The max voltage allowed between heater and cathode in a 12ax7 is 180v. One 12AX7 is in a "cascode" circuit instead of using an EF86 as was used in the London. Schematic of pre-amp to tone-stack attached (original and modified). The tone stack is not included in the simulation as it is buffered from the preceding sections by the cathode follower stage. The overall gain is about 8 times. Node A in Figure 3 is between R10 and pin 1 of 12AX7A in Figure 2. A lot of folks like the Tung-Sol or the Mullard RI in the gain slots (V1, V2). Designing an AC Cathode Follower Negative voltage between grid and cathode can also be obtained by connecting the grid to ground voltage and by elevating the cathode voltage. 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 is a common buffer stage design and combines high input and low output resistance. The NFB circuit is connected to the 8 ohm secondary winding of the output transformer, as usual. Usually 12AX7's have a limitation on the heater voltage (heaters are inside the cathodes) and the voltage on the cathode. Nice little boost pedal. By Plate-loading the tone circuit, we will get a lot of extra gain: of course we have just freed up 1/2 tube (V3b) which is available for an added pre . As a solution, I've added a 12AX7 tube and cathode follower circuit to add back the lost gain. Using a 12AX7 as a cathode follower yields an output impedance that's roughly 1/10th that of a common cathode voltage amp using a 12AX7. The Cathode follows the signal on the Grid without amplifying it. 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. The circuit posted is a common cathode (1'st stage) stage feeding a cathode follower (2'd stage). Most of the New Sensor Tubes can only handle . May 10, 2017 - A Direct-Coupled 6V6 Tube Amplifier with Cathode Follower by Raymond H. Bates from Radio & Television News magazine November 1949. A 12at7 will also work fine. 12AX7 of the low noise type selected carefully is adopted as the first rank and the 2nd . The MOSFET doesn't even get warm as there is only about 1.7mA of current draw. This circuit works well and sounds superb. The biggest difference between these power tubes is the amount of gain they will provide. When they are set up as cathode followers, the cathodes can see 150 to 200 volts depending on the design. Tung-Sol 12AX7's had a great record with very few complaints or returns. The Hughes & Kettner Coreblade, for instance, uses a cathode follower circuit to conjure clean tone, but the circuit's role in shaping distorted sounds is limited. The schematic for the original Model 65 preamp and datasheets for the 12AX7 tubes are easy to find online. This Russian tube has strong lows and bright highs. 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 =∞. of the follower is about 700 ohms, so cable capacitance and low input impedance will not result in high frequency losses.-----M4. The 68K resistor on the cathode is fine for a U7 and a bit lower than most cathode followers with an X7. Tubes with higher current capability than 12AX7s are usually preferred in cathode follower applications, but as your circuit parallels them it should be ok. Three Triode Phono amp, RIAA eq with NFB The above phono preamp schematic is virtually as simple as the two triode phono stage with NFB eq but has an additional cathode follower buffer output. You can sub a 12AU7 in the cathode follower buffer spot with no changes to the circuit. There is a switch on the 12AX7 cathode with two negative feedback options: one with a treble cut, one without it. See example schematic. The 12AX7 has a much higher "plate resistance" than the 12AU7. The design has a voltage gain that is slightly less than 1, less than 1k Ohm output with a nice high input resistance. The signal at the Cathode has extra current enabling it to drive a lower impedance load without the . The AKG C60 and 61 were cathode follower circuits. Typically 12AX7's have a heater to cathode limit of 150 to 180 volts. As a solution, I've added a 12AX7 tube and cathode follower circuit to add back the lost gain. The ECC83 / 12AX7 is a weakling in this regard, although for guitar applications it is often 'good enough'. The 12AX7WC also has a spiral wound heater which minimises mains hum coupling into the cathode from the heater in amps where the heater circuit is AC volts. I wouldn't recommend a DC coupled cathode follower for hifi applications But I love them and always use them in in guitar amplifiers. The Sovtek line of tubes including Electro Harmonix, Tung Sol, Mullard, etc cannot handle high voltages on the cathode, that is why those tubes are a no no in cathode follower circuits. Draw the Load Line. The plate follower (a.k.a. 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. 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. There was a lot of PWM noise, but I got it down to an acceptable level with the 2 R-C filters. There was a lot of PWM noise, but I got it down to an acceptable level with the 2 R-C filters. Fig. Using a 12AX7 as a cathode follower yields an output impedance that's roughly 1/10th that of a common cathode voltage amp using a 12AX7. The overall gain is about 8 times. Zetex makes a TO-92 package MOSFET worth some consideration for replacing 12AX7 followers. After the profiling process, i can build a new circuit in the housing and start profiling again. Despite its detractors, the 12AX7 is actually intrinsically very linear, even more so than the 6SN7 in the test I did. Figure 3 shows a schematic view of the cathode follower. ). You might want to consider designing a cathode follower circuit. Remember there is LOTS of BS posted on other forums . In AB763 amps (with reverb and tremolo), you already have 6 preamp tubes. 12AX7 cathodyne phase inverter. The LTSpice file contains the 12AX7 model and should run with the standard library installed. The cathodyne operates with 50% internal feedback, so it is extremely linear before clipping, much like a cathode follower. Amplifiers Tubes / Valves 12AU7 Cathode Follower Preamp markw51 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. The schematic shows a 12AY7 tube but a 12AX7 is a common substitute for higher gain players. A 6SN7 is an octal tube that requires a different tube socket. One can convert the cathode follower stage into a high impedance plate driven gain stage that uses only 1 triode i.e. Free Profiles Download . The cathode follower circuit does not drive the transformer any harder, it has less gain then the plate 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. If nothing else will satisfy your curiosity, go ahead and do it. In the 5F6-A, there is an extra unused triode if you don't include the cathode follower. But if we want a lot of gain from the previous stage then the ECC83 / 12AX7 is the obvious choice. T2_12AX7_tube_preamp_with_cathode_follower.png T2_12AX7_tube_preamp_with_cathode_follower.asc Oct 6, 2016 - 12AU7 (ECC82) Cathode Follower Tube Preamplifier Schematic. The 12ax7 triodes are connected in parallel for more gain and slightly lower noise. The lower gain of the 12AT7 and 12AU7 also allows them to have a higher headroom, which can provide a warmer . Don't make the mistake of thinking solely in terms of voltage—current is equally important in a buffer's design. Yes the V2 position on the MAZ is a Cathode Follower which drives the Tone Stack, like most Tweed and Marshall style amps do. 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. 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. The reasonable wideband preamps using 12AX7 employed feedback (Dynaco, ARC, early CJ) or intermediate cathode followers (later CJ) to increase the bandwidth to the minimum acceptable for hi-fi. In the part schematic you posted you can see that the 12AX7 cathode follower is running with zero bias (180V on grid and cathode). 180V across 56K means that the tube is running 3.2mA, that is a hell of a lot for a 12AX7 and to get that current the bias is down to zero. 10.2.3: Intermediate amplifier with cathode-follower; family of output-characteristics of the 12AY7, U The problem would seem to be the "maximum heater relative to Cathode, negative Voltage" in the case of the 12AX7, this is rated at, in many peoples eyes optomistic, 180Volts. In Shure's original preamp, each audio channel was a series pair of common-cathode amplifiers (because the design needed lots of gain). 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. A "classic" design that's typical of 1960s hi-fi equipment uses a high gain triode such as a 12AX7 as a grounded cathode amplifier, coupled to another triode acting as a cathode follower. Now, a 12AX7-based plate follower will suffer the same fate with an idle current of only 1mA, as the same physics apply. Pin 1 of the 12AX7 in Figure 3 is attached to Z in Figure 2. I try to simulate a simple gain stage-> Cathode follower using a 12ax7. 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 . Cheers Ian It's like a lot of things: it's not the use, but the mis-use, that causes detractors. The old Shure preamp used two 12AX7 dual-triode tubes (one for each stereo channel). However, the input to the lower valve must be inverted, requiring an external phase splitter. By taking the output from the cathode, it lowers the impedance and is able to drive the signal over longer distances. Because the AC cathode follower is normally used only as a transparent buffer for driving difficult loads, it makes sense to use a medium- to high -current valve that can provide plenty of drive. If I simulate this, the Uk of the cathode follower is not what I assumed. 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. March 5th, 2019 - Nobsound 12AX7 12AU7 Vacuum Tube Preamp HiFi Preamplifier Shigeru WADA Japan Circuit Assembled by Douk . anode follower), like the common-cathode amplifier, is a much-too-overlooked circuit. Cathode follower circuit puts the cathode at a very high voltage, likely well over 100 volts (I haven't measured it). So the circuit for a 12AU7 will typically have much lower resistances in it. Specifically, in a 12AX7 tube used as a "cathode follower", the voltage difference between that present at the cathode, as compared with the heater voltage, can be withstood or tolerated by certain types of tubes, whereas other tubes will fail. triode such as a 12AX7 as a grounded cathode amplifier, coupled to another triode acting as a cathode follower. All music composed and performed by Jan Honkys. The cathode follower has a lower impedance output. The lower "Lead" channel has a 2.7k cathode resistor and 220k plate load. 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. Stone Pony - 25 or 50 Watt (2x7591), tube rectified, EF86-12AX7 cathode follower. The ECC99-based driver stage will develop a good amount of Miller-effect capacitance, which the 12AX7's meager 1mA of idle current cannot cope with, but which the 5mA of idle current from the 12AU7 can. Its lows demand attention when playing full bar chords. This circuit works well and sounds superb. L Lee boatright 3 followers More information 12AU7 / ECC82 Cathode Follower Tube Preamplifier Schematic Find this Pin and more on cool by Lee boatright. The MOSFET doesn't even get warm as there is only about 1.7mA of current draw. because i have only two enclousures. 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. Two stage 12AU7 preamplifier with low impedance output stage. 9 pin miniature preamp tube (Amplification Factor = 100). This was inserted in the boxed area of Figure 2. Gain 8,6. . This circuit is necessary to "sculpt" the tone that you desire while playing a guitar. This circuit is necessary to "sculpt" the tone that you desire while playing a guitar. 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. Like any triode, the 12AX7 operates with more linearity the larger the plate. The SVT's preamp uses the 12DW7 (half 12AX7, half 12AU7) with the "12AX7" side as a gain stage and the "12AU7" side as a cathode follower. Input is a healthy 200Vpp. But Peavey, Orange and many others use a cathodyne Phase inverter circuit that puts the high voltages on the cathode. Most use a 12AX7, but other tubes will work. The price paid is power dissipation, lots of it. Without negative feedback, this type of circuit has very high gain (around 80) and distortion, and high output impedance. . We'd need to add yet another to include a cathode follower for the two tonestacks. 10.2.3). My point is that the RCA and Sylvania tube manual specify close to 300 volts of cathode/heater differential, which it's doubtful any guitar amp exceeds. Usually 12AX7's have a limitation on the heater voltage (heaters are inside the cathodes) and the voltage on the cathode. The cathode voltage is elevated by connecting it to ground through the resistor Rk, generally called the cathode resistor, as shown in Figure 12. Why? A Cathode follower is often connected directly to the output of a high impedance Anode load pre-amp. When choosing a load we are really only concerned with output signal swing. The extra 12v above ground in the original design takes a little of this potential off the way the tube is operated. With-out negative feedback, this type of cir-cuit has very high gain (around 80) and distortion, and high output impedance. Ef86-12ax7-12ax7+cathode follower-12ax7+cathode follower-EL84. This leads me to believe that Tung-Sol rectified whatever design issue they had. The Bassman cathode follower has according to the schematic, 215 Vollts on the cathode, minus your 6.3 heater, still leaves you in excess. 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. The failure of a "cathode follower" tube will cause sound dropouts or signal loss.
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