As far as I'm concerned the jury is still out that the 12AX7 LPS can not tolerate the higher Cathode voltage from a Cathode Follower circuit. The 68K resistor on the cathode is fine for a U7 and a bit lower than most cathode followers with an X7. I think the design "visually works better" when the output from the source is either direct to source or after the first source resistor (R33) - the waveform more closely replicates the wave of a 12AX7 cathode follower. A cathode follower with R k =∞ would have the same gain, and because the lower valve strives to produce exactly the same signal as a standard cathode follower if it saw R k =∞, there is no voltage difference between the two valves, so the upper valve does see R k =∞. That means the gain won't go crazy even with a 12AX7, most likely. Now, a 12AX7-based plate follower will suffer the same fate with an idle current of only 1mA, as the same physics apply. The max voltage allowed between heater and cathode in a 12ax7 is 180v. The 12AU7 has the smallest, at just 20. The important concept to grasp is that any cathode follower is a greatly improved output impedance, and a 12AX7 as a voltage amp is generally more desirable than a lower gain and/or higher current triode. The 12AX7 has a much higher "plate resistance" than the 12AU7. The extra 12v above ground in the original design takes a little of this potential off the way the tube is operated. Fig. With-out negative feedback, this type of cir-cuit has very high gain (around 80) and distortion, and high output impedance. i have heard from various sources that Sovtek 12ax7wa, Sovtek 12ax7wb, Svetlana 12ax7s, and JJs are rugged enough / meet the specs for a 12ax7 Cathode Follower. of the following stages: the 12AY7 preamp, 12AX7 voltage amp, and 12AX7 cathode follower (as in [1]) driving a nominal load resistance R L. The circuit schematic is shown in Figure 1 with component values listed in Table 1. In Shure's original preamp, each audio channel was a series pair of common-cathode amplifiers (because the design needed lots of gain). This was inserted in the boxed area of Figure 2. Usually 12AX7's have a limitation on the heater voltage (heaters are inside the cathodes) and the voltage on the cathode. According to the designer of the schematic he had used a particular value cathode resistor on the el84 to achieve a grid bias of -7.5v although the 12ax7 srpp stage was directly connected to the grid of el84. When a Tung-Sol 12AX7 dies in a cathode follower with 30 volts on the cathode, or in a phase inverter where the cathode might be 80 volts over ground, then it's the tube, not the circuit. Schematic of pre-amp to tone-stack attached (original and modified). Fender re-branded this valve and called it the Groove Tubes 12AX7. A second gain stage consisting of one half of a 12AX7, leading to a 12AX7 cathode follower that drives a tone stack with independent treble and bass controls and local negative feedback. Yet, with no feedback or followers, we can easily make a three-stage preamp (two-stage phone and one-stage line) with bandwidth to several hundred kHz . 12AX7 cathodyne phase inverter. I was looking at a schematic last night which is the same as what I would like to build but it used an el84 as a cathode follower. Remember there is LOTS of BS posted on other forums . One 12AX7 is in a "cascode" circuit instead of using an EF86 as was used in the London. Yes the V2 position on the MAZ is a Cathode Follower which drives the Tone Stack, like most Tweed and Marshall style amps do. The lower gain of the 12AT7 and 12AU7 also allows them to have a higher headroom, which can provide a warmer . The unit functions by taking the signal from a guitar amp's Effect Loop send into the unit's Input. If I simulate this, the Uk of the cathode follower is not what I assumed. Valve Amplifier Audio Amplifier As a solution, I've added a 12AX7 tube and cathode follower circuit to add back the lost gain. Cathode follower circuit puts the cathode at a very high voltage, likely well over 100 volts (I haven't measured it). The tone stack is not included in the simulation as it is buffered from the preceding sections by the cathode follower stage. Cathode follower = classic tones. Usually 12AX7's have a limitation on the heater voltage (heaters are inside the cathodes) and the voltage on the cathode. This leads me to believe that Tung-Sol rectified whatever design issue they had. The following circuit impedance is made up of two 1M volume pots in parallel. This SRPP output circuit can take a large Output voltage compared with a cathode follower output circuit, and has the special feature that are that the distortion property is excellent, an input impedance is high, and an Output load impedance is low. Don't make the mistake of thinking solely in terms of voltage—current is equally important in a buffer's design. When they are set up as cathode followers, the cathodes can see 150 to 200 volts depending on the design. March 5th, 2019 - Nobsound 12AX7 12AU7 Vacuum Tube Preamp HiFi Preamplifier Shigeru WADA Japan Circuit Assembled by Douk . In the part schematic you posted you can see that the 12AX7 cathode follower is running with zero bias (180V on grid and cathode). However, adding this circuit reduces the gain and quality of the amps sound. Two stage 12AU7 preamplifier with low impedance output stage. Building, troubleshooting and testing of these amplifiers should only be performed by someone who is thoroughly familiar with the safety precautions around high voltages. In AB763 amps (with reverb and tremolo), you already have 6 preamp tubes. Free Profiles Download . There is a switch on the 12AX7 cathode with two negative feedback options: one with a treble cut, one without it. Pretty standard 12AX7 gain stage with an AC coupled cathode follower to drive the cable. Without negative feedback, this type of circuit has very high gain (around 80) and distortion, and high output impedance. Lots of overall negative feedback is used to get the gain down to close to 10 or 20, Cathode Followers were introduced in the 50s by Fender in their Tweet Series, those amps had a NFB (negative feedback) loop, but with fairly high on 56kOhms and so they need more signal to drive the power amp circuit - a cathode follower was needed, to prevent the signal drop because of a higher damping of the tonestack. For example, don't expect a 12AX7 based Cathode Follower to drive a 600 ohm load to 10 volt peaks just because it has a 600 ohm output impedance. We start with the DC plate load line. If nothing else will satisfy your curiosity, go ahead and do it. Also, it is non-inverting. It can run at substantial currents meaning you can use a much lower cathode resistor and yet still draw no grid current. The bottom point of the line is the plate . Designing an AC Cathode Follower The first attempt to drive the 12AX7 used a cathode follower: The 6N3P triodes were paralleled, and connected as a cathode follower to drive the transformer. By taking the output from the cathode, it lowers the impedance and is able to drive the signal over longer distances. There was a lot of PWM noise, but I got it down to an acceptable level with the 2 R-C filters. This reduces the volume of the signal by at least 6dB and lowers output impedance to 600Ω. It is a 450V/0.07A device with the type number ZVN0545A. than the 12AX7. The schematic shows a 12AY7 tube but a 12AX7 is a common substitute for higher gain players. This squares with the empirical evidence of listening to it - it sounds the same as when a triode is used as a cathode follower. Recorded and mixed in Backyard Studio. According to the designer of the schematic he had used a particular value cathode resistor on the el84 to achieve a grid bias of -7.5v although the 12ax7 srpp stage was directly connected to the grid of el84. As a solution, I've added a 12AX7 tube and cathode follower circuit to add back the lost gain. May 10, 2017 - A Direct-Coupled 6V6 Tube Amplifier with Cathode Follower by Raymond H. Bates from Radio & Television News magazine November 1949. We'd need to add yet another to include a cathode follower for the two tonestacks. 12AX7 vs 12AT7 vs 12AU7: Differences Explained. But if we want a lot of gain from the previous stage then the ECC83 / 12AX7 is the obvious choice. Ef86-12ax7-12ax7+cathode follower-12ax7+cathode follower-EL84. When a Tung-Sol 12AX7 dies in a cathode follower with 30 volts on the cathode, or in a phase inverter where the cathode might be 80 volts over ground, then it's the tube, not the circuit. Tubes with higher current capability than 12AX7s are usually preferred in cathode follower applications, but as your circuit parallels them it should be ok. The cathode follower has a lower impedance output. Most use a 12AX7, but other tubes will work. Most of the New Sensor Tubes can only handle . 12AU7 (ECC82) Cathode Follower Tube Preamplifier Schematic. Cathode follower 1/2 of a 12AX7 or a 12AU7 can be used as a Cathode follower. The calculator calculates a cathode follower's unloaded voltage gain (the gain without being connected to the next stage) and output impedance based on the triode's characteristics and the cathode resistor value. The biggest difference between these power tubes is the amount of gain they will provide. A 12AU7 can take a much higher amount of plate current than a 12AX7 so by substituting the "12AU7" side, you increase the gain and lower the headroom, and an SVT already has TONS of gain. According to the 12AX7 calculator , for a plate supply voltage of 378V, a 220kΩ plate load resistor, and a 2.2kΩ cathode resistor, the DC grid bias is -1.75V. It is possible for a 12AX7 to have LESS gain than a 12AU7 in certain circuits. Yes the V2 position on the MAZ is a Cathode Follower which drives the Tone Stack, like most Tweed and Marshall style amps do. This circuit is necessary to "sculpt" the tone that you desire while playing a guitar. For more information about our 12AX7 tubes, see our tech corner article on 12AX7s. The MOSFET doesn't even get warm as there is only about 1.7mA of current draw. This circuit works well and sounds superb. Tung-Sol 12AX7's had a great record with very few complaints or returns. You can sub a 12AU7 in the cathode follower buffer spot with no changes to the circuit. The AKG C60 and 61 were cathode follower circuits. The signal at the Cathode has extra current enabling it to drive a lower impedance load without the . Oct 6, 2016 - 12AU7 (ECC82) Cathode Follower Tube Preamplifier Schematic. Nice little boost pedal. This is a common buffer stage design and combines high input and low output resistance. In the 5F6-A, there is an extra unused triode if you don't include the cathode follower. After the profiling process, i can build a new circuit in the housing and start profiling again. We use the 12AX7 triode as the input tube and the 12AU7 as a cathode follower. In such a point of view, the sensitive tube, 12AX7 would not to be the best tube for using as the cathode follower circuit, and so the engineers at Audio Research obviously made a decision to use a 6DJ8 instead of 12AX7 for V3. Plate followers. The overall gain is about 8 times. The overall gain is about 8 times. 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