifier applications. In many tube amps, the bias on the power tubes is adjustable. It taps pin 3 for plate voltage and pin 8 (inserts a 1 ohm resister) for bias current (actually a bias voltage but across a 1 ohm resister equals bias current - anyway, don't worry about that as it is built in to the probe). So amp runs ok and sounds the way it was running with 6L6 gc's,but a slight Hoffman Amplifiers Tube Amp Forum, Calculating SE 6L6GC CLC choke current rating? I'm working on a single-ended 6L6GC design that will have a voltage of about 350VDC on the power tube plate. pot. The neg voltage means very little without other readings that you will be taking. 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. The unique problem is this: I like ajustable bias but the xformer haven't a tap to low voltage bias ( +- 40v ) A friend has a Mesa 50 cal plus, 6L6 powered. Weber Speakers will be closed to spend time with our families -- Dec 24, 25, 26 and Dec 31, Jan 1 Have a wonderful holiday season and Safe and Happy New Year Celebration. The other tube shows 36VAC. 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. This lesson demonstrates the process of biasing a 6L6 Class A amp for optimal performance. Leran how to bias a guitar amplifier so it sounds like new. PRR wrote: Plotting loadlines is only ground. All readings (initially) seem reasonable (6L6 344V plates, Bias at 43mA). Green is the baseline and blue is the max. I've run the amp at wall voltage (about 120-122 typically), 117 and 110. Fixed bias does not have the cathode voltage difference of 20v. 45 transconductance. I'm testing at 1 kHz, and yes, the output transformer was recovered from a 1940 RCA Victor floor standing phonograph/radio. 46 volt drop across So for the 6L6-GC in triode mode, max plate dissipation is 30 watts; 60% of that would be 18 watts. The cheapest solution. If you are going to do this mod, you must first check that the schematic and your MV are the same. It is fixed bias but doesn't appear to have adjustable bias. Welcome To the Hoffman Amplifiers Forum December 02, 2024, 11:04:02 am : Welcome, I would assume they are about 25watts, if so you'd want about 42mA but remember as the bias goes up the plate voltage goes down. I don't know if 470Ω will work with your circuit but I would try before switching to the 345VAC source. and measure the range of bias voltage as you turn the adjustment. Level 3; Posts: 964; I love Tube amps; NOT a SELF biased amp fixed bias is the VOLTAGE at the control grid (G1) that Per the Onix SP3 WIKI, the bias voltage for the 5881 tubes is 1. 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. If you increase the value of the 22K R to ground it will cause the dcv to go up. diyAudio The conclusion is that the 6L6GC data sheet operating conditions don't point the user to the plate/screen voltage, bias and load which yield the max power available from the tube; they treat the 30w 6L6GC same-as the 19w 6L6. If your Mesa amp has this option and you are considering venturing into experimental mode, there are a few important things you should know. 25? I think, the valves with higher bias voltage should be avoided, IMO the 15V is the reasonable max limit, and while Now, the issue with the small (+) voltage where there should be (-) bias voltage could indicate that a coupling cap going from driver to 6L6 grid is leaking high plate voltage and overriding the bias voltage. The KT88 have 220k and 6L6' bias circuits I remember use about 100k. Either needs a resistor connected between grid and B+ to form a fixed bias voltage divider or needs to be bootstrapped to the cathode by adding a 47K resistor between the junction of R2 and For 6L6 valves then yes you would be around the 70mV range. 6L6 and 5881 tubes are types of beam tetrode tubes used commonly in guitar amplifiers. 5V). 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. vryo snx zmzhr nijnd qau idcjqqx cay hfcil zqtt ghfhnt