Cathode bias. And it should be around 1/40 of capacity in mA.

Cathode bias. Jan 7, 2014 #2 ; soulman969 Telefied.

Cathode bias Other amps, instead, put a negative charge on the power tube's grid. What would be a ballpark value for the cathode resistor? The other common type of bias circuit is known as fixed bias, and will not apply to the 5E3. However, when we designed the output stage, we considered the effects on bias after recovery from distortion. Cathode bias requires the correct supply voltage -- usually rather low, in order to maintain bias. Thirdly, unless you're stuck with 12AX7 for the voltage amp and splitter, you could do better IMHO. Oct 28, 2023 · Yes, if his amp is cathode-biased, it should have a fixed cathode-tied resistor --- to read voltage drop across. The cathode voltage can also be stabilized by means of a very large cathode cap and/or zeners. If you used a paraphrase inverter (instead on an LTP), you could get quite a funky sounding blues amp. A 100 ohm linear pot will work fine too. I want to bias my push-pull circuit at 6V. We offer entire commercial audio systems and commercial installation guidance. Have you heard of ‘biasing’ but perhaps are a little confused about what bias means and in particular what cathode bias means? Don’t worry – you’re not alone! Biasing is a highly confusing subject for the owners of valve guitar amplifiers. The power xformer can handle up to 5A of heater current May 20, 2008 · So if I go for (say) 1/2'n'half fixed/cathode bias, I would have a 250R resistor on each tube (or 120R for both tubes in parallel) and then use the bias adjustment pot and take measurements of the cathode voltage divided by the resistance to get the tube current. This substantially increases gain. Cafe Sound by Cathode Bias speakers. And, your output transformers will love it. I know I wouldn't need the input caps to the grid but in the grid leak version the grid one is grounded. And you can measure the quiescent current of your circuit by measuring the voltage at the cathode (between cathode and ground), and dividing by the total cathode Anyway, I'd like to change the grid leak bias front end of a couple amps to cathode bias. Although it may be fine as-is, it's still best to check. In DC terms, the result is that the two 10-ohm cathode resistors effectively double in value, acting as if a single 40-ohm cathode resistor were used to cathode-bias the triode. Nov 5, 2023 Practically all preamp tube circuits use cathode bias. It's easy to fix the design mistake and the amp doesn't sound bad when the bias is corrected. 6g3 Deluxe and 6g2 Princeton. Capacitor C6 is a cathode bypass cap that helps decrease local feedback and increase V2's gain. If you are committed to using cathode bias, then it would be best to stick to a class A design so that the effects of the cathode bias resistor can be effectively discounted. Note that each KT66 draws 1. At this grooving swing the second tube "run out" anode current, so it became necessary to grooving current too (within certain limits due to the DC coupling of first stage). Cathode Bias was born from vintage HiFi to bring a new experience to US Listening Bars with our Cafe Sound speakers and sound systems Instagram In 2023, I decided to get more serious about my audio hobby and incorporated Cathode Bias. Fixed bias amp are more usually biased cooler in class A/B. We will disc The cathode resistor in a typical triode preamp is bypassed with a large capacitor to eliminate a form of negative feedback known as cathode degeneration. V2's bias is set by R8. They're talking about rms watts, I believe, not tube dissipation watts. Most cathode biased amps are either deeply toward Class A, or completely to Class A. Many older amplifier designs used cathode bias with rheostats or ceramic power resistors with sliding wiper arrangements. Describing both sound and feel is, of course, a very personal thing, but here are a few common terms used to describe the differences. All Tweeds through the 1954 schematics were cathode-biased. This raises the cathode voltage and lowers the bias current leading to crossover distortion. With a "fixed bias" amp - which is actually not fixed in the same sense a cathode biased amp is - the grid current is controlling bias of the tubes, so because that current fluctuates considerably, you need more head room. To see for yourself, consider increasing the value of the cathode resistor in your AC30, eg to 82 Apr 2, 2018 · Hello. Oct 6, 2014 · The higher load impedance coupled with the presence of the resistor is why cathode biased designs always produce less power output than a corresponding fixed bias design does. I've biased amps for years, but know little about cathode bias, and/or what, if any, difference a hotter tube would make, or if they could possibly be too hot. On Fender amps it often connects to 2-220K resistors. But don't chase your tail on obtaining the exact sound of your friend's amp. Right now I have 388v on the 300b anode, 24v on the cathode with a 407 ohm cathode resistor bypassed with a 100uf polypropylene cap. Since I deal with mostly guitar amps, I like to use stompboxes to boost the front end, and have heard this is dangerous with grid leak bias. Being cathode biased, she lifts up her dress earlier with less effort and gives up the candy « Mar 29, 2017 · I built a Trainwreck with a cathode biased output stage like Dave H's. Jan 7, 2014 · Just for clarification, I do mean Cathode Biased Power Tubes . The difference is usually to make the grid more negative than the cathode. There are a lot of factors that determine the sound. In electronics, cathode bias (also known as self-bias, or automatic bias) is a technique used with vacuum tubes to make the direct current (dc) cathode voltage positive in relation to the negative side of the plate voltage supply by an amount equal to the magnitude of the desired grid bias voltage. (EL34's max out at 800VDC, so 375 is rather low. I’m having an issue checking the dissipation on the EL34s in cathode bias mode. 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 Tube Dissipation Using Cathode Resistor Voltage Drop Jun 20, 2024 · For cathode bias the grid is grounded. The cathode is positive relative to ground because of the cathode resistor, thus the cathode is positive relative to the grid. Oct 14, 2015 · I have never seen a single zener diode used to self (cathode) bias an amplifier. a zener) for cathode bias, rather than a CCS, may constrain PP stage distortion better. But note that at 800 pate volts, only 400 for the screens. 5 mA. Most of the larger amplifiers tend to be fixed bias. This is what happens when we apply the input signal: Jan 5, 2018 · For a cathode biased amp, it's quite common to bias them on the hot side. Started with lower value cathode bypass caps but ended up where Dave did at 220uF. Any of the fixed bias setups can be run using cathode bias instead. I also installed a switch for fixed or cathode bias. Jul 14, 2020 · The Origin 20 is cathode bias, and the Origin 50 is fixed (adjustable) bias. Amp is a 6v6 Plexi, AO43 chassis, Hammond original PT, 630v and a 30 watt OT. All good fortune, Chris Apr 2, 2012 · Therefore, if cathode bias is used, the bias voltage across the bias resistor will start to rise and fluctuate significantly as well. The swinging im talking about is basically when loud tone is played from a frequency gen it pulls the bias voltage up. Ce sont ici des composants situés au niveau de l’élément du tube qu’on appelle « cathode » qui assurent le bon fonctionnement de ce dernier. I suggest you leave things as they Feb 23, 2020 · Specifically then, it is the increase in cathode voltage in cathode biased designs, produced by the increase in average current draw of Class AB output stages at high levels of power output, that the Zeners act to clamp. To keep the cathode bias voltage as stable as possible -- very necessary to achieve the lowest possible distortion -- a capacitor is placed across the resistor to minimize the fluctuations. Mar 17, 2010 · I had the idea of a medium power combo amp that could be played out (occasionally), or at home based on a 6V6 output section. Do yourself a small favor and insert a small amount of fixed resistance say 1 - 10 ohms to make it easy to With the bias network changed to the 195R/100uf, I am dissipating 23. That's called fixed bias, and it has a similar effect. 😀) Works fine. Some people say using an LED for cathode bias makes the tube 'sound more solid-state. For reference, it is a 2x EL84 amp (a first for me); the schematic shows a 150 ohm cathode resistor. I had insufficient HT, so I had to look at combination bias. The conventional cathode bias choice for the lower valve would be a 430 Ω resistor bypassed by an appropriately sized capacitor. To measure this you will need a 1 ohm 1% 3 watt resistor that goes in series to ground from the cathode resistor . Feb 1, 2023 · Plate Voltage is low in the 250-ish range, and the bias reading was a crazy cold 45-50% dissipation. 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 One more thing: some fixed/non-adjustable bias amps have a "bias switch" to select for a specific tube type. Are these drawing grid current with the 47kOhm load of an MM cart? What about 1kOhm for a high-output MC cart? Checking the bias current in a cathode biased amplifier is easy, just measure the voltage across the cathode resistor and divide by the resistance value to obtain the cathode current. Jan 7, 2014 #2 ; soulman969 Telefied. 5 watts on those EL34's now at about 95%. These are still fixed/non-adjustable bias amps as there is no internal bias trimmer pot to fine tune. Cathode bias often lends a natural compression or 'squishiness' to the sound, due to the increase in bias voltage when one valve enters Class B conditions, though the larger the bypass capacitor, the less will be this effect. over a wide range of grid bias voltage (or in the case of grounded grid configuration, cathode bias voltage) as determined by specific performance requirements such as gain, linearity and efficiency. Dec 8, 2016 · There is an exception to all of this - cathode biased amps. Just erase the trem sections, assuming you don't want trem, ground the junction of the output grid leaks and use cathode So because you have the combination fixed bias/cathode bias you may be ok to run them at that current level. The 5F1's V1 and V2 use "common cathode biasing", also referred to as "self biasing" or just "cathode biased. May 3, 2021 · In practice, many folks will measure from anode to signal ground, and again across the cathode bias resistor, and just subtract. Rectifier Switch In a 5E3 Deluxe Open switch = tube rectification, closed switch = solid state rectification. LTPI vs Cathodine needs to be decided and it can be either. You lose control over biasing, there may be the odd pair of tubes that fall out of range that is ideal for the cathode bias and might benefit from manually setting the bias. 3A filament Apr 21, 2023 · My PSET 300B amp currently use electrolytics to shunt AC in the cathode bias circuits (both for the driver and 300Bs). With filament bias the grid is at ground potential (DC wise). This seems normal for cathode biased amplifiers and basically thats what grid bias is for. This is the 'musical power' kind of thing. Instead of connecting the center terminal to ground as shown, connecting it to a cathode biased amp's cathode resistor will elevate the center tap and reduce hum. Our primary objective as a company was, and is, to serve the growing vinyl and HiFi listening room market in the US. Same if it is a cathode biased amp and you measured across cathode resistor. 450 Volts hand-to-hand is no joke - some folks even die. Once you know your bias resistor's measured resistance (it won't change) you just measure plate voltage and cathode voltage to ground. In the article where I introduce Enhanced Fixed Bias (EFB™) operation for the Dynaco SCA35 & ST35 amplifiers (“A New Look At an Old Friend”), I show how any stereo amplifier with a common power supply and common cathode bias resistor between the two channels actually causes each channel to operate under conditions of regulated B+ with Jul 6, 2018 · The desirable quality of LED cathode bias is that you get the desired voltage drop with very little internal resistance. In electronics, cathode bias (also known as self-bias, or automatic bias) is a technique used with vacuum tubes to make the direct current (dc) cathode voltage positive in relation to the negative side of the plate voltage supply by an amount equal to the magnitude of the desired grid bias voltage. Sep 23, 2023 · One of the critical factors influencing the sound of a tube amp is the biasing method used, with cathode bias and fixed bias being two common approaches. And it should be around 1/40 of capacity in mA. Cathode bias, the increased current increases bias and tends to cut the tubes off. Fixed bias: The DC grid potential is determined by connection of the grid to an appropriate impedance that will pass DC from an appropriate voltage source. I frankly am confused by it,other than when you do the math it's not bad. Sep 25, 2008 · "cathode", "automatic" and "self" bias are all synonyms, meaning the cathode is lifted from GND by some amount, using the cathode current to create a drop across a fixed resistor, thus making the control grid negative with respect to the cathode without the use of an external negative power supply ( hence the names "self" "cathode" and Oct 13, 2024 · I usually change the resistor and re-bias the amp into the realm of reality. Having selected our operating point at 85V, we know anode, and hence, cathode current. The resistor value is determined based on design averages for plate voltage and current. Changing from fixed to cathode bias solves all the bias issues for the amp and it sounds fantastic. I’ve looked at the data sheet for EL34s and pin 1 is a grid3 but needs to be tied to the cathode, pin 8, which it is. We believed, and continue to believe, that growing Cathode Bias would require growing and promoting the industry as well. Late in that year, the 5E5A PRo went to fixed-bias also. The normal channel has too much bass response for my taste. Not only does this give better rejection of HT noise, but the 1M resistor is bootstrapped, making it appear much larger, giving a higher input impedance, which can sometimes be useful. It allows the Vette engine (see above) to draw what it needs to run right and discard the rest. This dual triode is not in my area of expertise so I'm not clear on how to go about the change to cathode bias. 5V heater supply doesn't influence the bias voltage? Nov 23, 2015 · In this 4th video in the Resistor Video Series, we discuss the purpose and function of the Cathode Bias Resistor. Cathode biased amps are often bias very hot and near Class A (tube amp rarely actually meet the standard for being true class a). Our first installation is slated in CALCULATE PLATE DISSIPATION (In Cathode Biased Output Stage) Based on Voltage and Resistance Readings. It should be tied to cathode it is correct to say. Prior to the current version of the IV all Blues Jr’s were fixed bias and run hot if not modified. This amp have a fixed bias power stage. One cathode biased amp I was recently worked on was designed for 90%, other amps are higher. The 'Q' current of each pentode is 6 mA (12 mA combined), and when either side is at peak signal the current will be 65 mA. Do you need to bias 6L6 tubes? Yes, if you are using an amplifier designed with a class A or AB power section, then you may need to bias your tubes. 350V to ca. Ad Free Member. Bias for a tube is the difference in voltage potential between the grid and cathode of a tube. Cathode bias amps tend to have the idle dissipation watts set higher. 4V from ground, then my job is done? The +2. And depending on how long each side cuts off, you may have to switch to fixed bias (cathode bias gets shaky when tubes are cut off a large percentage of the signal cycle). I just finished restoring a maggie 9302 6BQ5 amp, which I have set up using a 47uF cap and a 100 ohm/5W resistor as a common bias for all 4 output tubes, much like the OEM set up, but using a 47 uF cap instead of a 20 uF Instead, you'd take advantage of the higher voltage by using a lower load and class AB operation to get more power. TTBOMK, this is the primary difference between the two (I can confirm this after studying the PCBs of each), and must be how Marshall arrived at those output ratings. Also crappy vinyl DJ * @cafesoundsystems * @renolisteningroom" A quieter approach is to add a small cathode bias resistor in series with the main cathode load, then take a 1M grid-leak from there up to the grid. After discussion of the theory of its oper If your amp is cathode biased or fixed biased with a separate power transformer bias supply wire then this mod will work without any modifications. listening-rooms. -Is the cathode biased version exclusive to certain geographies (depending on wall socket voltage)? Is there fixed biased IV still being made? Aug 31, 2009 · I've been trying to find some info on switching cathode bias schemes. Deduct that from your bias probe reading. This placed the cathode at above ground potential and with the grid being returned to ground it was therefore negative with respect to the cathode. " V1A's bias voltage is set by cathode resistor R4 which is connected to V1A's cathode (pin 3). I haven't Feb 14, 2012 · If the cathode resistor in fact measures 1500 ohms, then the two tubes are only drawing about 5. In this article, we’ll explore the differences between cathode bias and fixed bias and how each method impacts the tone of a tube amplifier. To achieve that, my idea is: Reduce the value of the cathode resistor to 10 Ohm (Instead of 100 Ohm). Put a 130Ω to 150Ω high wattage resistor between the cathodes and ground and bypass the resistor with a big cap. There are several ways to do this. Fixed bias allows for correct bias at higher plate voltages. So either you run a lower supply voltage, or a higher impedance. I was wondering if I may be missing something. Cathode biasing is also called self-bias where the way the circuit is set up BASICALLY allows the tube to draw what current it needs. Amps with an adjustable (referred to as fixed) bias system, input a bias voltage on the input grid of the tube to regulate the current. Tubes like 12AX7 use cathode bias and do not need bias adjustment. Nov 28, 2017 · Le « cathode bias » ou polarisation automatique : de par son montage, le tube est polarisé automatiquement. Nov 24, 2022 · With cathode biased amps the most common way of setting the bias is adjusting the value of the cathode resistor. Although a CVS regulator (ie. I have been tinkering with the normal channel today. find the voltage supply that applies a negative voltage to the grid(s). Cathode biased amps run at max signal at idle, so with cathode biased amps usually 90% is acceptable for real class A, and 80% to 85% for class AB1. Oct 30, 2008 · Then a measure that is seldom seen: Place a large capacitor (these days 6,8 - 10mF is not that bulky) across the cathode bias resistor, and you will get mostly fixed bias performance from cathode bias because of the large time constant of the cathode bias network. May 28, 2023 · Cafe Sound is a new product and services line by Cathode Bias created to address the needs of the growing commercial listening room market with products with high integrity at an effective cost point. If that capacitor is stiffened to the point where the cathode bias isn't chasing the DC grid modulation then that charm is lost. 4W would allow cathode voltage to reach nearly 80V at 200mA, although the buffer resistor would be dissipating 11W by then. In my mind, a 6V 1 Watt zener should work as shown. Feb 3, 2021 · Hello. Oct 29, 2023 · A higher plate voltage on a cathode-biased tube will also increase bias voltage, keeping dissipation nearly the same. Dec 28, 2023 · The autobias is currently 20mA, with a 100 Ohm cathode resistor, which corresponds to -2V between grid an cathode. Minor variation, an output tranny with 3K4 Raa but the same B+ (well +380V). So for a cathode bias amp, the negative "grid" bias voltage is really the cathode's positive voltage. Jan 3, 2021 · You can indirectly measure it though by measuring the voltage drop across the cathode bias resistor. 250 ohm linear pot is used to balance the voltage on the two heater wires for minimum hum. And Madison's estimated 440V ain't very different from the Clubman running 430V. Essentially Those two Matchless amps are the only examples of cathode biased EL34 amps that I've seen. Jun 23, 2024 · The constant current generator will adjust the DC bias potential between Grid and Cathode in such a way as to keep the DC bias Anode to Cathode current at exactly 8. Have folks experimented with using film capacitors of similar value in this application, or adding a small film cap in parallel with the electrolytic? What affect does this have Jul 9, 2014 · Some also have Mid control. 'course, I ain't gonna be changing tubes every 3-6 months either. The clean to crunch ratio should be just about perfect for gigging. Since it affects gm1 more than plate gm, it also lowers the Mu and the effective gm1 of the tube. Selmerdave AK Subscriber. Often it is a switch that can select bias for EL34 tubes, or 6L6 tubes depending which is installed. If your amp has fixed bias without a bias pot (like the original 5F6A Bassman) you must either install a bias pot or replace bias resistors to adjust the bias. This is why you do not see bias adjustment on single-ended amps like SET audio amps or Fender Champ. 0: 348: January 11, 2024 Reno Listening Room build-out. Thus, no biasing is required when replacing tubes — but using a matched set of replacement output tubes is, once again, highly recommended for obvious tonal reasons. There are other factors too, such as the grid return resistor to ground but I'd leave that one alone. Dec 20, 2024 · When the battery is connected in cathode, for automatic grid bias, battery has some amout of trickle charging when device is working. ) I suspect that at 500+ plate volts, good cathode bias may be a problem. Recommended Bias Point: Cathode Biased (no re-biasing necessary) KT 45 V1: NOS EF86, V2: Sovtek 12AX7LPS, V3/4: JJ E34L (matched pair), V5 Solid State plug in or Sovtek 5AR4 Nov 9, 2010 · The Tweed PRinceton is single-ended and cathode biased as well. See full list on robrobinette. Low power mode might be Class A cathode bias. Even if the current through the 620K input resistor shown above was enough to shift the Grid DC level by 1 or 2 Volts positive or negative the constant current generator will Cathode biased for a touch responsive playing experience. On a cathode-biased amp, the value of the cathode resistor (stock 5E3, this is the 250Ω 5W from the 6V6 cathodes to ground, with a bypass cap across it in parallel) sets the bias point. Subscriber. The idea was often to maximize power. Jan 27, 2015 · Going by the GEC KT66 data sheet, a cathode-biased non-UL version could be designed running ~450 on the plates and ~300 on the screens with a 2 separate 500R cathode resistors running into an 8kPP OT. Jul 28, 2008 · Known around here as cathode bias as well. No negative feedback means there's no feedback loop from the output of the stage back to the beginning of the stage at the entry to the phase inverter, something used to tighten up and clean up many other types of amplifiers, most notably many Class AB designs, but If your amp has fixed bias and you'd like to try cathode bias. [2] [4] Cathode bias (self-bias, automatic bias) - The voltage drop across a resistor in series with the cathode is utilized. Mar 11, 2012 · Filament bias is meant as a another biasing scheme besides fixed bias and cathode bias. . On the other hand, cathode bias shows a more smooth amp - biased on the hot side ~ 100mA's per tube @ 430V on plates - with none of the agressive character of fixed mode. However, I'm wondering if it might be a good idea to make the cathode bias adjustable? 560 ohms is the planned cathode resistor, but what about changing that to, say, a 500 ohm resistor and 100 ohm pot in series? That would put the pot at roughly midpoint. Reply Delete Nov 20, 2008 · If you use a bias probe (that inserts a cathode resistor) you are seeing your plate and screen current together. With multiple tubes sharing the same cathode resistor, this calculator assumes the tubes are fairly well matched. Cathode bias not only automatically sets bias Mar 28, 2009 · WIth the amp I could adjust teh bias so no problem,, With a cathode biased amp like the 5e3, the 250 ohm resistor might fit some brands of 6v6 tubes where other would require a 330 ohm resistor to be biased correctly. Cathode Bias In later AC sets cathode bias was commonly used with the current flowing through the cathode circuit being used to supply a voltage drop that could be used to bias the valve. With huge cathode capacitors and a hi-fi mentality about clipping, it may be totally fine. I'm building this amp with cathode bias for a friend who wants lower output. When the capacitor is large enough, it acts as a short circuit for audio frequencies, eliminating the negative feedback, but is an open circuit for DC Sep 22, 2020 · Use individual self bias. Jun 22, 2012 · For a cathode bias g3 will stay together with cathode at the bias potential as time the bias is developed over cathode resistor. Fixed Bias A Vox AC4, like many amps, is designed to make the power tube's cathode slightly positive—a state that is commonly referred to in the guitar universe as cathode biased. Oct 4, 2008 · In fixed biased amps 70% is usually considered the max bias at idle. Cathode Bias In the absence of a cathode resistor, with both the grid and the cathode seeing the same voltage, the triode's r p offers the only controlling opposition to the flow of current through the triode. 5W dissipation. May 5, 2024 · Part of the charm of cathode bias comes from the squishiness of the bypass capacitor. In these calculations, 5% of the cathode current is assumed to be screen current. Combinations of bias methods may be used on the same tube. Hi everybody, The amp I have been working on lately is this cathode biased plexi with 6V6's. The better the current balance, the less chance you have of early core saturation. (adjustable tap vitreous enamel power resistors. Feb 5, 2008 · Secondly, EL84s behave so well (in both triode and UL) using cathode bias that I doubt you'll do any better with fixed bias, though it won't do any harm to try if that's what you want. Dec 6, 2016 · Bias: probably grid biased in 50 watt mode. Cathode-biased output tube tone can be “round” if no bypass capacitor is used, or it can be “crisp” if a cap is used – sounding quite similar to a fixed-bias output stage. Since I don't want any capacitors on the signal path, my only possibility to apply the bias is to elevate the cathode. May 16, 2023 · Overall, bias is just "a voltage between cathode and grid" that keeps the grid more-negative than the cathode to tame plate current. Between them and the cathode bypass cap, the cathode voltage is clamped pretty well. It is really only guessing what the circuit actually consists of. Feb 27, 2019 · Mostly there is no reason to manually bias as the cathode bias will take care of this for you. Feb 12, 2006 · I have a Ceriatone Aunt Peg which is a clone of one channel of an Ampeg B15, with a switch to select either fixed or cathode bias. 500 Ohm for each cathode, and 47uF to bypass each cathode. Note that if the output tubes share a common cathode resistor, you must divide the current reading by the number of tubes sharing the resistor. Jun 7, 2020 · What would be the difference between using a voltage regulator for cathode bias and a voltage reference for cathode bias? Aren't these both ways of creating fixed bias operation? SY's Red Light District EL84 amp uses lots of red LEDs in the cathodes of the push-pull EL84s, for fixed bias operation. measure the voltage at this point (say it's -32V) measure the current the tubes are drawing at idle using one of the ways to measure bias (say it's 44mA per Nov 30, 2009 · The diode in the cathode circuit adds in a clean 3/2 power resistance (adds in series to both gm1 and plate gm), which dilutes the 4/2 power g1 mistracking back closer to 3/2 power. So if you compare corrected fixed bias to cathode bias, the cathode bias actually is hotter. Mar 2, 2012 · I've just been doing some 300b biasing. com Negative voltage between grid and cathode can also be obtained by connecting the grid to ground voltage and by elevating the cathode voltage. Additionally, single-ended power amplifiers are mostly cathode biased also. That means they let a lot of cathode current flow compared to Class AB biased amps. Because of the modulation described above, the cathode voltage is not constant. 5V in the first stage of a phono preamp, using 12AX7 or 6DJ8. Cathode Bias (Self Adjusting) Jul 18, 2020 · OP killed the auto bias, so 300B plate-cathode voltage grooving from ca. 6 ma each which is much too low. You have to redo your math. You can always check the voltage across the 500 Ohm resistors, when the tubes are first put in, and later as they age. In other words, if I'm aiming for a -12. The 6L6 amps went to fixed biased in '55 with the exception of the 5E5 PRo. I would like to try some tube rolling - specifically EL34s. I would not be afraid to copy that schematic. It is a 5C5 circuit from 1953. But worrying that much about it is a bit like wondering how many of the cylinders you should disengage in your Ferrari so the engine will last a little longer. 4V above the grid. DC elevated heaters virtually eliminates heater hum, pushing the boundaries of silence. > pick one up that would give me around 400 Volts Aug 8, 2006 · Your arrangement seems like a reasonable compromise, although you might like to try using a higher value cathode resistor, to reduce the plate current of each tube to 4mA, and increasing the plate load resistors to 33k. Re: Cathode Biased 5881 am I in the Ball Park? « Reply #6 on: April 05, 2013, 09:24:39 pm » The screen voltage and all of your other measurements sound fine. Both of those terms are highly confusing Checking the bias current in a cathode biased amplifier is easy, just measure the voltage across the cathode resistor and divide by the resistance value to obtain the cathode current. Jun 5, 2024 · I believe the amplifier is cathode bias for several reasons. This technique is generally referred as cathode bias or self-bias. It's like a cathode resistor with a large value bypass capacitor in one easy to use part, which lights up to tell you it's working. Essentially, the cathode resistor sets the bias of the output tubes by dropping a small amount of voltage. Doesn't sound too impossible, but I'd like a little guidance before I torch something up unnecessarily. The 2 x 6V6 Tweed amps were cathode baised except for the 5G9 Tremolux, which was fixed. 410-415V, so also needed grooving negative bias voltage. (In triode anode and cathode currents are equal). The P–C difference goes in the top box, C voltage is also the 'voltage drop' (to ground), and you're done. I want to change it to cathode bias but I can't find any layouts that use cathode bias with a 6SL7. 192mA. The discharge via some leakage element is minimal, and when working battery is on slow trickle charge. You may want to look at the Brown era. Jan 31, 2024 · In fact, for cathode bias I like the 'voltage drop' method. Weber states, fixed bias and cathode bias sound different and also feel different, too. Cathode bias amps are often favoured by guitarists because of their sustain and overdrive characteristics. Running hot may be part of the magic. When we refer to an amplifier being “cathode biased,” it means that the output stage of the amplifier is biased according to the voltage drop across a resistor connected to the cathode element of the power tubes. 60mA of current, so 364v a-k and 21. Sep 25, 2006 · However, with cathode-biased amps, the surges in bias voltage across the cathode temporarily bias the tube colder while cranking, and help keep things under a degree of control. V1B's bias is set by R6. This gives you the bias voltage too, and lets you keep one hand in your pocket - especially important when getting started. Cathode-bias, the idle current and power consumption *must* be nearly as high as the full-roar power consumption. Jan 12, 2021 · 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. Jan 28, 2018 · The build thread ran over on amps and cabs at the end of last year. Cathode bias resistors 270 Ohms rather than 300 Ohms. The reason is because during higher drive each tube is cut off for part of the time so raising the average current through the cathode resistor. If it uses a fixed bias system, then the tubes will need to be biased with every change. Oct 24, 2013 · To keep from cathode biasing this amp with its high plate voltage into to class A so you do not burn up the tubes I would start with a 1000 ohm cathode resistor and then see how much cathode current your drawing. Sep 25, 2017 · In a cathode bias system you have a fixed, cathode resistor that establishes the operating point of the output tubes. Cathode bias is also known as self bias and is very forgiving. In most common guitar amp circuits, cathode biased amps will self regulate plate current, in many cases plate Nov 4, 2017 · In my forthcoming build cathode bias is the plan. That should be right around 4V as expected if your circuit is pulling 5. Measure your voltage drop across your screen resistor to calculate screen current. Changes in the bias voltage will vary the conduction angle (that being the portion of the 360° cycle of varying anode voltage during Aug 10, 2005 · JC Verdier: 400V on OPT, assuming 10volts less on anode, same 750R cathode resistor, about 60V & 80mA Va=390-60=330V Again, on curves, Va=330V/Vg=-60V, more than 120mA !!! There're several other design examples show higher than 400V on their anodes, 750R~1K cathode resistors for self-bias. The power transformer is 315-0-315 with diode rectification which should give a B+ in the neighborhood of 375VDC. With PRR's plan, most of the "voltage between cathode and grid" is developed by the 180-220Ω cathode resistor (value due to the note about increasing the value). After about a year of reading virtually every related article on this forum, I decided on an Express pre-section mated to a cathode biased output section roughly equivalent to a Deluxe, hence the Express/Deluxe. The 270V condition has a nice side-effect: the ~~150mA total cathode current is just about what a 12AX7's heater needs. The Jan 11, 2021 · You may be confusing the 18watt designation. In AC terms, the cathode is loaded by a constant-current source and whatever external load impedance is driven times two. The two sorts of bias are cathode bias and fixed bias. Anyhow its purpose it is not to stay at particular voltage as time all tube voltage operation is relative to cathode. Oct 16, 2016 · A 24V zener that could dissipate 2. I have reduced the cathode bypass cap from 220uf all the way down to 1uf but there is still too much bass. Jan 28, 2009 · 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. 4: 449: November 25, 2023 Jul 19, 2009 · I've seen cathode bias as low as- 0. Joined Mar 20, 2011 Posts 22,582 Location Sep 21, 2018 · In this video we'll take a look at how to use Ohm's Law to calculate the bias on a cathode biased Fender Pro Amp. While I have built quite a few amps, most have been fixed bias with an adjustable bias pot, not cathode biased like this little monster. Aug 31, 2012 · Here the cathode is biased at 1. Feb 28, 2023 · 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 – make the cathode more positive than the grid (grid stays at 0 volts, called cathode bias or auto bias) – make the grid more negative than the cathode (cathode stays at 0 volts, called fixed bias) This predefined voltage difference between the grid and cathode is called the bias. Best regards Thomas. The most widespread method to bias a cathode is to insert a cathode bias resistor between the cathode and ground, and connect the grid of the tube to the ground via a grid leak resistor. If you beat it anyway (such as e-guitar) the bias-shift does wonky things to the sound. Cathode bias is typically 90-100% rated current if it's done right. Fixed bias is associated with more powerful, more efficient ampifiers and is arguably the better choice for hi-fi use. Feb 10, 2010 · Cathode bias amps generally will produce less power with the same power tubes. Cathode Bias with a Constant Current Source A few years ago, I received many e-mails asking for help on how tube-based, push-pull amplifiers that used a constant-current source at the output tube's connected cathodes worked. 5V bias on the DHT (grid to cathode), and if I put two zener diodes in series to elevate the cathode +12. Jun 25, 2018 · Cathode biased means the output tubes are biased by a large power resistor that's permanently connected to the tubes' cathodes. I have seen a neat adjustable cathode biased system that used a 350 ohm resistor together with a 100 ohm 5W pot. but hard to tell without schematics. If your amplifier is cathode biased you must replace the cathode resistor to change the bias level. The cathode (filament) is raised to a positive voltage through the voltage drop caused by filament current through the filament bias resistor. The input signal is 2Vp-p. The clue here is that the bias voltage is much too hi for the B+ present. Modern players have the advantage of an extremely wide “acceptable” tone range to experiment within. May 11, 2019 · The biasing is really generous cathode resistor is 470ohms and the capacitors across that is a 50uF 50V electrolitic. It comes stock with 6L6GC power tubes. Apr 30, 2021 · Cathode bias is often referred to as “self-biasing,” as the design ensures that the tubes — wait for it — bias themselves. There is a 100uF cap on the board and a large heat-sink resistor underneath against the top plate where the "4" is on top and they appear to be connected to the tube the same way as cathode biased schematics I have looked at. Jul 9, 2022 · As Mr. 12K Followers, 2,296 Following, 2,437 Posts - Cathode Bias (@cathodebias) on Instagram: "I collect HiFi and build vintage style audio systems. Apr 19, 2010 · Doubling the cathode bias voltage quadruples the cathode resistor heat generated. Fixed bias: Firmer, punchier, crisper, and tighter, with a good level of clean headroom. Its a trade off. Just tie pin1 to pin 8 for both tubes and connect the 2 tube cathodes together. I was curious if a hotter pair would bump that to 60-70%, or stay the same. If your amplifier is cathode biased, then it will take care of the setting itself. The current which the output tubes need to flow is dependent on the operating class. ' Fender used an OT with a 6K primary for several cathode-biased 2x6L6 tweed amps with B+ in the range you mentioned, with a 250R cathode resistor. I used a 5w cement block wire wound Mar 18, 2024 · Ver IV: fixed bias: cream or black PCB Ver IV: cathode bias: black PCB-Why didn't Fender announce the cathode biased circuit revision? It's a major change that should have been labeled version V. 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. Apr 21, 2012 · cathode bias is the most tube friendly way to go. If DC (meaning in phase) modulating of grid voltage on a cathode biased amp was a good idea Leo would have done it. Jul 15, 2013 · Cathode vs. you can also push the bias to 100% that's anode plus screen rating put a 1 ohm r in the cathode circuit to measure ma over, just double check what you rigged as a screens supply and keep the screens some odd volts negative to the plates Mar 2, 2008 · Checking the bias current in a cathode biased amplifier is easy, just measure the voltage across the cathode resistor and divide by the resistance value to obtain the cathode current. 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. How many tubes share the same cathode resistor? Jul 27, 2022 · Fixed-bias, the cathode current can rise with signal to supply a low load impedance. ClassicTone and Mojo both sell clones of this transformer, and it's a great little unit that has 2-bolt mounting (3-9/16" between holes), so it fits on Doug's new, highly flexible Stout chassis. jhxmosa echaju syffem ikwxikky upmww yuxoas pvoss hfnifs gbydfavkc rgif