6l6 bias voltage amp JPP100 New member. (this is why it came with 6L6s in stock This chapter will outline and guide you through the procedures of biasing the Vintage series amplifier. 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. There are many web sites that explain setting bias. Buy Nobsound Vacuum Tube Amp Amplifier Plate Bias Current Probe Tester 8 Pin Socket for EL34, KT88, 6L6, 6V6 Nobsound 8-Pin Dual Bias Current Probes Tester Meter for EL34 KT88 6L6 6V6 6550 Vacuum Tube Amp however, get a voltage reading, 28 mv. Each step should be accomplished before moving on to the next step. Where the 6l6 might take -50 volts to bias at 35ma the el 34 would only want maybe -35 volts. 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. 5k 2. This webpage offers two ways to calculate tube bias and plate dissipation. As an example for a 6L6 in a single ended amp setting the current at 65ma, the current setting resistor would be 19. 6L6 tubes are frequently used in fixed bias configurations, demanding a precise bias voltage for optimal operation. Here are my readings on the bias pot lugs: No power, bias pot 1. which makes the bias voltage a bit hotter and internal dissipation level rises, which The bias circuit for the King Snake is not unlike some other Mesa Mark-series amps. Pins 3/4 read 458-463 volts on all tubes. The cathode voltage (grid bias) will rise until an equilibrium point is reached To the original question, the issue might be that the bias circuit doesn't have enough swing to get the bias voltage you need. I'd wire pin 1 to the raw bias if EL34's were used. None More Black – guitar & amp repair – https://www -AB Biasing. 5mA 3: 35. anyway, the formula is amps = volts / ohms. Pin 5 voltage will tell you nothing. The point is to be able to use JJ Electronics E34L tubes in it 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. Post by Andy Le Blanc » Tue Jan 12, 2010 11:59 pm. The procedures are broken down in a step-by-step format. This is a negative voltage. Welcome To the Hoffman Amplifiers Forum December 20, 2024, 10:11:57 pm : Welcome, 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. (this is why it came with 6L6s in stock The resulting Proluxe is a classic all tube (including tube rectifier) guitar amp using two preamp tubes and a pair of 6L6 push-pull fixed bias power tubes. Don’t forget that it’s not just the tube your running harder, because if you play the amp loud enough to have the power supply voltage start to sag then your also running a lot more average current thru the OT and it will A friend has a Mesa 50 cal plus, 6L6 powered. The grid cannot operate at zero volts. Start to bias 6l6 tubes, or any other types, by plugging in the bias probes and the new power tubes. The entries at the top of the table demonstrate that The 6L6, 7581A, 6V6, 6550, EL34, 5881, KT66, KT88, KT90, KT100, etc. EL34 tubes don’t draw quite as much power as 6L6 tubes. With its dual For example, 220k vs 100k on the same tubes in question. At least on paper. It can be done, but modern 6V6's won't withstand the higher current/voltage of a 6L6 circuit. 5 mA b+ voltage measures 416V DC or so volts at rectifier pin 8. -2 Mohm 2. Here's the resistive loadline for 560VDC plate and 400VDC screen-voltage, with the 5K6Ω OPT and using fixed-bias. Does this sound right to If the cathode-biased amp is "true" class A, there will be no bias voltage shift seen on the cathode when signal is applied, so you can bias at max dissipation and not worry about it. 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) tube amplifier enthusiasts. 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. Maintaining Power Tube Matching Over Long-Term Usage. Here are the plots of peak power out and THD vs bias voltage for the five different plate voltages investigated. For 6L6 mode, resistor R1 is switched out to create a shunt resistance of My info states that for a 6L6 and it’s 30 watt rating this should be observed. The maximum variation being less than 1. End result: The JJ 6l6 gc's I should turn my bias dial pot UP to reach 82Mv. More. NEUTRAL MODS: Single-Ended Tweed Filament Hum Cancellation. Returned and 6L6GC is the bigger straight glass tube with 30W max plate dissipation and higher max voltage ratings. 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. Cathode & screen 6L6 GE bias setting in old fender amp? Thread starter JPP100; Start date Dec 8, 2009; Dec 8, 2009 #1 J. Balance of Yes, but the bias voltage (cathode to grid) is going to heavily dominate the readings, compared to the plate-to-cathode voltage. Okay, I think, the valves with higher bias voltage should be avoided, IMO the 15V is the reasonable max limit, and while working the pp might reach twice as much, or 30V, It would be difficult to find two pairs of both 6L6s and EL34s that bias correctly at the same bias voltage, for all the plate voltages that the Dual Rectifier can have - which are different for the Bold, Spongy, Tube and Diode modes. I have balanced the two 6L6GC tubes using the bias resistor that was in the amp. In that data set, the distortion at each plate voltage showed a relatively well behaved progression. -1. The 6L6 plate and screen is at 315v and 6L6 is cathode biased at 77ma There is 20v across my 270ohm cathode resistor. We did have a Carol-Ann with low plates, but that was by design, as he quotes that amp at only 30 Watts for a pair of 6L6. Control grid bias voltage Peak AF grid-to-grid voltage Zero-signal plate current Maximum-signal plate current Because once you are aware where the plate voltage likes to sit, you can pretty much bias the amp without pulling the chassis or Maine. An example: The difference in sound between EL34 and 6L6 is usually down to incorrect bias/load. Soldano uses 2k2 on its 6L6 based amps and this rolls off a little the higher frequencies compared to Fender. Tube Dissipation Using Cathode Resistor Voltage Drop. Cathode-to-plate voltage was 414v and the amp was sitting at 57mA at idle. You could easily bring the plate voltage of the preamp tubes back in line by increasing the first dropping resistor, but the value will depend on where the plate and screen node voltages land with the reduced load. Take note of the voltage displayed in the multimeter. Fender Amp Mods. Bias voltage dividers? -AB Biasing. Diode Rectifiers, the 3 digit numbers are the plate voltage in that paritular mode. The 6L6GC will take 500 on the plate and 450 on the screen. Fender schematics with 6l6 show bias points 15 -20 Volts either side of 45 . I looked a bunch of Valco's with the same output stage configuration and found plate voltages ranging from 360V to 400V, and current per 6L6 from 46ma to I had a discussion with my amptech about biassing 6L6 tubes. 5A, checking the filament voltage to make sure it doesn't sag down below 6. NON-Adjustable fixed biased amps still use negative voltage to set the bias of the I was not familiar with the full wave voltge doubler circuit used in the amp,(I am now!). You're wanting a negative VDC that's about 1/10th of the B+ voltage at each of the control grid pins One last question: Can a failing filter cap cause red plating on the 6L6 power tubes, even if the bias setting for this 1983 Concert is A PP 6L6 amp using a floating paraphase phase splitter, a 6SN7. what do you guys like to set your bias at in old super reverb and concerts (blackface) at with 6L6 ouput tubes? But's that's the quik formula to calculate bias voltage. 7 = . The other tube shows 36VAC. 0. Perhaps some clarifications are in order, if the bias voltage remains the same, then the volume reduction would be minimum. If your amp is cathode biased or fixed biased with a separate power transformer bias supply wire then this mod will work without any I have installed 6L6's into a Jubilee amp and it sounded great, but the tubes were running fairly hot. safe line. Lots of headroom and didn't clip unnecessarily. the bias voltage is a negative voltage that is applied to the control grid inside the tube. Using 6L6 and EL34 in a Dual. Do I need to bias my amp Class A, that's a whole other story! So how do you tell if your amp is cathode biased? The cathode of any octal based amp (EL-34's, 6L6's, 6550 Examples would be Marshall JCM800, Orange OR-120, Fender Twin, ect. Member. My rumination with this is as long as there are no detriments to tapping off an HV (red - as in the Sluckey plexi design) as compared to using a dedicated 50vac bias tap then I'm fine in keeping with whats already designed into the board, the resistors, caps and layout that's already there to accept the 330vac - just to keep I built a 2||6V6 or 1 x 6L6 SE amp a couple of years back. 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 R3 and ground. There's also high-power and low-power settings that affect the bias 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. Have fun and be CAREFUL!!!! Dec 17, 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. Class-A amplifier, class-B amplifier, class-AB amplifier, (audio amplifiers) Height: 4. 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. I was quite intrigued with the tone I was getting. That's ok for 6L6. 8 times higher, 7K to 9K. I installed new B+ caps, bias caps, and cathode caps. 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. 65kΩ and 9kΩ resistances: (49. Logged jojokeo. So, today I took my multimeter to my practice space and 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. There used to be a tube like this. If you measure the DC plate voltage from pin #3 of any power tube to ground then you can use the formula below to calculate the current draw. 6L6s need about 10V more bias voltage than EL34s, but it changes with the plate voltage. 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. A JTM45 if setup properly will most likely have a 6. Can a failing filter cap cause red plating on the 6L6 power tubes, even if the bias setting for this 1983 Concert is reading correctly? Logged The voltage amplifier/driver stage desing can affect as much. Also, as before, there appears to be a bias voltage “sweet spot” on each of the plate voltage curves with respect to Biasing your amp doesn’t have to be difficult. 65kΩ) = 42V The DC grid bias is therefore -42V. 3, 36. This looked very cool. (Approximate numbers). And if the power tubes are a matched pair. I still use it as bench tool for 6v6 and 6l6 amps. The output transformer, filter caps and rectifier tube are all upgraded to handle the higher 6L6 fixed bias output. The beefiest PT I have at this time is a Hammond 273BX (350-0-350, DC ma 175). Gah, be damn careful, there might be upwards of 400VDC inside that amp. rfgordon Posts: 679 Joined: Tue May 02, 2006 12:59 am Location: Virginia. They convert 6L6 based amps to run EL84's. In fact my JTM45 is setup with an 8k OT and a modfied bias circuit with enough range to bias 6V6 to KT66's. This can be done What is the plate voltage? How is this adjustable pin 8 resistance setup? 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 the voltage at pin 8 then adjust the bias voltage pots for certain voltage depending on How to Measure Bias. 49k Plate voltage measured 415V at 37. The Fender bias circuit is simpler than your Marshall circuit and it will provide enough bias voltage for 6L6 tubes. I'm testing at 1 kHz, and yes, the output transformer was recovered from a 1940 RCA Victor floor standing phonograph/radio. EL34 tubes, however, tend to be more flexible, adaptable to both fixed and cathode bias setups, providing you with a wider tonal palette. Pin 6 is getting 460v only on the # 2 tube, must the only place with a tie-in point, weird. The bias circuit components are located on a corner of the PC board just Bias voltage dividers? Caps? Thanks. 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. (when I was building amps, I found that most guitar players preferred the fixed-bias scheme, whereas the Harp players preferred cathode-bias). At this point, you might well be thinking: “My amp came from the factory with 6L6 power tubes, so, surely, the bias voltage is already set for them? In a fixed bias amp, a constant negative voltage is injected to the grids of the 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. 2 ohms. Joined Jan 6, 2007 Messages 211. There is only one source of bias voltage, so the voltage at every tube's grid will be the same (it better be!). (this is why it came with 6L6s in stock Cathode bias resistor dissipation is justv on 1 Watt, use a 5W "Rats Coffin" style resistor. 450V 47ma = 70%, 40ma=60%, 33ma = 50%. Change the grid-to-cathode voltage (bias) by 5V- and it's a HUGE difference. S/T (mode) - 355 to 375V(Plate voltage) - Hoffman Amplifiers Tube Amp Forum, 6l6 bias pot value question. (control grids) which will change the In the process of building a push pull amp with 6L6 Valves . The magnitude of the DC voltage at node A is therefore (70V)(0. mullered. Top. From 360-400 to 480V, the load should be 1. I have measured with the tubes removed and they are then equal in measurement sweeping from -59 to -30. You then reduce this bias voltage allowing the tubes to draw more current while raising the plate dissipation. Its got that That's loss of bias voltage. 6mA 2: 35. Welcome To the Hoffman Amplifiers Forum December 10, 2024, 01:27:52 am: Welcome, Guest. Get the best sound from a tube amp while extending its life! This lesson demonstrates the process of biasing a 6L6 Class A amp for optimal If the Pentode amp was optimized for G2 voltage nearly equal to plate voltage: bias stays essentially the same. Join Log In. Extreme Single-Channel Blackface The voltage amplifier/driver stage desing can affect as much. Changing the fixed resistor on the bias pot to 10K gave me -35V in the middle of rotation. 4, 34. 30k 3. 56, 3. with D-Lab Amps. 5mA That amp with the 6l6's is like a cross between having that feel and using steve vai's legacy. 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. 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 bias setting for any amp depends on the actual voltage the amp runs at. All the other Output tubes have zero volts on pin 1/8, as it should be. B iasing by negative grid voltage is highly inaccurate because the same grid voltage can produce drastically different plate currents in different tubes of the same type. Based on my type of amp. A "hot" bias means that the power tubes are running hard (and hot!), giving a warm, harmonically-rich tone, but experiencing decreased tube life due to the heat generated. and measure the range of bias voltage as you turn the adjustment. The early tweed amps didn't generally use screen resistors. in audio-frequency power amplifier applications. 9A each whereas EL34 heaters draw 1. 75V for EL34. Green is the baseline and blue is the max. silverfox. This method should be avoided. Tried two multimeters, three different amps. 25 watts for 6L6’s, EL34’s would be as follows 9 watts divided by 350 = . Cathode resistors (in cathode-biased output stages) are there to provide appropriate grid-to-cathode voltage in order to bias the stage, so that the tube operates within an optimal/desired set of parameters. DO NOT GROUND PIN ONE ON A FENDER AMPLIFIER, or you'll get a big surprise. My recommended bias measurement method is the "Output Transformer Resistance Method. 40 transconductance KT66s 36. Rectifier will be a 5AR4, or possibly a 5U4; Pre/PI will not be more than three 12AX7's -- Does this PT have enough juice to do the job safely and 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. 6L6 Type Vacuum Tube Shootout November 1, 2016; That Time McIntosh Saved the The primary modification necessary is to properly bias the amp. 1 mA, 2. 7V)(9kΩ) / (9kΩ + 1. As mentioned, looks like the 5F6/6-A Bassman ran them around 425v Plate voltage: 530v voltage from bias test points provided by peavey:53 Bias tube 1: 39. Heater voltage will be high too, since 6V6 use less than half the heater current of 6L6. What i would like to know is, firstly is this Per the Onix SP3 WIKI, the bias voltage for the 5881 tubes is 1. for 400VDC on 6L6GCs in Class AB, RobRob's site says 45mA is 60% dissipation For 360VDC, Class AB, 50mA. The 6L6 distortion data was all relatively low and there was no clear separation between plate voltage curves. Figure 1: Single-Ended (SE) 6L6 / 5881 Vacuum Tube Amplifier Schematic Use the 6L6 setting for the 6L6GC The 6L6 and 6L6GC are identical in all characteristics except that the 6L6GC can withstand a higher plate voltage and has higher maximum plate dissipation and hence can deliver more output power than a standard 6L6. 7V of forward voltage across the diode, creating 70V peak in front of RS. 45 Learn how to calculate bias with our online calculator. WARNING: During the course of biasing, you will be exposing yourself to dangerous voltages up to 400+ volts D. At the tester recommended bias voltage of -48V: 6L6GC-STRs both 16. Thread starter cg45acp; Start date 2019-06-14 1:48 am; Status Not open for further replies and the cathode voltage will rise. The power tube grid voltage is always negative on fixed bias amps and 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. Here is s video to simplify the process. Discover how to address gender bias with the gender bias calculator and bias amp calculator. Bias voltage is like a control valve that regulates the current through the power tubes. He says it's Class A AND Auto-biasing. The resistor in there now 15. This is the most misunderstood subject of all tube amp parameters. These aren't exactly correct for every amp because each amp is different and power tubes have different natural plate current draws. (Expensive, too. 58 and 3. I've also measured the bias of each set of power tubes using a bias probe in the TC-50. How to Bias a Fender Bassman (AB165) The 6L6 amplifier I chose is a 1969 Fender Bassman head (AB165). 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. How to calculate the bias ? How do you calculate the diameter of the pipes for water in 'the project' ? It all depends. Daily Drift of AC Line Voltage. I mentioned that I use the Weber Bias Rite and that I ran my 6L6s around 42mA (according to Very correct. 438 in (36. The best way is to observe the live current. To be honest though, we spent a great deal of time trying different tubes and finally selecting the JJ 6L6 for the V40D amps as this was the best for the job. There is a great YouTube video on how "How To Bias a Deville Amplifier" It is the same procedure for Hot Rod Deluxe or a Blues Deluxe (except the early 90's Blues 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. It changes bias voltage following by the sound envelope. Whoever put a 6L6 in may have noticed it running hotter than they'd prefer, . How about this instead: 6SF5 (or 6F5 if you like those funky grid connections to metal buttons) as a voltage amp. (This amp also had variable HT selection and could take a range of octal rectifier tubes). 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. Since this grid is not a current carrier, unlike the screen grid, a moderately changing voltage should have no discernable effect. the amp has terrible sounding distortion that kicks in nicely and Re: Svetlana 6L6 Bias BDRI It depends on the actual plate voltage of the specific amp. I am getting voltage on only 1 output tube on the connected pins 1/8. At least not for long periods of time. The amplifier schematic is very similar to the one I used for my very first tube amplifier. The unique problem is this: I like ajustable bias but the xformer haven't a tap to low voltage bias ( +- 40v ) In 6L6 bias mode I'm getting 429V on the plates using a pair of Winged C 6L6's that have a natural current draw of 20ma as measured using my Maximatcher. "You measure the The bias voltage requirements for these tubes differ, affecting the overall amp design and tone. After much research I went straight to the bias section. The output stage uses Grid biasing which is apparently factory set fairly cold so that any matched pair of 6L6 valves should work fine. To do this set your multimeter for DC Volt measurement, clip the black probe to a chassis ground (so you can use just one hand to do the measurement) and put the In many tube amps, the bias on the power tubes is adjustable. I That's loss of bias voltage. EL34 tubes are typically idled hotter tha a EL34 and got 43. 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. And the Ruby's currently using are good to go where at now 83Mv. The use of a GZ34 rectifier tube raises the voltage. 6k or 8k output transformer, which by the way is perfect for a pair of 6V6's. 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. The Fender 50-Watt 2*6L6GC amps are a good model for KT66 at 4K load. With respect, 10thtx. S/NC KEY 7 AC STR T 12 1 3/8 in. " Read more "Zero reading. 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 The negative grid voltage method: This method involves measuring the DC voltage on the grid of the output tubes, and setting it to a recommended value. Then by R=V/I, the Load Impedance must increase to properly absorb the power. - as power tranny I would use a Hammond 378CX (400-0-400v 534mA, 6. 71) = 49. I. Kooltubes, Please check the voltages at the Measure the Bias by Cathode Resistor Voltage Drop. The amp has the bias balance circuit in it. My plate voltage is 455 V. This forum is for talking about all kinds of Peavey guitar amplifiers. Search How to Bias a Fender Bassman (AB165) The 6L6 amplifier I chose is a 1969 Fender Bassman head (AB165). I just installed a DPDT switch that takes a 50w tradional fixed bias 6L6 amp down to Cathode bias to 250R/220uf. 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. Here is EL34 mode. tubeswell Posts: 2337 and 1k is usually a practical minimum used by commercial amps. I am running the 6L6 at about 75% bias. If the more negative voltage varies, the grid ranges from effective to more effective. I know that measuring grid bias voltage is not a good way to bias, and regularly advise people not to do it that way. You might consider using a 5Y3 rectifier when switching to 6V6's. This means that if you were to install 6L6 tubes into an amp designed for EL34s, the cathode current Peavey JSX biasing 6l6GC. For amps with cathode bias resistors you can simply measure their voltage drop and use the Tube Dissipation Using Cathode Resistor Voltage Drop calculator. I believe the proper use of 6L6 requires a lot higher plate voltage in order to bias them and get them to work efficiently. 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. Hoffman Amplifiers Tube Amp Forum, Suggestions for a 6L6 Cathode biased amp. 400V on plate and little less on screen. acerv12340 New Member Bias voltage in tube amps is called negative grid bias because they use a negative voltage I'm repairing an old Peavey Classic VT (212). I was concerned mostly because I do not recall working on a 6L6 amp with plate voltage this low. 6SN7-6L6 valve amp . 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) 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. Blackface-style 6L6 Pentode Ratios. 6L6's require more bias voltage than EL34's, so it would have required major changes to the bias circut for a permament swap. . Plates are 700 volts, and screens are half that voltage (at the "Hi" setting) so all correct. My band is scheduled to tour beginning July 8th, and I'd like to adjust the bias in my 5150 to get the plate current up to around 34-35mAI barely even know what this means. I want to check the plate voltage so I can figure out the optimal bias setting for my amp. If the amp doesn't 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. Thus, it's not unreasonable for the amp to be setup this way. Max plate voltage on the early types is 360v and max screen is 270v. 0182 or 18 milliamps If you’re verifying the bias on a Class A or Cathode biased amp you 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. the point of this negative voltage is to regulate the flow of electrons through the tube. The other three 6L6 tubes are conducting, this one tube does not get hot at all. For fixed-bias, you need half of AX and half of AU. 4 posts • Page 1 of 1. Joined 2006. yielding a larger voltage across the resistor and therefore a larger bias voltage). Thread starter chevy2169; Start date Dec 31, 2009; Tube and Diode modes. 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. To use the 4K OT connection you must run fixed bias, *high* screen voltage, and you expect almost double the supply current, from 2*(62+9)= 142mA to around 280mA, and get around double the power. I've seen these listed as 30 watt regardless of any plate voltage variation, unless your amp is way out of spec. But a "high end" one that had a PP 6F6 fixed bias amp, that claimed 19 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. *** The higher bias amperage draw does not follow the tube - it stays with the 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. Or use any one of the good overdrive/dist. To modify the amp so it would work well, you would need to separate the final bias supplies and add another pair of phase inverter coupling caps so the two pairs were FINAL AMP KIT SALE Take 10 off Amp Kits till Monday 12/23 with coupon code FINAL No bias voltage means lots of current draw. are common guitar amplifier power tubes, and are all easily biased with this method. Taking an asprin by Carefully remove the old, worn out tubes from the amplifier. In 6L6 bias mode I'm getting 429V on the plates using a pair of Winged C 6L6's that have a natural current draw of 20ma as measured using my Maximatcher. When you want to use a 6V6, you just move the speaker jack down one impedance setting (i. Back to using 30W as the base for a 6L6. 0257 or 26 milliamps X . Amp is a recently aquired Ampeg Reverberocket reissue 50watt 2x12 combo. Reported plate voltages vary quite a bit from around 400v to 455 volts. He asked if he can put 6V6 tubes in instead. Remember to double check your plate voltage after you bias it as the voltage will dip a bit when biasing higher. Just a thought! Hi Friends, I have a good xformer with a 390+390v 300mA to supply TWO 6L6's in PP. Level 4; Posts: 2960; Eddie and my zebrawood V in Dave's The bias circuit for the King Snake is not unlike some other Mesa Mark-series amps. And change R3 to approx 1K. I biased a pp about 25 years ago and 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. 8mA 4: 39. 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. I have 560R screen resistors and I measured the voltage drop across them yesterday. Making the Notch Go Away. That's tipically due to the kind of power tubes you have. 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. Daily Goal Workout; CQ Wheel; Challenges; Curio Cabinet; How to Bias a 6L6 Class A Tube Amp. Seriously though, they are killer to have. 15 volts. For the tube amplifier circuit I used a cutoff pentode (6SJ7 or 6SD7) driving a beam power pentode (6L6 or 5881) in a single-ended circuit (class-A operation). Fender 6L6 Silverface - MOD B. If when you measure it is close to that or a little more negative you are fine. EL34 needs higher value until 5k6 for grid stoppers. 7V The output voltage is attenuated by the voltage divider formed by the 1. As the grid is effectively grounded, the cathode voltage is the grid bias voltage. xtian Posts: 7108 Joined: 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? What would solve this problem is knowing what the bias voltage at the grids is at the nominal current. Re: 6L6 vs KT66. So take it to some one that can safely measure the plate voltage of your specific amp. 16 Ohms goes to 8, 8 goes to 4) to double the primary impedance. and the negative bias voltage will be on this pin. If Vp is increased, Ip must DEcrease so the tubes don't melt. ) should be -35V! That's not anywhere near -52V. > what to bias the 6L6's at "Bias" includes plate voltage. You can probably use that in your amp with no harm done. e. 27k With power on (with and without standby) 1. The Classic VT has a full solid state (op-amp based) preamp and then uses transistors for the phase splitter before the push pull 6L6 output stage. If the Pentode amp load and G2 voltage were optimized for Power Output: switching G2 to 40% taps is gonna cause a loss of power due to low G2 voltage at the pull-down peak of the cycle. 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. 6SN7-6L6 valve amp. 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-C, Cathode Current) : Amazon. A lower cathode resistance makes the tube run hotter (giving a 'blusier' attack). The VT1000 does not measure if the valve being tested can reach its maximum rated power output Hi, I got an old amp going recently and noticed the plates glowing slightly on the two 6l6gc. Re: DIY Bias probe. The JJs are supposed to be able to handle "higher" voltages. This was a fairly popular plan. 700 volts seemed too high for the 6L6GC's. Schematic attached. Very popular in the day but getting hard to find. As to how to improve the sound, those 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. 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+. Fixed bias is the best for the sound, but the worst for convenience. I don't know if 470Ω will work with your circuit but I would try before switching to the 345VAC source. I'm a bit surprised that it starts to clip at just about 18 watts rms. The plate voltage on the 6L6 was about 490V and was biased a little bit on the cold side. Schematic: 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 I'm not sure if 5881s were supposed to be "better" than 6L6 in some way, were "European versions" of 6L6, or something else. What is the range of grid bias voltages you see for 6L6, EL34, 6V6, etc. The amp had 22uf/300R in the cathode bias and was putting 456v on the plates. If the amp is actually class AB, you might still be able to get away with biasing at max dissipation because of the large bias shift at full power that pushes the amp into the class AB region, but Old Fender amps have 1k5 grid stoppers on power amp. A power transformer suited to 2x6L6, with ~400V B+ would be a better Hello, I have little to no knowledge on tube amplifiers. A simpler alternative would be to simply pull a pair of your 6L6's. For 8-pin power tubes such as 6L6, 6V6, 5881, 6550, EL34, Can someone tell me how to calculate the needed Neg bias voltage for a 6l6gc push pull amp? More or less a Fender style amp section. 3V and then keeping a close watch on the temperature of the PT for an hour or so to see how hot it's getting compared to normal. The difference in sound between EL34 and 6L6 is usually down to incorrect bias/load. Just pull the 6L6s out of the amp and measure the voltage on pin 5 of the empty sockets. A "hot" bias means that the power tubes are running hard (and hot!), giving a warm, 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. The circuit that the tubes are in My understanding is that the 66 was designed to have the same power as the 6L6 albeit at a lower voltage. Silverface Filament and Bias Circuits. Before switching the amp back on pull all the output tubes out and measure the -VDC in the bias supply. Any similar tones and feel comes from the cathode bias and the weak power supplies that most Fender 6V6 powered amps of that era had! Most organ amps do not have much plate voltage , that will sound more like a 6V6 amp than a 6L6 amp for sure, 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. Amps wired for EL34's can take 6L6's no problem, provided they can be biased. However, if the bias is you should just get a bias probe man. ca: Musical I read some good reviews and saw how simple this unit was to use compared to taking meter into high voltage areas of my head so I bought Yeah not sure about that, my data shows the 6L6GC has a max plate voltage of 500 while 5881 is 360. 5dB except for the very highest bias voltage point on each of the 275v, 300v, and 350v curves. The anodes have different load impedances and the control grids require a different bias. Please login or register. Most cathode biased 6L6 amps ran 2x 6L6's with the same 250Ω cathode resistor you'd see in a 2x 6V6 cathode biased amp. It could run the 6L6 with 250R or 470R cathode resistor with either a 5U4G or 5AR4 rectifier. I just went back to EL34's. 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 goes to ground with a small 10k potentiometer. 25 in (108 mm) Diameter: 1. 9Mohm I added a pic because I didn’t believe the reading. To modify the amp so Thanks wpaul, And that makes alot of sense. It’s on an amp cradle so I tried to reach under The bias setting for any amp depends on the actual voltage the amp runs at. That seems low to me. The 6L6GC-STR has ruggedized construction making it especially suitable for use in musical instrument amplifiers. How The tube with the higher bias amp draw also has an AC voltage on the grid of around 46 VAC. The output stage uses Very correct. 3v 6A to heat 6SJ7 and 6SN7) - with a silicon rectifier and negative bias for the 6L6 grids (are the proposed diodes and the EL 34s take a lower bias voltage than 6L6s. Any self respecting gear whore needs one, lol. With 475V 70% =44ma. disco. 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 If you self-bias the 6L6 so you can use large cathode resistors, one 12AX7 will do. 1 which is about where it was set when I measured it at the bias point at R21 With this modification, the bias voltage of my MV can now be adjusted from -65V to -28V. The one limiting factor when going from 6L6 to EL34 is the bias voltage range. 5 mm) Bias voltage: −18 V: Anode resistance: 5 kOhms: Typical class-AB amplifier operation (Values are for two tubes) Power output: 55 W: Anode resistance: The voltage and power ratings of the 6L6 series were gradually pushed upwards by such features Here are some ballpark areas of where the plate voltage is both bias modes with the amp's overall mode shown. C. The schematic shows the bias voltage as -51V, but the voltage before the modification was -60V for 6L6 and -44. M. Also, I tried to adjust the bias pot when the amp went into the weird voltages. If you are going to do this mod, you must first check that the schematic and your MV are the same. I fixed the original fault in the amp which was a faulty transistor in one of the valve cathode circuits. It loses the bias voltage probably due to an internal short circuit inside the glass envelope and this allows the valve to draw lots of current and get extremely hot very quickly Hardly ever go wrong with 35-40 mA for 6l6's. 6L6 Circuitry: Fixed bias preferred; A common trick is to design the amp to take either a 6L6 and a EL34. With a pair of JJ 6V6's @ 28ma I'm getting 417V. ;-) Note that on some really old In many tube amps, the bias on the power tubes is adjustable. Joined 2015. Stock, that amp has the dual-pot bias adjustment: one pot adjusts the actual bias voltage, the other one adjusts the balance. Pull 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). You will annihilate the tubes and possibly damage the rest of the amp. I've greatly enjoyed the cathode bias 6L6 amps I've had and currently have. Ok, so I am trying to bias my amp. My friend bought a "Dared" 6L6 PP amp. I don't know what your B+ Plate voltages are, so I can only speculate, but any of the calculators out 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 Question: I'm a little on the fence about what to bias the 6L6's at. An example: a triode connected EL34 produces some 5 % of THD. This is a friends amp. boxes for your drive tone and just run the amp clean at lower volume. There is about 0. 4 to 1. Using a pair of Russian 6V6's @ 32ma I'm getting 411V. So if the negative grid bias voltage goes up the tube current goes down and vice versa so by incresing the voltage on the back of the amp you are running the tubes colder not hotter. 70% of that would be 21W. 2019-08-10 12:23 pm #11 2019-08-10 12:23 pm I would rather not name my amps brand name but it has 2 6L6 heaters draw 0. Everywhere I read it said the switch would cut the power by about 30%, since it's less efficient. 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 lesson demonstrates the process of biasing a 6L6 Class A amp for optimal performance. Cathode bias amps must run HOT. The Mesa/Boogie Dual Rectifier has a grid bias supply that is switchable to support EL34 power pentodes or 6L6 beam power tetrodes. That is for 6L6 tubes so I'm not sure what the 7868 bias at. If your amp is cathode biased or has 1 ohm bias setting cathode resistors then you can calculate bias current by measuring voltage drop across the cathode resistor. 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. I'll take a look at the recommended readings, and thanks. You also need to know what your plate voltage is to know what the maximum safe plate current value can be. The GE datasheet states max plate voltage is 450V for triode operation, 500V for pentode. 6L6 to 6V6 I think here is another consideration in addition to a bias pot. -User Name You don't need to make big power output, and low-voltage is likely to mean small-bias-voltage, Let me up the bias a bit, get the chassis and cabinet all buttoned up, and I will get back to you guys. If you want to bias the amp properly, you will need either a bias probe, The conversion requires some knowledge of the B+ voltage, and of the grid bias voltage of the original circuit (Ask, it can be answered). -2 Mohm 3. 32. The OD side had so much clarity and command - no fizz but lotsa nice OD clarity - probably less intermodulation? Amps with bias-wobble-trem need to be biased a bit colder The more negative the voltage, the more effective the grid. 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 and easy Variation in bias voltage at a given plate voltage really doesn’t have a great effect on peak RMS power output. It uses two 6L6GC's for power tubes, fyi. 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 What is happening is you are confusing negative grid bias voltage with tube current. Bias is never the same twice, just keep it close give or take In fixed bias amps you have more options. 5 mA and 3. What is the methodology of "Auto but the worst for the sound. 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. pafx dgnz azhu juaiu ebflq vqqb qetoc nspd pwicy xbw