El84 fixed bias. The 70% figure refers to fixed bias amps.

El84 fixed bias The plate voltage is 400V, and the plate current is 50mA. But even with the big cap, under sustained signal the cathode voltage still comes up a fair way and cools off the bias under load. After having built EL84 amps using both Fixed and Cathode bias, I’m convinced that fixed bias is the way to get the absolute maximum performance out of them. The solid fast and acurate lower end of the fixed bias is obviously better than the original. Essentially, you want to balance the voltage between the then checking across that might give 16 volts, though honestly I'd seriously doubt it for a fixed bias amp. But it the bias is still fixed. I have one fixed bias EL84 amp with separate 1K screen resistors. Cathode-biased power section will have more harmonic content, compared to the fixed-bias version. The amplifier is idle-biased to say 80% or 90% of max dissipation, unlike a fixed biased amp that would be biased between 50% to 75% of max dissipation. Is there even possible to run two EL84 in fixed bias with the type of transformer used in a 18W amp? Anyway, if anyone have a schematic I'd love to see one. Hi, about diyparadise. On the The smaller Mesa amps using EL84’s are also fixed bias but are not as cold so by using hotter tubes they sound real good and the JJ EL84’s are by far the best EL84 being made today. For example, assuming the supply is designed for use with a fixed-bias push-pull EL84 output stage, with a nominal current draw at idle of around 68mA. Cathode-fixed bias and OT impedance? In the tube specs for the EL34, the (Mullard) data sheet lists operating conditions for two valves in push-pull, for both fixed and cathode bias respectively. Joined Jul 16, 2007 Messages 2,828 Reaction score 2 Location SoCal. I have a fixed bias using a 250 ohm resistor. The Scott 299A and 299B used "fixed" bias on the 6BQ5s. It may be due to the ratio of Ia peak to Ia idle is somewhat lower with typical cathode bias than fixed bias designs, the crossover kink occurs at higher levels, rather than closer to the zero crossing type that's tends to be seen with fixed bias. Historically this wasn't done for EL84 amplifiers because they were considered to . A friend mentioned to me that fixed bias was for guitar amps and i should convert to auto bias. This is a fixed bias amp that uses 4 x EL84, but the bias is not adjustable at all, the amp has to be modified to enable this. If your using 2K5 for you KT88, then the EL84 may be happy, but the impedance match (It isa fixed bias amp. TrANsfer ChArACTerisTiCs pLATe ChArACTerisTiCs A. Accordingly, we can use a grid leak resistor R l of 200K Ohm to give some margin to the resistance of the bias voltage circuit. The core tone IMO should be Push-Pull fixed bias – but with whichever power output you desire. 5 volts at idle on top of the cathode R and a 100uf bypass cap. Member. In attach the schematic: it use a 6922 with the first section at gain and the second as cathodyna. It's sound stage is deep and wide with a sparkling highs. Output power is 5. Part 2 of the clip deals with the theory and out EL84 may be more linear, but it has a reputation for short life probably because it just runs too hot. I've looked at some of the 20w blackstar designs and they are cathode-biased push-pull with a I like to use 150Ω for shared cathode resistor. Red light district Oddwatt el84 pp Baby huey Tubelab simple pp Tubecad journal (john broskie) I built one that is basically the broskie circuit with a few tweaks from other amps like a CCS on the el84's cathodes for fixed bias current and using a non bypassed red led on the 6n1p. in the fixed bias Fender Pro Junior or cathode biased Dr Z Carmen Ghia. On the There are many tried and true el84 pp amps an this site. Yes, you get a smaller voltage drop with EL84 Ceux équipés d’un bias fixe (fixed bias en anglais) ou ceux qui fonctionnent grâce à une polarisation automatique (cathode bias). g. FACTORIES can get away without individual biasing and save valuable Tech/Assembly time by setting a fixed negative bias voltage *because* they are BIG buyers and can get pre-sorted/matched tubes in batches or dedicate personnel for that task ,aluxury an individual buyer does not have. The James is flat to ~65 kHz and the Hammond is a mess above 10 kHz but to 100 Mhz both were clean as a whistle save for the normal 120 Hz. Tube Dissipation Self-Biasing Amplifiers: Yes they do exist. I replaced the fuse and threw in some test el84's and it works and sounds fine, but I'd like That's called "fixed bias" but it's adjustable in the sense that you can design a circuit that allows adjustment of the negative bias voltage. Amps with the cathode resistor you mentioned are cathode biased and generally run at a higher idle dissipation. e. For example, a tube requires -10V of bias to conduct 50mA, then the cathode can be raised to +10V with the grid at 0V. In terms of circuit design, the EL34 is frequently used in amplifiers with a fixed bias configuration, allowing for more precise control of the tube’s operation. plate voltage is higher for the 7189, but the max. EL84 datasheet specifies that maximum grid to cathode resistance, with fixed bias, is 300K Ohm. That can help for those who want to push the limits of the amp and don't care for an overly warm, mushy sound when that happens as the fixed bias amp will not compress the signal as smoothly as the Actually the spec. Sv. regards As the original circuit of the Geloso is a Fixed Bias and many guitar amps that uses el84 are Cathode Bias it was think to swap to the Cathode Bias alternative Knowing that to have both the Bias versions on the amp is possible and ispiring to the Weber 6d30 Kit, we have think to have both options as to be able to have different tone responses For power amplifier tubes cathode bias and fixed bias are the most common. There was a post recently about 4x EL84 SE. 5W per tube at its current +B I have a variant of my SPP board (modified for fixed bias) that runs "EL84's" from 425 volts and makes 25 to 30 watts per pair into 6600 ohms. 6K spec. 1 el84 power pentode p e n t a l a b o r a t o r i e s 9740 cozycroft avenue * chatsworth * california 91311 (800) 421-4219 * (818) 882-3872 * fax: (818) 882-3968 EL84 is 18. Don't try this with a self biasing EL84 amp though. Mesa claims that if you buy their mesa tested and branded tubes they In fact, this is what we are doing whenever we adjust the bias on a fixed-bias amp! If we bias hot enough (i. ones too) and is a great valve to achieve superb tone at lower power levels. Biasing without them is clearly a disfunctional way. This system, designed by David Gillespie, uses the LM337 to control output bias, and "track" any power supply fluctuations that may happen under heavy load conditions, such as like "heavy bass" and such. And the operating class is an entirely different issue from bias method. Should i keep the fixed bias or convert it back to the williamson? Your thoughts, opinions and past experiance would be greatly appreicated. 44Ω, respectively. The tube will reach a stable point of equilibrium and stay there. This modification helps extend tube life and reduces hiss in these amps. Typically you will find cathode bias amps in lower power EL84 configurations although there are exceptions ie: I have a PP EL84 amp. Cathode Bias 6V6 power tubes in the same amp. If you look at the notations on the schematic, the "EFB" stands for Enhanced Fixed Bias. To see for yourself, consider increasing the value of the cathode resistor in your AC30, eg to 82 Short story, my plan is to build a Marshall 1959SLP clone with a pair of EL84 at the end fixed bias, at 450V B+ yes is kinda high but has anyone tried it before? btw the screens are on the range, at <300V and plates idle at 24mA, with those voltages the amp gives me 25W-ish (10%-ish distortion) and 30W full clipping. You can get close by Fixed bias means the valve is likely to be either too hot or too cold. Those tubes were removed from a Baldwin tube organ that was pulled from church use It is fully possible that with fixed 6 V bias voltage the current of one pentode is 5 mA and 7 mA of the other, I used 3 or 4 in series for each EL84 in an SPP board. I really like the idea of a 9V battery, but have never tried. The intention was to make a simply solution with a very good OT trafo. Formulacoop. for fixed bias. Can this also be done with a fixed bias amp and how should I connect the zeners ( about 12 volt , I think) ? Can I connect the zeners to ground after the coupling caps and before the 220K bias resistors ? We also use a 1K Ohm screen stopper resistor R screen to connect the EL84 screens to the ultra-linear taps of the transformer. Vacuum tube EL84/6BQ5 is a miniature pentode with equipotential cathode, designed to amplify low frequency power in the output stages of HI-FI audio. ~-40V). M. Do they both need biasing when Forums. 8K 2w resistor then filtered again with another 300 x 200 You might find that a fixed bias EL84 amp does provide greater punch and get an extra clean watt or so out of the otherwise identical amp design. , 40% screen tap EL84 Page 1 of 8 Telephone: +381 18 550 741 EL84 is A. EL84 is 18. The schematic you showed is for blending EL84's and 6V6's. 6 may be a better approximation. It's been running off and on for about 5 years on the same set of tubes. My ST-70 issue on my bench right now getting 2mm test jacks. One channel is slightly louder than the other, making the soundstange about 12-18" off center. Most push pull EL84 amp currently built are fixed bias. Power Stage: 2x 2 EL84 power vacuum tubes in Push-Pull Ultra-Linear configuration and fixed bias; Phase Split Stage: DC coupled Concertina with 12AX7 vacuum tubes; Input Stage: 12AX7 vacuum tubes with self bias and bypass capacitor; Class AB with 28 mA anode current bias; Output Transformer: Hammond 1650E Primary: 8K Ohm C. To control the current flow you have to make the control grid Present El84 crop are very inexpensively produced tubes. At full tilt the voltage on the cathode R rises to about 18. Glad I did. The schematic is kinda of a mess but if you page down to page 7 you will see the power amp section and the 4 El84 power tubes. 5K for B+ = 450V. I therefore tried ultralinear plus cathode feedback for the 6V6s and 6SL7 for the diff amp front end. In an EL84 HiFi Amp I actually use separate bypassed bias resistors for each tube but with a small unbypassed common resistor (equal to 12 to 15% of what is the normal individual cathode bias resistor value of 270 Ohms) under those for the link back Re: Cathode Biased EL84 Amp vs Fixed Bias Depends more on the circuit itself than the tube compliment trying to compare 2 amps by bias type is like trying to describe the driving performance of 2 cars only using the kind of tires each has. In a fixed bias output stage the K is grounded with no R between ground and the cathode. Thread starter razorrick1293; Start date 2011-11-20 12:14 pm; Status Not open for further replies. The With a common cathode bias resistor the feedback becomes common mode which changes its effect. Due to the high +B in this amp, Ive reduced bias on the EL84 to 30mA which equals 10. I have about 2,000 hours on a quad of the 7189 version(6P14P-EV's), in my Fisher integrated. If nooone has a manual to verify, you might have to open it up. Joined 2004. (0. Most of the time a PPP EL84 design will be a little fiddly, with more compromises compared like a PP EL34 or 6L6. Note that a fixed bias amp with a cathode resistor of 25 Ohms or less wants the idle wattage to be less than 70% per tube. No - fixed bias is no cathode resistor and (therefore also) no bypass cap (so that the cathode then sits at 0V potential) - and the grid then has to have a negative DC voltage applied to it to drop it about 10V or so below the cathode (i. F. How about configuring your monoblocks for fixed bias so you can run EL34's or 6L6s just give yourself lots of adjustment room with the bias voltage (~-55V vs. This page is a step by step guide on how to adjust Black Ice Fusion tube amplifier bias. 25% @ 1W) and response is 10 Hz to 44 kHz +/- 0. Whether or not you cathode bias EL34/6L6/6550/KT88 is purely a matter of preference. I do check the bias on the amps with fixed bias. 6 volt filament trans wired to the 6. With fixed bias; 270V B+ the AB power can be stretched close to 10W and no-one should underestimate the simplicity of fantastic listening power and simplistic design. Amps with a single EL84 output tube are probably class A and will run normally at around 100% max. It can be easily fixed but it’s best to avoid by installing and removing your tubes correctly. Newsletter. My first real tube amp was triode-connected 6V6 PP ( with 330 V B+ and fixed bias with low current ) with tube rectifiers in mono-block configuration The dual EL84, fixed-bias power section delivers up to 28 watts in full power mode, and just 2 watts in low power. Thanks to David Gillespie for suggesting using the OPTs in this fashion. Nor does fixed bias dictate class AB! Most push/pull EL84 amps are cathode biased, so there is no adjustment. I made an effort to try to google and search various forums to no avail. Paid Member. 2022-08-04 The fact that there is bias current to measure in AB indicates that the tubes are not "shut off". The ST35-BCU Bias Control Upgrade Kit is engineered to fit the new or original Dynakit® ST-35 amplifier. Some time ago I read about the Paul Ruby zener trick for cathode biased amp to reduce the harshness when diming the amp ( el84 tubes). Most EL84 amps are cathode bias – well fixed bias is BETTER, 18W from a pair of EL84’s. I ran into an issue with el34 and el84 and the fender circuits, the control Super, Bandmaster, Bassman, Twin, 5E series) and the new the 5E11 Vibrolux (Concertina with fixed bias), the Deluxe received the upgrade of only the Concertina Splitter (make sense to have the girls wire essentially the same PIs in all these models I have an old Edison-12 poweramp, with EL84 push-pull output stage, that was originally cathode biased. Bias R was usually 10M, resulting Yep, that's definitely a cool feature in "fixed bias (aka, adjustable bias)" EL84 amps. Running the EL34 at ~420. That's unusual, because low powers are more Max. 2011-11-20 12:14 pm #1 2011-11-20 12:14 pm #1 Hi there, Just wondering, would it be ok to run - Bias warmer (lower cathode cap) - risks burning out tubes - Stabilize bias voltage The warmer biasing approach used in vintage Marshall 18W and many Vox amps is an interesting one. (2) to evaluate the sound quality of EL84 as driver I've used output transformers with primary Z of 7. You may want to consider splitting the cathode resistors, ie, The tube will reach a stable point of equilibrium and stay there. Formulacoop; Aug 14, 2024; Vintage Amps (Before 1991) Replies 0 Views 317. I saw this thread and thought I'd chime in (EL-84 pun intended), since I did a lot of cathode-bias experimentation over the last month. These are still fixed/non-adjustable bias amps as there is no internal bias trimmer pot to fine tune. (EL84/6BQ5 dans le cas des Vox) sans avoir à régler quoi que ce soit. Plate Voltage mA . I don't know if it is significant enough but I tried to keep it as close as I could. For this reason, the bias should always be checked, even with cathode-biased amplifiers. Another bonus is the amp runs Bias Settings For Safe Plate Dissipation Class AB: 70% Class A: 90%: Class . 5K, whereas for cathode bias, it specified 3. One more thing: some fixed/non-adjustable bias amps have a "bias switch" to select for a specific tube type. 425V plate voltage and 40mA (as per schematics) EL84 2K. De nombreux autres amplis à lampes fonctionnent eux avec un bias fixe, la plupart du temps un potentiomètre a été prévu par le Hi all, i want to make a class A pp amp with fixed bias. Joined 2010. Output Pentode Quick reference data Grid No. Of course, Much better a fixed bias for each 84 . The typical tube amp is a fixed bias amplifier. Jul 24, 2008 #5 I think by "bias selector" he means can you switch between EL-84s and 6L6s. F. Ive made a couple of bias-blocks (CCS sub-circuits) along the lines used by Gingertube in his Baby-Huey amp, one per output valve. Along the same line you gain about 15 extra volts if you use a fixed bias rather than a cathode bias. The bias voltage is set at -20V, and a fixed bias is used. How many of you have compared the exact same amp using the same In this clip I complete an objective test comparing Fixed vs. 3V current capacity. Of course, the bias point must not be pushed above the I've never seen a schematic with two EL84 in fixed bias. Take a look on it on the Tronola Electronics site, and you can have individual bias pots if you wish. EL84 A. 24VAC for a total of 760mA for all four tubes, but you are losing 4. 8K 2w resistor then filtered again with another 300 x 200 Cathode-fixed bias and OT impedance? In the tube specs for the EL34, the (Mullard) data sheet lists operating conditions for two valves in push-pull, for both fixed and cathode bias respectively. Super cool amp, crazy smooth and you get such contact and feel from the amp when you dig in it's nuts. You'll cook the remaining two power tubes. To avoid having to use matched EL84s I would like to institute separate cathode biasing. ) When I started to set the bias, I couldn't get them to go much below 60ma and I was shooting for 40-45. 2dB. Dave Gillespie's great low cost EFB mod should work and is sonically better than the tradition fixed bias with a transformer supplied bias voltage. EL84 amps typically run somewhere around 12 volts of bias, Bamadawg Would a fixed bias vs cathode bias affect tube life span? Is one harder on the tube? Say cathode biased at 100% vs fixed at 70%? So would a pair of EL84's at 300V in a cathode biased amp then last longer if they were biased at 80% instead of 90-100% of their max plate dissipation? Yes Adjustable "fixed" bias removes the cathode bypass capacitor time constant, gives complete independence of bias from other valves' current, and has the lowest distortion. Cathode Bias (Self Adjusting) Class AB Fixed Bias 50% Cool = DC milliamps 60% Average = 70% Max Safe Dissipation = * Cathode Bias 85% Cool = DC milliamps 95% Average = 100 % Max Safe Dissipation = *Merlin Blencowe believes up to 85% is OK for Class AB Fixed Bias Amps. Mesa Triple Rec Issue. output pentode Ug(V) Ia(mA)-15 -10 -5 0 0 40 60 80 100 120 140 160 180 20 Cathode vs. (PSU is on a separate panel currently with a plastic laundry tub over it to keep pets away) LED biasing for 6CG7 input gain stage and for concertina phase splitter. AC30 are cathode biased, Blues Juniors are fixed bias; it’s a world of difference! With fixed bias, it can be moved over its range without affecting the dynamic conditions too greatly. And it's only 263mm wide – smaller than an A4 sheet! Chappers: "I wanted something that was affordable, doesn't compromise on tone and is made in England. Superb clean tones with the gains back and master up. Mine is a fixed bias. Hi EC8010, I've only tried cathode feedback with a James 6123HS/EL84/fixed bias and a Hammond 1628SE/EL84/cathode bias and haven't seen anything so far to suggest instability. I have never seen a EL84 version but have hear about them! Reply. This is a fixed bias amp so not much to do after you measure it but it is a nice check to see where the tubes are operating or if you have one that is way mismatched. Aug 14, 2024. It seems that EL84 tend to be cathode bias, but not exclusively (Mesa has 6L6 and EL34 amps that are, and I'm sure there are fixed-bias EL84 amps). Bruce Heran outlines the design and construction of monoblock Push-Pull EL84 (6BQ5) valve amplifiers. A Classic 30 is a FIXED bias amp - not like most EL84 amps, but you might do some reading. They are fixed bias and not adjustable - big difference. 6k6 load is nice for either -- and will give slightly lower distortion on EL34's afaik. These may have an adjustment pot or a fixed resistor for the bias voltage setting, but either way they are called a fixed bias amplifier when there is no cathode resistor on the output tubes. I IMHO EL84's are TOO spongy under heavey OD with cathode bias. It is desired to have a bias supply range from -9V to -13V, nominally -11V. I made a autobias based on 30 mA CC source with 200 muF on a EL84 (in fact ECL86 on a Revox The tube will reach a stable point of equilibrium and stay there. They are different size tubes. This is to enure that any grid leakage current does not add to the bias votage in the direction of increased anode current. Unfortunately, it comes off of a dedicated PT secondary. For the purpose of this thread "Choosing a good EL84 Amp" I've added: With regard to bias types I've read opinions that state: 1. Cathode bias is accomplished by raising the cathode voltage above ground and keeping the grid at approximately ground. fixed bias self - bias (at: filament voltage 6. Exactly The 70% figure refers to fixed bias amps. The Tung-Sol specs 23-45W depending on setup and voltages. 3 V plate voltage 250 V,first grid voltage minus 7. Seperate resistors and two large caps give a feel even closer to fixed bias. But besides sounding good and lasting well they are the most rugged modern production EL34. So, following what you wrote, as long as I'm not getting red plates I could bias hotter and hotter until the crossover just dissapears and actually USE the grid conduction as a part of the bias? Why not I guess. for Cathode Bias which many EL84 amps use is 9K for Pentode hookup of 7591's. 8W @ 2% (0. The ST35-BCU employs an adjustable fixed bias circuit which allows for user adjustment of the bias setting of each (EL84) The tube will reach a stable point of equilibrium and stay there. You gain a little more power,, longer tube life and better sonics. If you are planning to play out much failed tubes are inconvenient at the least. Tube Type . They aren't self biasing. European AM/FM radio sets of the 1950's and well into the 60's inevitably had an EABC80 for AM/FM detection, the Triode (ecc83 like but mu of 70) in grid-leak bias driving an EL84 output. Or at least when it hits the point chosen by the zener voltage. 2. S. CALCULATE PLATE DISSIPATION Based on Plate Voltage and Cathode Current Readings. I have 355 VDC on the plates of Bias Calculator and plug your figures into the formula for cathode biased amps. Have you an idea about LED CCS ? Thanks. I think fixed bias with EL84's is much better for overdrive. It looks from the data EL84s aren't used with fixed bias often because they need less bias voltage the effects of cathode biasing are reduced. May 6, 2009 #2 ; B. output pentode max 22,0 max 71,4 max 77,8 Base: NOVAL uf = 6,3 V If = 0,760 A Typical Characteristics: ua = 250 V ug2 = 250 V ug1 = -7,3 V ug1 fixed grid bias 2) Ig1 = 0,3 µA 6BQ5. BUT they ran the tubes really hot. That can help for those who want to push the limits of the amp and don't care for an overly warm, mushy sound when that happens as the fixed bias amp will not compress the signal as smoothly as the This page has the proper tube bias settings for Black Ice Amplifiers such as the EL84, EL34, KT77, 6550, KT88, KT120, KT150, and KT170. After recapping an old amp, made in Argentina, cathode bias, which came with 2 shorted output tubes, installed new tubes (JJ EL84, matched 4), and while checking voltages against the User Manual voltage table, I realize that with those values the EL84 are biased at almost 14Watt. Distortion at Ig1=+0,3µA FAX: +381 18 550 806 Postal address: Bul. The bias current is set purely by the input bias voltage. Jump to Latest R. If to OP had EL84s, it could be different. Even the Magnavox 9303 EL84 PP amp has split cathode now. I also like to use two separate 100Ω for screens. I know this is TUBE AMPS 101, but I am confused. I don't think it would hurt sonics going from Tung-Sol's 6. He also said you may notice a need to change them more often as EL84 amps run the tubes hotter. Then the output stage with fixed bias and a To adjust the voltages of a "fixed bias" amp is the same principle for all push pull amps. I talked to the dealer who suggested that I increase the resistance in the bias circuit and I did, multiple increases, and could never get Fixed bias , is useful on high-power tubes (amplifiers) to avoid the wasting of power (and heat) on big cathode resistors (KT88 , EL34 , 6L6 ,etc) BUT for low power tubes like 6BQ5/EL84 , 6V6 , ECL82 , I always prefer cathode bias . 8K - 10K PP with triode connected EL84 and have noted only very small differences in performance with real speaker loads and So I completed my first "real build" using ideas from Sluckey's November schematics, pairing it with a 12at7 PI tube, a presence control loosely ripped off from an HRD, They have a Hammond 270FX, which looks like a 550V PT with ample 5V and 6. Originally I used jacks that use the test leads with banana plugs I had on EL84's can sound a bit harsh when driven hard, maybe more so in fixed bias. Both negative and positive voltages (fixed BIAS +CCS and Drive voltage) are Due to low HT supply I was stuck with fixed bias for the 6V6s. What are the sonic differences? If negligible, why not make them all that way for ease of use? My tech said EL84 amps are "fixed bias" meaning there is no adjustment needed when changing tubes, you can just swap them out although some may run hotter, temperature wise, than others. Does anyone have a link on how to measure bias? Each pair of tubes shares a common (cathode?) 150 Ohm resistor. Just sayin' Chris Hornbeck. Yes, you get a smaller voltage drop with EL84 From what I learned you'd like a voltage rating for the caps that is 1/3 more than the actual voltage there for a 50% extension of life so an EL84 SE is typically 10 volts or less for the cathode bias so a 16 volt would be fine. A cue to performance is the 1KHz square wave shown in pic. I can't seem to find an el84 datasheet with loadlines beyond 300V, most of the amps + all datasheet recommendations I EL84 fixed fixed bias. My question is have you guys built an EL-84 P-P output with fixed bias? My plan is to take unused 5VAC filament winding and rectify with FW bridge to create negative bias for output pair. Either can cause poor sound. 3 V, (at: filament voltage 6. ) So a mild 6V6 at 250V (which is hardly worth fix-bias): 250V/10 = 25 V, the most you could need to stay cold. No. It is full wave bridge rectified with Schottky diodes. "Half on" is about half that. Because they draw differently they need to have that sophisticated switching/cathode cap arrangement to prevent the tubes from Rebiasing fixed bias amps can be an ordeal, but with this simple modification, it becomes much easier and more accurate. For fixed bias, the A-A load is listed as 3. They rival most all of the classic EL84/6BQ5 tubes Bias can ALWAYS be adjustedtry a different grade or brand of tubes! Practicality can sometimes decide whether you adjust the circuit or change the tube. If its fixed bias are you rebiasing it before you compare tubes? Theres a variety of reasons that a The Baby Huey PP ultra linear EL84 is a design by member Gingertube. T. Description. The output is filtered with a 300uf x 200v cap then to a 6. , push the Class A load line up far enough) it will become pure ClassA. 47uf Jantzen Superior Z It doesn't take much to build a good EL84 amp with some proven tweaks. It is a conventional setup where the EL84 anodes are directly connected to the OP transformer while the screen grids run from HT via a 1. EL84 sounds like a Vox; Most of the character ascribed to the tube type is actually originating in the circuitry of the classic amp people associate with it. Adjustable Fixed Bias amplifiers (adjustment potentiometer to set the "fixed" bias point) Cathode Bias amplifiers (also commonly referrred to as self-biasing; Cheers, Billy . You should use a resistor for each pair of output tubes or better yet a resistor on each EL84 for best results. All there is available upstream is 12Volts for all four tubes. Below is the schematic of the EL84 Tube Amps: 15 – 30 watts; Circuit Design: Biasing and Amplifier Classes. Appreciate your opinion. 2 Ω (basically 4 Ω). Can this also be done with a fixed bias amp and how should I connect the zeners ( about 12 volt , I think) ? Can I connect the zeners to ground after the coupling caps and before the 220K bias resistors ? For an inexpensive EL84 option it is hard to beat the Russian 6P14P tubes. Cathode bias amps have more sustain and dynamics but a spongier response particularly with bass notes (more sag). I replaced the cathode bias resistor for a 1R - used a small mains transformer and biased the EL84's manually (fixed ?) at Well, it IS biased at the cathode I guess. I already made the voltage source from reverse installing a 12. . Cheers Long time gone and this one is sorta off the reservation. Ive been modifying an amp that uses P-P EL84. 5V through the 2 x 3R resistors in series with the EL84 heaters, so you must've el84 Bias 9V at grid enough? 07-14-2009, 04:15 AM. Too hot also means shorter life. With a SS rectifier, is 550 too much for the EL84. Joined Mar 26, 2008 Grid bias is a fixed voltage, but the bias supply is often adjustable, Max. Essentially, my EL84 amp IS "fixed bias" for all intents and purposes. They're both heads so I have used them with the same speaker - honestly I've gotten mine pretty close at this point, but I don't feel it's as loud and it's definitely not as smoothhis sags and compresses instantly a very soft but rich and cutting attack. The first (auto bias), which is probably the one newbies should build and a fixed bias version. It's true that cathode bias is self adjusting, for better or worse. Play your amp to warm-up the components. Or certainly you could put EFB on it, too. The results are impressive - 20 W at 1% THD, and sound that compares well I'm afraid I'll chase my tail on this one forever, and I'm ok with that - when I nail it that's like the day my life begins. Low Hi, I've been looking around a little (on the web that is) for a good bias point for el84 triode-strapped SE; currently my amp is running at 300V with 200ohms cathode resistor. The Baby Huey PP ultra linear EL84 is a design by member Gingertube. You wont be able to guess an amps headroom completely by spec alone as many things interact to determine it. Francois G. I have one that has a blown fuse and a dead output tube. More thought; the EL84 (10mA/gm in triode) with such CCS as a load direct AC coupling to the GM70 is´s a sure Bias can ALWAYS be adjustedtry a different grade or brand of tubes! Practicality can sometimes decide whether you adjust the circuit or change the tube. Actually the spec. I've switched tubes around, but the problem persists, and it is even throughout the range of the volume pot. razorrick1293. My latest creation has about 9. (1) to compare the typical cathode bias for EL84 versus fixed bias. dissipation. And crossover distortion due to grid loading becomes very pronounced. Operating in Fixed Bias requires attention to the resistance between the grid terminal and the filament. New posts Search forums. Your #4 example would that also lead to bias excursion of the 6V6s causing a shift to a more negative bias I decided to use two of my Heathkit AA-100 output transformers to build a fixed bias, pentode (not UL) power amp modeled on the Fisher SA-100 amp. To control the current flow you have to make the control grid Suppose you are designing an amplifier that utilizes a 6V6 tube. A long time friend approached me with this idea a while ago and I wasn't nuts about the idea but finally came along. Cara Konstantina 80-86, 18000 Nis, Yugoslavia Hey, I was looking into making a self bias using a CCS for an EL34 amp, but I just found a 24v transformer that I ripped out of an iced tea machine (the fruits of dumpster diving), and I noticed that after being rectified, its just around the right voltage for a fixed bias on an EL34. Normally they are used in a very simple circuit that features no negative feedback and are run in the cathode biased mode which gets close to them being run in class A, which makes for a very squashy Mine is a fixed bias. The pairing is quite magical, and the original Dynaco ultralinear output transformers have the damping factor for the Quads. 5K resistor. This video explains how to convert f I know it's fixed fixed bias and I've read that they come from the factory HOT. So far the only tubes I've had to adjust over time was a quad of KT-77's that I used for a few years. There's no need to have huge 10 Ohm resistors on EL84 cathodes, as they dissipate 10-15 mW at idle and 150-200 mW peak. I have a quad of 6P14P-K (vibration resistant) and have had no issues. A single EL84 puts out 4 to 5 watts, so the speaker needs to be able to handle that (it shouldn't be I'm a noob to Marshall tubeagebeen using fixed bias Boogies and EL84 amps that do not need biasing Well, I'm always going to say Winged C =C= EL34s, especially in a Marshall. I bypassed a series resistor and raised the bias a couple volts but 9 volts seems dangerously low for fixed bias (at grid pins); no? The Peacey C30 has 14 Volts upstream and this Duncan has 12V—but the grid blockers are bigger Fixed bias can be used to bias an output stage to class A, AB or B, but its cost and lack of inherent safe operation traditionally limited its use to cases (deep class AB, class B) where there was no other feasible way to bias the output tubes. ; -10V potential). Of course there are EL84 amps with fixed bias, and some with Dave's EFB. Often it is a switch that can select bias for EL34 tubes, or 6L6 tubes depending which is installed. Joined 2011. This page has the proper tube bias settings for Black Ice Amplifiers such as the EL84, EL34, KT77, 6550, KT88, KT120, KT150, and KT170. Well heck folks, finally got my EL84 6CG7 project to a listenable state. This meant 100K for 6V6 grid to bias supply resistors. However, just as different tubes from different manufacturers will draw varying amounts of current in a fixed-bias amplifier, the same is true of a cathode-biased amplifier. Cathode bias resistors are typically 200 to 270 Ohms for each EL84 (6BQ5). 10V and 754. Fixed Bias A Vox AC4, like many amps, is designed to make The AC4's 8"speaker is rated at 3. It utilises 12 EL84's in total cross the two channels but the method and amount of biasing is not obvious from the physical layout. Would someone explain the difference between cathode bias and fixed bias. Joined Jul 16, 2007 Messages 2,828 Reaction score 1 Location SoCal. (and often more expensive) A lot of people use a large shared cathode cap (~1000uF) to get more of a "fixed bias" sort of feel. JJ tubes and Thorsten's recommendation of fixed bias with secondary winding as EL84 cathode feedback helped substantially but still not where I'd want to build it out permamently. The original Dynakit Stereo 35 6BQ5/EL84 amp modified with Dave Gillespie's Enhanced Fixed Bias (EFB) circuit is my favorite choice, especially for driving a pair of Quad EL57's. The EL84 cathodes are commoned to ground via a single 100R bypassed by 50uF. This with the lower gm of the 6V6 (compared to EL84) suggested that the Baby Huey circuit would not be suitable. The amp I built in 2006 justifies that quite well. The circuit can be made to resemble a neat perfect zener with a good high beta transistor. plate dissipation is the same as the 6BQ5/EL84. None of the GM-70 data sheets give aa maximum value for R(g→k) but that does not mean that high values are OK. Not unusal to see conjunctive filters across the OT primary to counter that, e. The AC 30 used a single cathode bias resistor of 50 Ohms for all 4 EL84s bypassed with 220uF. com El84 amplifier schematic ,what sound difference between resistor chatode bias and LM317 ccs. US$795 including shipping. 25 and 35 volt work but their life is reduced by 50% and using a 10 volt at 10 volts is at the caps rated life. Hello! This is the last circuit I have developped around a push-pull of EL84. Adding a bias pot to the Fender Blues Junior and Pro Junior is becoming a very common modification these days. So long as you don’t mind the extra components needed. Fixed bias EL84 Ian, My fixed bias has been running for almost 2 weeks and I did some comparison with the original baby huey last night. Once this is known, the value of R1, the back-bias resistor, can be easily determined using Ohm's law. And when there is fixed bias, and the max grid resistor is 300k, when you parallel 3 tubes, the max grid resistor is 100k. You might find that a fixed bias EL84 amp does provide greater punch and get an extra clean watt or so out of the otherwise identical amp design. El84L power section with fixed bias. In these calculations, 5% of the cathode current is assumed to be screen current Some amps are cathode bias, some are fixed. There are many tried and true el84 pp amps an this site. Lower anode Re: Cathode Biased EL84 Amp vs Fixed Bias Depends more on the circuit itself than the tube compliment trying to compare 2 amps by bias type is like trying to describe the driving performance of 2 cars only using the kind of tires each has. Hi all, i want to make a class A pp amp with fixed bias. 2 driving voltage measured with fixed bias 2. "Self-biased" usually refers to tubes configured with cathode biasing. Take the Peavey Classic 30 (the three preamp tube model, not sure about the new ones with only two). 5K for B+ 375V, and 6. There are no "bias resistors" in there - that amp has fixed bias. If you are trying to get the maxium power out of a EL84 tube fixed bias, controlled by a 10 turn screwdriver adjustable pot is the way to go. Billm RIP. 50 Caliber EL84 hot rod mods. AFAIK there are 2 major versions. Using the "Tube Bias Calculator," the calculated bias voltage and resistor value required to achieve the desired bias voltage are 32. The most accurate way to scale down an EL34 design is probably to set up EL84s identically (fixed bias) The Soldano Atomic 16 is a nice little beast that uses 2XEL84s with fixed bias at ~-21VDC. Considering this tube was destined in small sized TV sets, it's done well to survive against the EL84/34 stuff. 3v filament line. And even "self biasing" amps should be checked - there IS a bias setting, it's just not as commonly adjusted as with a fixed bias amp. The OPTs arethe excellent James 6123, which I know you've used and liked. Regarding the EL84 output tubes, do I really need to re-bias or can I simply swap out the old EL84's with a new set of EL84's (as long as the new tubes are a matched set)? Thanks for any help. 10W is a reasonable goal but don't expect that with a low b+ and cathode bias. But that alone doesn't dictate class A. . The Mini Block tube amplifiers use a 5751 in the driver position and the EL84 output circuit is biased into Class-A, ultra-linear operation. I agree. 3 V The EL84 is the classic output valve for smaller British style amps (and some U. Hello. Make sure it's unplugged first. Not all fixed bias are adjustable fixed bias amps. There are some differences between true pentodes like the EL84 and beam power tetrodes like the 6L6 and 6V6. MrMarkIII Well-known member.