12ax7 Cathode Resistor

5k with a 220k plate resistor. The voltage measured at the plate will be lower. The cathode resistor can be bypassed to reduce or eliminate AC negative feedback and thereby increase gain; maximum gain is about 60 times. Now, place a resistor in series with the plate. Now the grid will be a 0v, so the C1 coupling capacitor is not needed. 5K cathode resistor on the gain stage with a. 5 volt curve and place a dot where the plate load line intersects. The bid is for 10 russian tubes. Description- Vintage Allied model 935 tube amplifier with Japanese Matsushita tubes. 0 550 tap volts amp. Sometimes, depending on the batch of tubes, the tension on the septar socket springs and the heat generated in the amplifier, there can be malfunctions of sorts. I bypassed the cathode resistor with a 100-µF/250-V high-quality audio electrolytic capacitor. This calculation is based on Langford Smith's formula from Radiatron Designers Handbook 4. The negative feedback is introduced in this stage, but we will ignore this until after we understand the power amplifier better. RCA and Sylvania first introduced the 12AX7 to the world in the late 1940s. Put a 2k 1/2 watt resistor from cathode to ground. The heater and the inside of the cathode shield tube form a capacitor, and hence ac voltage variation between portions of the heater and cathode will cause a capacitive current to flow. The additional output section octane allows the feral power of the 5879 preamp tubes that are at the heart of the Cherry Bomb to fully reveal their explosive performance. 85mA = 1176 ohm. 12AX7 – Wikipedia It has relatively high Miller capacitance, making it unsuitable for radio-frequency use. In order to implement this method in your amplifier, you’ll need to install a 1 ohm, 1% resistor between ground and the cathode of each of your output tubes, as illustrated in the diagram below. But the 12AT7 and 12AU7 are rated for a max of around 1/2 that. diytubes) submitted 2 years ago by frosty1 So, I'm working on step 1 of my plan outlined here to improve the sound of my Blackheart Killer Ant amplifier (2 x 12ax7 tubes, 1/4W, Class A) by replacing the 12ax7 "power" tube (V2) with a 12au7 for more clean headroom. prevent the likelihood of oscillations at HF when the cathode of the follower becomes virtually directly connected to the input tube cathode at HF, via the capacitors in the NFB network. The DC voltage on the output is restored by a chain of Zener diodes and the DC operating point is controlled by adjusting the cathode resistor of the. The output voltage is the raw B+ minus the voltage dropped across the pass tube. I've also tried this circuit a but found I prefer the cathode bias of the 5F2A later tweeds in a 12AX7 pre-amp. I'm wondering if I can drop the gain on 1/2 of a 12AX7 to get it to around the gain of a 12AY7 (having, let's say, 100K plate resistor, 820 Cathode R ) by messing with cathode resistor alone? I'm in a situation where I have one socket and I'd like to have 1/2 12AX7 in one half and a "simulated" 12AY7 in the other half (i. (I know Ohm's law, can find datasheets on the web, and can read a schematic; I just need to know what information I need to look at and what formula I need to use. This doesn't mean that the stock cathode follower cannot be modified or improved. The load resistor becomes about 12k5, the cathode resistor is 830 ohms, the bias voltage is -5. Instead, a 1 Ohm resistor is. the CCS; this signal is reversed in phase with respect to the amplified signal. 5mA = 200k ohm. 5 to 3 uF bypass cap, and a 120K resistor for the CF's cathode resistor. Ik: Cathode current/Plate current; measured through a precision 1 ohm 10W resistor in series with the cathode. If I change that to a 10K resistor and keep the 1UF cap, how might that effect the sound? Am I correct that the higher the resistor value the "colder" the bias? The only other thing I've noted is that a cathode cap, the higher the value more "thicker" it'll sound - why is that?. 1 day ago · The module will come preset to Synergy's preferred setting which for the SAVAGE module is position 1 (1. Boutique Amp Goodies: Three 22uF Samwha Power Filter Caps*, Four. The SRPP is coupled directly to a grounded cathode 1/2 12AX7, the voltage on the cathode of this stage is raised by Zener diodes. A single common cathode 12AX7 gain stage is designed that could be used as the front-end stage of a tube power amplifier or a tube guitar amp. and Vishay/Dale RN60 or RN55 for cathode and grid. bone SCT-800 tube mic, tha basically is another Apex 460 variation, but with a different PCB layout and – more important - the 2nd half of the tube is not in use, the cathode follower stage is omitted. 6sc7 is a dual triode octal socket with a common cathode. From the resistor junction now in the ear solder a wire. The 12ax7 is a miniature high-mu twin triode, each section of which has an individual cathode connection. ‘Short out’ the parallel combination of the 470K resistor and the. It was released for public sale under the 12AX7 identifier on September 15, A center-tapped heater permits operation of the tube from either a 6. Calculate Plate Dissipation In Cathode Biased Output Stage Based On Voltage And Resistance Readings In these calculations, 5% of the cathode current is assumed to be screen current. ALWAYs check the bias and idle power of the tube with the new rectifier to be certain that it is running within spec. After that, the 12AX7 starts producing serious amounts of distortion. The effects of the various capacitors on low‑frequency response combine: If the input coupling capacitor causes a 3‑dB. The voltage measured at the plate will be lower. At idle current of 0. Guitar & Studio Help with re-biasing an atypical DC-coupled cathode follower (self. The nice part about this was I have 2 modern Tung Sol 12AX7 tubes that hummed under any normal arrangement. Distributed star 0V distribution. 5 to 3 uF bypass cap, and a 120K resistor for the CF's cathode resistor. 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. At pin 2 use a 470 micro-Henry choke and a 30 to 60 pF bypass cap to stop RF from getting into the audio input. Reverb on both channels. 5pF, which is likely to be similar to 12AX7. I had genuinely screwed up the order, and wasn’t going to order 2 components online, so I ran over to Radio Shack during the week for a missing resistor, and a capacitor for the tone-stack. changing plate and cathode resistor values. The gain of this stage is approximately the ratio of the anode resistor to the cathode resistor, which is 100/1. So the cathode voltage would be about 35 volts RMS, or 50 volts peak, or 100 volts peak to peak. The difference in modern 12AX7's sound would probably swamp out the sound of the substitution. The Verb s ground reference resistors; R62 and R63 are 47ohms. • New Tung-Sol 12AX7 in V1 • New JJ ECC83 in V2 • Groove Tubes 12AX7 in V3 • I modded the preamp to JCM 800 specs, except for the V1 plate resistor which is 220k. 02 caps and 56k slope resistor). The gain of this stage is approximately the ratio of the anode resistor to the cathode resistor, which is 100/1. I don't see how this could have even worked! Plus, there was a 12au7 instead of a 12ax7 in the driver cathode follower stage. Cathode Bypassed, plate resistor 100k, cathode bias resistor 1. The second important revision in the RH84 schematic is the replacement of the cathode resistor (automatic bias) with a CCS (actually, current sink) fixing the current draw of the tube. And I'm sorry you lost sleep over this. The cathode resistor at the ground end is a few volts negative to the cathode. Fixing the current draw of the tube with a simple circuit (LM317 + current setting resistor) allows a constant current draw regardless of the tube inserted. 6SL7 SOCKET LUG #3. In this schematic, the AC source couples through C1 and superimposes AC voltage across L1. Those are D. Servicing information and voltage and resistance data included in this manual are intended for use by the knowledgeable and experienced technician only. The output tranny built for me by heyboer has the correct 3. (314) 599-3636. So a typical 12AX7 likes a bias of 1. This impedance is then in parallel with the actual cathode resistor, so if you used an 820 ohm cathode resistor, the actual cathode resistance would be 1609||820 = 543 ohms. Now best practice would indicate to fix the blown cathode resistor and test the valve, the testing of the valve usually being the hard part. Which would approximate the correnct combined cathode / plate current. Cathode biased power tubes will have a resistor and bypass capacitor connected in parallel from their cathode to ground, as seen with most preamp 12AX7 triodes. It-11 Audio / Tonegeek. Use a bare piece of wire, or move the. Because the value of the cathode resistor is so small it will not make much difference. Too little bias current and it’s turned up automatically. This calculation is based on Langford Smith's formula from Radiatron Designers Handbook 4. The first stage cathode should not have a bypass capacitor. If the cathode resistor is unbypassed, negative feedback is introduced and each half of a 12AX7 provides a typical voltage gain of about 30; the amplification factor is basically twice the maximum stage gain, as the plate impedance must be matched. The 12AX7 is basically two of the triodes from a 6AV6 – a double diode triode. Just reduce the PI tail resistor, to around 22k, like Alexo suggested, and increase the power valves' cathode resistor to about 250 or 270 ohms. At the moment you have the cathode going to ground. The cathode bypass capacitor C59A smooths out the variations and provides a steady bias for the tube. I'm trying to get my head around the way a cathode follower (CF) works. The first preamp gain stages on each channel of a JTM45 have a shared cathode resistor, but serarate plate resistors. Note that there isn't much difference between the two, but one thing I did on the 12AX7 circuit was to pre-compensate the gains so that they matched between the low-pass and high-pass sections. 12ax7 Tube Datasheet - GuitarKitBuilder. Should drive Grado's with no problem what so ever. 5, not 100!. Directly-heated tubes are. The 12AX7 is basically two of the triodes from a 6AV6 - a double diode triode. Ik = Ek / Rk cathode current equals cathode voltage divided by cathode resistor value (Remember to convert milliamps (ma) to amps (A) by dividing ma by 1000. Another tube you can try out is the JAN/GE 5751. All models are small, portable, and lightweight. I simply soldered in a lower value resistor. Grid Resistors - Why Are They Used? General. The cathode bypass cap can be 22 or 47 uF, 10-16VDC, I generally use those common values. The larger the common cathode resistor, the better the balance. The 100 ohm resistor in the cathode does not contribute any significant amount of bias voltage, it is there so negative feedback can be applied from the output transformer secondary. The reason is that the V1 and V2 tubes share the cathode cap and resistor (25 uF/1500 ohm) and pulling one of the tubes will changes the effective value of the resistor they both share. I’m looking to get some more headroom from my amp, so I was looking at changing the cathode resistor as per mod 3 from this. The load resistor becomes about 12k5, the cathode resistor is 830 ohms, the bias voltage is -5. The meaning of the -1 term is as follows. In a cathode biased output tube things are different. On the (AC sweep) simulation, it will be a voltage gain of ~36dB. The static plate. The 12AX7 was originally intended as replacement for the 6SL7 family of dual-triode 12zx7 tubes for audio applications. lation breakdown, since the cathode is operated at an elevated voltage. Most use heaters which can be optionally wired in series The amplifier is in a perfect condition. 12AX7, a high-gain valve used in practically every guitar amp ever made. The output tranny built for me by heyboer has the correct 3. Each of the two channels was mixed together via the volume knobs and shared a single tone control before proceeding to the next shared gain stage. The same thing happens when a tube is operated with a plate load resistor (RP), its rp value is “loaded” in the same fashion as resistance-B “loaded” resistance-A in the example above. The 12AX7 tubes make use of a miniature dual triode to produce their unique sound. Many of the ‘70s master volume Fender amplifiers provided a balance control instead of a bias voltage adjustment. of V3, a 12AX7 as a free-running multivibrator circuit. With the 12AX7, there is enough excess gain that the circuit will still work, even at very low frequencies, but it may be a problem with other. The 6n2p is very similar to the ECC83 /12AX7 but it has a slightly different pinning. It powers on tubes light up otherwise untested. Not for the Concert II amps from 1982-1987. A (12AX7) cathode coupled phase splitter is used, where V1 (first triode) is a voltage amplifier and V2 (second triode) can be considered as a grounded grid amplifier fed from the commoned cathodes of V1 & V2. 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. A cathode resistor (for cathode biasing) makes the cathode voltage stay within a range positive with respect to grid, as opposed to 'fixing' the grid negative with respect to cathode (fixed bias). 001uF capacitor. Which would approximate the correnct combined cathode / plate current. Part 1 Multisim is used to create and simulate a. The YJC is the YJS with an added ground wire. 5k with a 100k plate resistor or 2. 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. The cathode bypass cap can be 22 or 47 uF, 10-16VDC, I generally use those common values. Circuit Changes. As the diode has the ability to change its forward current to maintain the voltage drop, it wouldn't fight the output so much, increasing gain for small-signal tubes and power output for output tubes. Grid Resistors - Why Are They Used? General. The ratio of plate to cathode sets the bias of the tube. You will need to look up the data for the particular tube your are using. The original Fender Champ type AA764 power stage uses a 6V6 beam power tube, biased at about -20V, which will need an input swing of up to 40 V peak-peak to be driven to full output power. A 12AX7 triode can't. A 12ax7 is not a 12ax7 I thought that, I bought a Tung sol goldpin 12ax7 to replace a ruby 12ax7 that was in my blackstarthe tung sol has definitely more gainif the "tone" is the same between a tung sol 12ax7 and a tung sol 5751 then I'll try also a 5751. With multiple tubes sharing the same cathode resistor, this calculator assumes the tubes are fairly well matched. • 12AX7 and tone stage moved to general input stage, with feedback taken just to 6BM8. The Original Vibroverb runs a 12AX7 with a 1k 1w-cathode resistor as its Reverb driver. We'll be drawing the load lines for the second preamp stage V2A (center left). Adjust the bias trimpot located farthest to the right (looking from the front) on the GB-1 and BB-1 circuit boards until you read. The load resistor will dissipate about 1. 5, not 100!. 85mA, the cathode resistor value = 1V/0. The first stage cathode should not have a bypass capacitor. 12AX7 - Wikipedia. Two 1-ohm 1/2 watt resistors are used to tie the cathodes together, and to the bias resistor (Rk). This service manual was prepared for use by Authorized Warranty Stations and contains service data for the Marantz Model 7C Stereo Console. The voltage across the cathode resistor is at 23VDC, 18VDC called for in the specs. Subscribe If you enjoyed this article, subscribe to receive more just like it. 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. 5) Unground the cathode of the 6AU6 modulator and insert a 330 ohm resistor with a 30 uF 15V capacitor in parallel across it to ground. 8K resistor in series. In this schematic, the AC source couples through C1 and superimposes AC voltage across L1. Today, it is by far the most common preamp tube used for electric guitar amplification and there is an almost overwhelming supply of current production options to choose from. I also modded the phase inverter to a LTP, as the amp had an obnoxious cathodyne phase inverter. Effect of changing cathode resistor values - for a 12AX7 preamp valve the cathode resistor may typically be 1. If the cathode resistor is unbypassed, negative feedback is introduced and each half of a 12AX7 provides a typical voltage gain of about 30; the amplification factor is basically twice the maximum stage gain, as the plate impedance must be matched. Cathode resistors are another poor use of CC. 6K cathode resistor and a 1UF cap. 5k Ω cathode resistor is marked by the brown, green, red, and silver bands. But the 12AT7 and 12AU7 are rated for a max of around 1/2 that. Lets say I havea gain stage with a 5. 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. “If your DSL20 is in fact cathode biased and you want to raise the dissipation you would have to lower your bias resistor value and vise versa for lowering dissipation. 5pF, which is likely to be similar to 12AX7. The load resistor becomes about 12k5, the cathode resistor is 830 ohms, the bias voltage is -5. A typical amplifier stage like this has a resistor connected between the HT and the anode. 02 caps and 56k slope resistor). Cathode Bias (Self Adjusting) Cathode bias amplifiers shouldn't require any adjustments and will work with a wide range of tube plate currents, as the circuit is "self adjusting". The grid resistor also acts as the load that the coupling capacitor sees, and so it is used to calculate the value of the coupling cap. Designing an AC Cathode Follower For the following example we will design a cathode follower to act as a buffer for an effects loop. Removing the 82-ohm cathode resistor on this socket, and grounding the cathode, will drive a top quality NOS USA 6BQ5 to approx 38-40ma plate current, plate dissipation approx 8 watts, and an average Sensitivity setting of "40" represented a reading of "100" for these best-quality 6BQ5. At this operating point, the 6AU6 is providing about 50 volts of swing, which is just over the 46 volts we need for the 12B4A's full output. 0 550 tap volts amp. :: 12AX7 Vacuum Tube page. Also, any heater-cathode leakage can cause the valve to amplify the small leakage current, leading to much higher background hum levels. 2 times the original. The first preamp gain stages on each channel of a JTM45 have a shared cathode resistor, but serarate plate resistors. Before to insert the EL34 tubes in the sockets switch on the amplifier with only the two little tubes (12AX7/ECC83 and 5814/ECC82) and set the trimmers to get about -50V from the grids of the EL34 and the ground. A single common cathode 12AX7 gain stage is designed that could be used as the front-end stage of a tube power amplifier or a tube guitar amp. Cathode Bypass Capacitor: Without the cathode bypass capacitor, the voltage developed by the cathode bias resistor would fluctuate with the varying plate current and produce negative feedback, lowering the gain of the stage. It shapes the electrical signal as it passes through based on the voltage and current. If I put the 12DW7 in place of the existing 12AX7 (cathode follower), do I need to change the plate and/or cathode resistors, and will there be too much voltage between the cathode and heater if I don't (change them)? Or do I just swap the tubes out and hope for the best?. Is my assumption correct? The KT66 is a beam power tetrode. Consider the quiescent current thru each tube as a function of its plate resistance and cathode resistor, and the voltage drop through the plate load resistor relative to the B+ voltage. The value is 1 ohm. I'm wondering if I can drop the gain on 1/2 of a 12AX7 to get it to around the gain of a 12AY7 (having, let's say, 100K plate resistor, 820 Cathode R ) by messing with cathode resistor alone? I'm in a situation where I have one socket and I'd like to have 1/2 12AX7 in one half and a "simulated" 12AY7 in the other half (i. The trade-off is that slightly more grid drive voltage is needed to achieve the same output level. It is used when a transistor is an amplifier that always has plenty of collector to emitter voltage so that it is never saturated. A portion (about 1/11) of this signal is picked off using a voltage divider, and is fed to the grid of the other half of the 12AX7 twin triode, which forms the phase inverter. This is because electrons randomly run into the grid wires on their way to the plate. 2 Introduction: A valid alternative to very expensive Caddock is to use a parallel of two normal 1% metallic oside resistor with inverted phase to compensate some of their inductance. It shapes the electrical signal as it passes through based on the voltage and current. The ground wire is used to bypass the amp's original cathode bias components so that the Yellow Jacket® converter's internal bias components are used instead. Both can accept RCA & XLR inputs Push-pull circuit with deep class A bias to get the sonic benefits. A 12AX7 'Shootout ' ". It was released for public sale under the 12AX7 identifier on September 15, A center-tapped heater permits operation of the tube from either a 6. 0 550 tap volts amp. cathode Power tube cathode screen Resistor Capacitor Electrolytic Cap. With 12ax7s in the CF application, things start sounding compressed when you drop the cathode resistor (47k) below about 100k, and I don't personally care. The amp was built with a conventional R-C coupled first gain stage with 220k plate resistor and 2k2 cathode resistor, into a cathode follower driver, into the 6B4G. Be careful if the cathode resistor is left completely unbypassed. The 12AX7 has a Mu of 100. The larger plate resistor allows the tube to achieve more gain. The cathode resistor can be bypassed to reduce or eliminate AC negative feedback and thereby increase gain; maximum gain is about 60 times. Most use heaters which can be optionally wired in series The amplifier is in a perfect condition. 5 volts DC across the 10 OHM cathode resistor. A 12ax7 is not a 12ax7 I thought that, I bought a Tung sol goldpin 12ax7 to replace a ruby 12ax7 that was in my blackstarthe tung sol has definitely more gainif the "tone" is the same between a tung sol 12ax7 and a tung sol 5751 then I'll try also a 5751. This allows us to focus our is set by three resistances: R1 is the anode resistor, R2 is the cath- attention on the dynamic plate-cathode resistor. The 12AX7 requires the highest value plate load resistor for acceptable gain and lowest distortion, the 12AU7 requires the lowest value. Now the grid will be a 0v, so the C1 coupling capacitor is not needed. This is the biasing point. The cathode bypass capacitor in tube amp forms a filter that reduces the low frequency gain of the a common cathode amplifier. Models and comments will be added as I find time. Many of the ‘70s master volume Fender amplifiers provided a balance control instead of a bias voltage adjustment. The preamp is just one 12AX7 but using the two triodes in that single tube. For example, if the original was 170 ohms, increase to 390 ohms. For the cathode and plate resistor values shown in Fig. This is achieved by choosing a plate resistor of twice the internal plate resistance of the tube, which for a 12AX7 is approximately 50k requiring a plate resistor of 100k, then choosing a bias resistor so that plate voltage is 2/3 of B+. The other blue wire runs between 6SL7 lug #2 and the turret terminal board junction of a. The chassis inside the "Junk Box" modulator/ PS. Grid Resistor. 2 show anode current, Ia, on the y-axis: this is simply the current flowing from anode to cathode at any time. Mu*ed - Ztp*id = Mu*Rd*id - Ztp*id. While not as accurate as the output transformer shunt method (detailed below, after the cathode resistor procedure) it is far and away the safest of the two methods, and can be successfully done with medium- and even low-quality test equipment. Reducing the value of the 1K8 cathode resistor causes more anode current to flow so that the tube runs physically hot. V1=12AX7 V2=12BZ7 and 47k resistor from Cathode of second stage at pin8 of V2, to ground. RCA and Sylvania first introduced the 12AX7 to the world in the late 1940s. 02uF coupling capacitor. Then solder the free end on the new resistor R37 back to PCB from where you lifted R13. The cathode resistor bypass capacitor is directly in the signal path and should be considered as so. The negative feedback is introduced in this stage, but we will ignore this until after we understand the power amplifier better. With a 12AX7 when using typical resistor values for plate and cathode resistors, gain will drop by about half (or something-other-than-half if you're not using a 12AX7 & typical parts). 022 µF 25 µ F 25V 470 1W 8-450 8-450 16-450. 0 550 tap volts amp. The x-axis shows anode voltage, Va: this is the voltage dropped across the valve between anode and cathode. Five graphs of distortion by component (2nd thru 9th) are presented for the following conditions: 1. The nice part about this was I have 2 modern Tung Sol 12AX7 tubes that hummed under any normal arrangement. The grid resistor closes the current path between grid and cathode so that the grid discharges across the resistor and. The total gain is roughly +20dB (1:10); the open-loop gain is closer to +34dB (1:50). The 12AX7 is the most common member of what eventually became a large family of twin-triode vacuum tubes, manufactured all over the world, all sharing the same pinout EIA 9A. cathode Power tube cathode screen Resistor Capacitor Electrolytic Cap. It powers on tubes light up otherwise untested. also if you change the value of the bypass capacitor (the capacitor that is paralleled with the cathode resistor) you obtain a change in gain one other tube to be mentioned is the 12ay7 click to enlarge look to V2/a look to the cathode arrangement, you will have a progressive baypass cap insertion. Calculate Plate Dissipation In Cathode Biased Output Stage Based On Voltage And Resistance Readings In these calculations, 5% of the cathode current is assumed to be screen current. Q: I want to audition different anode resistor values. On the (AC sweep) simulation, it will be a voltage gain of ~36dB. The output triode's cathode resistor is made up of three sub-resistors, which works as a voltage divider to present the feedback to the bottom 12AX7’s grid. It took mere minutes to solder these in place. The cathode resistor at the ground end is a few volts negative to the cathode. 5k cathode resistor. Because the value of the cathode resistor is so small it will not make much difference. The ten megohm resistor allows the grid to develop about -0. I bypassed the cathode resistor with a 100-µF/250-V high-quality audio electrolytic capacitor. A screen resistor is used not so much to reduce the voltage on the screen, but to allow the capacitor to make the screen voltage constant with respect to the cathode. Lets say I havea gain stage with a 5. A check list for the parts you need for your JTM 45/100 build. The 12ax7 is a miniature high-mu twin triode, each section of which has an individual cathode connection. It can also be set up to deliver negative feedback if the cathode resistor is not bypassed. They all should operate but they will sound different, I predict the 12ax7 should distort earliest out of these tubes because it's starving the most in this application. Is my assumption correct? The KT66 is a beam power tetrode. In a cathode biased amp, the bias voltage varies up and down (that’s the ‘variable’ bit) automatically to keep the bias correct. As I don't have a schematic, I'm working a little in the dark with this one. Put the meter on the highest DC volt range. Ok, we're using a 12AX7. 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 DC voltage on the output is restored by a chain of Zener diodes and the DC operating point is controlled by adjusting the cathode resistor of the. Adjust to mV. Where each cathode line crosses the loadline is where the tube will idle with the resulting bias point. Unlike the 45, which uses a shared cathode, the 50 watt uses a separate cathode arrangement on the first tube, which adds pre-amp gain. and powered from the cathode resistor on the 6L6 power tubes. Adding a 3 prong grounded cord is a necessity. EL34 PSE (Parallel Single Ended) ver. end to new resistor R37. This substantially increases gain. 6K cathode resistor and a 1UF cap. 5pF, which is likely to be similar to 12AX7. After finished my simple 12AX7 tube preamp, I decided to try other medium mu tubes, including this 12AU7 and also the 6922. It is especially suited for use in resistance-coupled voltage amplifiers, phase inverters, multivibrators, and numerous industrial-control circuits where high voltage gain is desired. A 12AX7 can be driven from cutoff to positive grid voltage with a couple of volts of signal, so the grid resistor never has a big enough signal to be distorted appreciably. The 6DJ8 has to cathodes, two grids, two plates, one heater and one internal shield, for a total of eight elements. 12AX7 has a 1M pulldown on grid input while NuTube 6P1 has a bias circuit to adjust the bias voltage on the grid between 0V and 3. That causes it to distort, which is the purpose of this circuit. • DC elevated heater with tuned humdinger pot. 5mA = 200k ohm. on high power it has a 62ohm cathode resistor and on low power adds another 68 ohms to cathode resistor value. While not as accurate as the output transformer shunt method (detailed below, after the cathode resistor procedure) it is far and away the safest of the two methods, and can be successfully done with medium- and even low-quality test equipment. The same thing happens when a tube is operated with a plate load resistor (RP), its rp value is “loaded” in the same fashion as resistance-B “loaded” resistance-A in the example above. The 12AX7 requires the highest value plate load resistor for acceptable gain and lowest distortion, the 12AU7 requires the lowest value. 5V, the voltage gain 61, and the pk-pk output voltage about 182, representing an RMS value of about 64V. Only a small part of the full RC curve is trav-ersed with each sweep (around 1 to 2 volts) in-suring good linearity. The voltage gain of the phase inverter is set to just under 10 by the ratio of plate resistor to (unbypassed) cathode resistor. This resistor was lacking in the Cheyenne (and indeed, its lack prevents the AC signal on the Cheyenne's cathode-follower cathode from going below about 50 VDC). 1 uF and one. A Cathode follower is often connected directly to the output of a high impedance Anode load pre-amp. The output can either be taken directly from the cathode or from the junction of Rb and RK as shown. This doesn't mean that the stock cathode follower cannot be modified or improved. Individual isolated heater supplies are required for the 6080 and 12AX7 because of the high voltages on each cathode and the limited heater cathode break down voltages. Reducing the value of the 1K8 cathode resistor causes more anode current to flow so that the tube runs physically hot. 02 caps and 56k slope resistor). Since this current does not flow through the resistor we have a steady cathode voltage. Immediately following are equations for calculating cathode voltages, currents and resistances. Re: Calculating base resistor for controlling common cathode of 7 segment display hFE (beta) is never used for a saturated transistor switch. – The measured voltage is squared and divided by the load resistance of 8 ohms to calculate power. I bypassed the cathode resistor with a 100-µF/250-V high-quality audio electrolytic capacitor. The stage is most definitely in a muddy region of operation. Contains one 12AX7 and one EL84, with a solid state rectifier. 2k, the grid bias is 1. Adjusting the value of the resistor connected to the cathode (the main filament-like part that forms the core of a vacuum tube) of any of the gain-stage preamp tubes can greatly affect the overdrive capabilities and headroom. The cathode bypass capacitor in tube amp forms a filter that reduces the low frequency gain of the a common cathode amplifier. Sometimes, depending on the batch of tubes, the tension on the septar socket springs and the heat generated in the amplifier, there can be malfunctions of sorts. The lower resistor in the divider (R2) should not be excessively high or the maximum heater-to-cathode resistance may be exceeded. Can I just change them? A: From 47k up to whatever you fancy (1Meg is a practical maximum), but you must re-bias the section (see tables above). 6 mA (cathode resistor 4700Ω), then a larger resistor, say 47k, could be used. Circuit changes The junk chassis used a 12AX7 tube for preamp and phase. Cathode follower 1/2 of a 12AX7 or a 12AU7 can be used as a Cathode follower. Stock tube in the cathode follower position is best. This is the maximum voltage that the valve can see between the heater and the cathode (known as the "heater-cathode voltage"). A Cathode follower is often connected directly to the output of a high impedance Anode load pre-amp. It works but there was a bad hum. The green wire from the 12AX7 output connects to C30. Ik: Cathode current/Plate current; measured through a precision 1 ohm 10W resistor in series with the cathode. 5 to 3 uF bypass cap, and a 120K resistor for the CF's cathode resistor. The reason for this is because the tone stack is a fairly low impedance. 5k cathode resistor. The resistor at the left side of the board is the 10k (brown, black, orange) V1b cathode resistor & is the correct value.