The bypassed resistor in a common cathode can rise the even harmonics to higher levels. And sanded it down and refinished it! For the 12AX7, the limit is 10mA, but it would not be sensible to operate the valve at much above 3mA. cathode-to-ground impedance () New! Thank you. This tool demonstrates a step by step calculation of the amplification factor of the basic triode voltage amplifier and plots the amplitude and phase vs. frequency. In this case it is used as a "treble enhancer"; at high frequency the cap tends to be byapssed (its impedance going lower) so the resistor of 150 is in (almost) parallel with 1k5 and the gain increase, I suppose that what I don't quite get is that the lower value cap is reducing the amount of bass frequencies available to all the subsequent preamp triodes. plate, grid and cathode. Preamp tubes are cathode-biased (well, after the '50's anyway) but a big jump in plate resistance will put the tube out of bias range. Tube selection is CRITICAL in these stages. Arrow points to the insertion index notch. This must be nailed down first. (3) Now, the bias-point is selected based on the balance desired between headroom/input range and non-linearity/harmonic distortion. One of the cathode follower's virtues is its low output impedance, so it seems a shame to raise it by adding series resistance. 11K would be way too high, and the tube would be in cutoff. I would think that would give you a greater palette to work with. Alternatively, if your schematic shows (say) a 1k cathode resistor and you know you have 1.6mA tube current, then the bias voltage would be: 1,000 x 0.0016 = 1.6V In the plate characteristics chart above, we can see that at an idle voltage of 175V, the load line shows 0.83mA tube current (where the 1.5V grid curves crosses the loadline). Trust me: a 12AX7-based cathode follower sounds dreadful driving a 600-ohm load beyond a few tenths of a voltand I mean only one or two tenths of a volt, not three or four tenths. 3. There is typically about 1.5pF between socket pins, making a total or 3.1pF for both Cga and Cgk. A circuit designed for a 12AX7 does not provide the ideal operating parameters for a 12AU7 even though the 12AU7 will work in the 12AX7 circuit. We'll call this variable x. I do not see a problem with putting a 10k pot in series with a 470R. Now there is an exception, the 5751 was designed to have a mu of 70, and a plate resistance identical to a AX, the 5751 tube was specifically designed to be a lower-gain, plug-in replacement for the 12AX7. Know it works by measuring performance at every point in the amplifier. It has relatively high Miller capacitance, making it unsuitable for radio-frequency use. Location: Australia. I plotted the 2.7k cathode resistor voltage gain and it drops from 58 with 1.5k cathode resistor to 52 with the 2.7k resistor. I am using the schematic from a Gallien Kruger bass amp to make a preamp for an amp I'm building. Here's a chart showing how to calculate internal plate resistance on the plate characteristics chart: Here's how transconductance is related to the characteristics chart. You are here: naval museum pensacola; samsung mu6500 refresh rate; 12ax7 gain stage calculator; January 20, 2022 . Low cathode resistor means low bias voltage which means greater risk of grid current on signal peaks. It may not display this or other websites correctly. Yet schematic assigns fender's 1.5k to both, with 10uf cathode bypassing. I'm just using the 2nd and 3rd valves with midrange treble and bass controls. . Using the average anode characteristics graphs, we see that a grid voltage of -1.5V, with respect to the cathode, is needed to obtain a current of 0,75 mA. "Cold clipping." From "woody" to narrow panel to brownface to blackface Reverb, an in-depth exploration of Fender Deluxe circuitry. In this role it is widely used for the preamplifier (input and mid-level) stages of audio amplifiers. Transconductance expresses the relationship between voltage change on the grid to the change in plate current. You must log in or register to reply here. The cathode resistor sets the bias for the valve, as long as you're under the maximum current for the valve you should be fine. Specifically, in a 12AX7 tube used as a "cathode follower", the voltage difference. (1) proper bias and setup is first dictated by intended use of the circuit. Yes, that would make the clipping level stable rather than depending on what happened in the previous few seconds. ( or you can measure the voltage across the cathode resistor) you can calculate cathode current using the cathode resistor value. Using the numbers from the chart above we divide 250V supply by .00246A = 101.6k ohm plate load which is very close to our actual plate load made up of the 100k plate load resistor and 1.5k cathode resistor. What 820ohms does is reduce impedance? The tighter spacing of the grid voltage curves at the bottom of the chart incorporate the changing internal plate resistance into the graph. Triode Calc. So what if the operating point changes and you get different distortion characteristics. the voltage gain of the stage changes very little for the rather large change in cathode resistor. Sorted by: 2. The calculator plots gain versus frequency based on the characteristics of the tube, resistor values, and the capacitor value. All the other triodes have standard 1.5K bypassed with 22uf capacitors. An advantage of using 12at7 or 12au7 is that the preamp hasn't that much gain anymore You have to turn your vol. anode follower), like the common-cathode amplifier, is a much-too-overlooked circuit. The defaults are typical for a 12AX7 You can type in the boxes or drag the sliders (1) mu, from datasheet (2) Total effective plate load resistance, k ohms Toggle navigation Amp Books . Rather, what is created is a shelving filter, with two wide flat gain regions, and a small rise of a few dB between those two plateaus. Our Books; Amplifier Calculators; Tube Calculators; Tutorials; back You already know your cathode voltage, as above. between that present at the cathode, as compared with the heater voltage, can be. . To calculate the amplification factor, standard values for the parameters "S", "mu" and "Ri" are often used. Life is a daring adventure or nothing at all! From system design concepts to individual stage operation, It may not display this or other websites correctly. What happens if it isn't? The human brain is an interesting thing. It can do both. In this example let's pretend you got 20V at the cathode. Well, technically, there is no gain increase in parallelled triodes, if you scale the plate and cathode resistors to account for the resulting change in rp. The K2-W used two 12AX7 dual triodes, with one of the . Eddie and my zebrawood V in Dave's basement '77. Now, a 12AX7-based plate follower will suffer the same fate with an idle current of only 1mA, as the same physics apply. If needed, the purpose of a cathode-stopper resistor is to buffer a capacitive load from the (slightly inductive) output impedance of the cathode follower. It is getting clearer. This is another indicator that gain is falling (because rp is going up). Then turn off the amp and measure the actual resistance across the bias resistor. New! Triode Tube valve amp Gain Calculator. Sodo we know who were the ones that Dumble refused to build amp for ? This calculator uses the power equation P = I V P is the power rating of the power tube, I is the bias current, and V is the plate voltage of the power tube. Well how bout, thank you for giving a clearer definition. To steal from many is research. We see below that moving from -1 volt on the grid to -2 volts results in plate current going from 1.39ma to .16ma or 1.23ma per volt. Total Cathode Current =. All valves have a maximum plate dissipation too, and again, exceed this at your peril. Triode is one order simpler, give it a shot and see how you like it. However, there is also some capacitance between the pins of the valve socket and associated wiring, which adds to the capacitances inside the valve. This corresponds to a bias current of 0.75 mA. That current minus the SG current will be the plate current _____ To be a man, Be a non-conformist . Use 1 ma when figuring a power supply load for 12AX7. I experimented with a pentode using pots in the cathode, plate and screen positions to see the effects of changing each. pot up means less resistance to the grid and more brilliant tone In this 4th video in the Resistor Video Series, we discuss the purpose and function of the Cathode Bias Resistor. Here's how the 12AX7's mu fits into this graph. Effects of resistor values on a v1 cathode/cap and controlling gain, Re: Effects of resistor values on a v1 cathode/cap and controlling gain, http://www.valvewizard.co.uk/gainstage.html, Go here and read this post first, then post your problems in the forum - Click here, Quote from: jukelemon on October 25, 2013, 09:50:18 pm, Quote from: jukelemon on October 26, 2013, 08:24:00 am, Quote from: jjasilli on October 25, 2013, 09:30:59 pm, Quote from: Willabe on October 26, 2013, 08:29:18 am, Quote from: sluckey on October 26, 2013, 08:45:04 am, Quote from: Willabe on October 26, 2013, 09:02:44 am, Quote from: jukelemon on October 25, 2013, 08:56:46 pm, Quote from: jojokeo on October 26, 2013, 01:09:03 pm, Quote from: tubenit on October 26, 2013, 10:12:26 am, Quote from: LHPcope on October 27, 2013, 09:18:28 am, Quote from: HotBluePlates on October 26, 2013, 09:46:01 am, Quote from: printer2 on October 28, 2013, 08:08:59 pm, Quote from: jjasilli on October 26, 2013, 07:59:35 pm. LED Current Limiting Resistor Calculator; LED Parallel/Series Calculator; Operational Amplifier (Op-Amp) Tester; . (2) Next, appropriate B+ voltages and loads are chosen, to fix the slope and position of the load line. Some guitar amps use the opposite approach for the same result. Cathode Resistor: ohms. Typical currents are in the range of 0.5-2mA, although operation down to a few microamps is perfectly ok for some applications. The correct way to do this is to use the required cathode voltage divided by the desired cathode current to get the resistor value. You are using an out of date browser. The operating point is set by the cathode resistor and appears at a particular point on the loadline. This calculator assumes the conductor is round. Well, splitting hairs here, but a grid resistor or any dc path from grid to ground is the path for grid leak current, therefore it is by definition a grid leak path. A custom 12AX7 switching box was constructed to allow for a quick switch comparison of six tubes at a time while plugged into the V1 socket of a Marshall JCM800 2203 guitar amplifier. My question is about the cathode bypass on the second 12ax7 , which has a 0.22uf capacitor in series with a 150R resistor bypassing a 1.5k resistor. From Dave Funk's Tube Amp Workbook: The K resistor is the main gain setting component: may range fro 820R to 50K; lower values and the tube won't operate. There are also other differences than just the gain, or mu, between a 12ax7 and the rest of the 12a_7 family of tubes, like plate resistance. I plotted the 2.7k cathode resistor voltage gain and it drops from 58 with 1.5k cathode resistor to 55 with the 2.7k resistor. Number of Tubes that share a cathode resistor: Voltage Across Cathode Resistor: volts. Above 50K is unity gain. Hence on low power the cathode resistor value is doubled - the effect I notice is that it has . The voltage gain of the stage can be determined with this chart using the yellow AC load line vs a 1 volt grid voltage change as seen below. 10,2,38,23,38,23,21 Resistance of a Conductor Use the following to calculate the resistance of a conductor. Draw the Cathode Load Line Cathode Load Line - Stock generic AB763 preamp values .1-.047-100k-250pf - 12AX7 - 100k plates sitting at about 250v +/- - cathode resistor values are 1.5k (effective) on V1a-V1b. Recommended configurations for the 12AX7 are below: Since the cathode resistor is bypassed for audio frequencies by the 100uF capacitors, gain will be set by the transconductance of the tube under the operating conditions, as shown in the above diagram. It seems to be "common internet wisdom" that decreasing a preamp tube's cathode resistor will warm the bias and lead to, Anode current is not determined by 100k anode load, but mostly by transconductance characteristics (transfer characteristics). This could cause a bias shift in a coupling capacitor. Click any Calculate button to see the recommended tube bias currents in milliamps. 0 x Octal Tube Socket Numbers Plate is pin 3 and Cathode is pin 8. It considers both reactive loads and resistive load. This technique is often used for the preamp's bright channel. Or maybe the book I have does. The first dings in a new nitro guitar are painful, Tinnitus is ruining my life. Resistors in series calculator Provide all of the resistance values in series separated by a comma "," and click the "Calculate" button to determine total resistance. It's a distorted sound that simply sounds good to someone's hearing. Two triode circuits sharing one cathode resistor will pull twice the current through it so you have to cut the resistance in half to get the same voltage drop across the resistor. The plate follower (a.k.a. You must log in or register to reply here. The Gear Page is the leading online community and marketplace for guitars, amps, pedals, effects and associated gear. It does not account for coupling capacitor bass attenuation . Used Plate to Cathode Voltage = volts Change Plate to Cathode Voltage. If you have a 12AX7 stage with each triode going to a different channel, and then remove one of the channels so only one triode is being used, should you then change the value of the cathode resistor on this tube in order to keep the used triode running in the same manner as before? 12ax7a cathode bias resistor variance discussion 12-26-2015, 02:17 PM By mere accident discovered 1st issued vc3112 during 1994 installed 820ohms halfwatt on both stages channel one. For a better experience, please enable JavaScript in your browser before proceeding. withstood or tolerated by certain types of tubes, whereas other tubes will fail. 0 Members and 1 Guest are viewing this topic. The 12AX7 is a high-gain (typical amplification factor 100), low-plate-current triode best suited for low-level audio voltage amplification. In real grid leak bias, the resistor can be much larger than 1 meg. BUT, if you just parallel, say, a 12AX7 into a circuit without changing any of the component values, then it's the same as if you double the plate and cathode resistors for a single triode. For a better experience, please enable JavaScript in your browser before proceeding. For a better experience, please enable JavaScript in your browser before proceeding. Hi value cathode resistor to bias cold enough to reach cutoff clipping. A separate . Remember, a bypassed cathode resistor of a half-12AX7 does not create a typical high-pass filter response (with bass rolled off more and more as you go lower and lower down in frequency). 12ax7 gain stage calculator. please help me, Down in this low-current region, there are, Big changes of internal plate resistance correlate with. Solution: Effective plate resistance = 260k paralleled by 220k, paralleled by 470k, or about 95k. Let's say it's 475 for this example. You can: choose among most common vacuum tubes (I am slowly adding more), Suppose we use a 12AX7 vacuum tube and we want to set the operating point at the red spot in Figure 8, using the green loadline. Some of the add-ons on this site are powered by, Amps/Cabs Tech Corner: Amplifier, Cab & Speakers. - 22uf cathode bypass caps What would happen tonally and theoretically if I jumped that cathode resistor value up to 2.7k or 3.3k. I have used very low cathode resistor values, down to ~380 ohms on a 12AX7 preamp tube-second stage, with what I think are very pleasing distortion/gain results. You will need to look up the data for the particular tube your are using. You are using an out of date browser. My last post assumed the cathode resistor was bypassed. From "woody" to narrow panel to brownface to blackface Reverb, an in-depth exploration of Fender Deluxe circuitry. The secondary effect that we most often look at is this same resistors loading on signal source, be it a guitar . But I have read that much lower than 800 is bad and will 'hiccup'. There's a reason we see 12AT7s used for reverb drivers and PIs many times. We see below that moving from -1 volt on the grid to -2 volts results in plate current going from 1.39ma to .16ma or 1.23ma per volt. It might depend on what you are driving it with. Coupling Capacitor (C) Load . Try replacing a the K resistor with a 50K pot, in series with a 820R fixed resistor (to maintain bias) and you'll have a practical answer to your question. on high power it has a 62ohm cathode resistor and on low power adds another 68 ohms to cathode resistor value. Related Ohms Law Calculator Effective Cathode resistance = 333 (1/gm) + 1600 or about 1933 ohms. You must log in or register to reply here. In many ways it is the inversion of a cathode follower: where the cathode follower takes its . A signal wave too high above the operating point will saturate; too far below and it will clip. First, select 6V6GT and measure you plate *and cathode* voltages. Whether there will be distortion (and how much) can greatly depend on input signal size relative to the operating point. The tighter spacing of the grid voltage curves at the bottom of the chart incorporate the changing internal plate resistance into the graph. A deep dive into the electronic design of the ultimate high-gain rig-of-death! 12AX7 draws little plate current, usually around 1 ma. Stage gain = 49.1. working on another patent idea jukelemon? Here's how the 12AX7's mu fits into this graph: Note also that the spacing between grid voltage curves narrows at lower plate current. It may not display this or other websites correctly. This calculation is based on Langford Smith's formula from Radiatron Designers Handbook 4. .. You are using the plate voltage and plate current to calculate a cathode resistor value. Please enter Tube Type and Plate-to-Cathode Voltage above before calculating Tube Dissipation. 1 Answer. JavaScript is disabled. . This is not entirely correct since these values depend on the DC bias. For 9-Pin preamp tubes like the 12AX7 the plates are pins 1 and 6 and the cathodes are pins 3 and 8 (measure between pins 1 and 3 for A; 6 and 8 for B). Differential amplifier (phase splitter etc) made from a 12AX7. For example, 12AX7-based cathode followers that are expected to drive 600-ohm loads, because of the 600-ohm output impedance the 12AX7 produces in a cathode follower. The. Your same 1v change of grid voltage results in a shorter span across the plate voltage axis when those grid curves are closely-spaced. Design your amp using a structured, professional methodology. You are using an out of date browser. From system design concepts to individual stage operation, an all-new examination of Bassman electronics. Excellent graphic and post HotBluePlates, thank you. pot up to get the same volume as before (you all know that) Here's the point --> turning the vol. Thanks! After discussion of the theory of its operation, we will take measurements from. The dc-30 schematic has high low power switch. Plate followers. Note that if the cathode resistor were bypassed, the stage gain would be about 285 (95000/333). TUBE VALUE ; 12AT7 12AU7 12AY7 12AX7 6SN7 6SL7 5751 : Cathode resistor (Rk) Anode resistor (Ra) Cathode capacitor (Ck) Gain: Gain dB: Half Gain freq : Anode impedance : Roll Off Frequency. JavaScript is disabled. Master the basics of preamp, power amp, and power supply design. This calculator allows you to trace the anode characteristics graph, and to compute and tune loadline and operating point of various vacuum tubes. 1.5k cathode resistor = 250v / (100k + 1.5k) = 2.46ma 2.7k cathode resistor = 250v / (100k + 2.7k) = 2.43ma These values form the left end of the red plate load line in the chart below so the slope of the line (and therefore gain) barely change. The ECC83 / 12AX7 quotes Cga and Cgk as 1.6pF each (depending on which data sheet you read). No risk of grid current issues and still get asymmetric clipping. The voltage drop across the cathode resistor puts the cathode at a positive voltage compared to the control grid (normally around +1.7 volts DC). Thank you. I revised the chart but can't post it because this website is caching the chart jpg. Enter plate minus cathode in the box, and hit calculate. A deep dive into the electronic design of the ultimate high-gain rig-of-death! Thanks HotBluePlates, I re-ran the transconductance chart and came up with 1410 micromhos.
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