I'm working on the design for a high gain 50W head for modern metal. I've laid out my ideas for the preamp:
All tubes are 12AX7s. This is not really an original design, just a collection that I've put together. The first two sections are from Brian S.'s (AX84 forum) L3 amp, the 3rd (cold clipper) and some other tidbits are from the Peavey 5150, the final gain stage, the cathode follower, and the tone stack are from AX84's UberGain.
The schematic is preliminary, and will need tweaking. The coupling caps (C4, C5, C8) are probably too large, and the first couple of stages will probably need some caps across the plate resistors to decrease the gain at high frequencies.
Audio Research Tube Preamplifier Schematics
The power supply will based on an Antek toroid with a solid state rectifier, possibly with a cap multiplier for the preamp section.
The circuit looks fine. But, I don´t understand why grid stoppers are so high, 500K??? R21 and R22 are a 2:1 attenuator, and R15 and 16 are 2:3 attenuator. Why?
The attenuators are just used to drop some signal before the next stage. The values are high so as to not load down the preceding stage.
Ech83 Guitar Preamplifier With Tone Control On 12v Dc
If you want to reduce level to the following stage, then it is better to unbypass some part of the cathode follower, then lowering stage gain AND causes NFB that increases linearity. R17 is too high and is loading nothing. In fact, considering Miller capacitance, it is adding a low pass filter.
Osvaldo de Banfield said: If you want to reduce level to the following stage, then it is better to unbypass some part of the cathode follower, then lowering stage gain AND causes NFB that increases linearity. R17 is too high and is loading nothing. In fact, considering Miller capacitance, it is adding a low pass filter. I´m not musician, I´m talking only based in electronic theory and practice. Click to expand...
I understand where you are coming from, but increasing the linear operation of a stage is the exact opposite of what a preamp like this is supposed to accomplish. The values and circuit blocks I'm using here are pretty standard in all guitar amps.
Tube Preamp Inside A Guitar
OK, but explain me some questions I cant undestand. In the 4th stage, you gain say, 50 times. But the 2:1 attenuator consisting in the 2 resistors R21 and R22, then the signal to the 5th stage is 50 * 1/2, or 25 times. Which is the advantage of such circuit, versus adding a small cathode resistor unbypassed to obtain a 25 times gain in the 4th stage?? This way the 4th stage allows similar gain, but less distortion thanks to local NFB. I know that musicians like you use distortion deliberately, but help me understand you.
Osvaldo de Banfield said: OK, but explain me some questions I cant undestand. In the 4th stage, you gain say, 50 times. But the 2:1 attenuator consisting in the 2 resistors R21 and R22, then the signal to the 5th stage is 50 * 1/2, or 25 times. Which is the advantage of such circuit, versus adding a small cathode resistor unbypassed to obtain a 25 times gain in the 4th stage?? This way the 4th stage allows similar gain, but less distortion thanks to local NFB. I know that musicians like you use distortion deliberately, but help me understand you. Click to expand...
The entire goal is more distortion, not less. So, one stage amplifies the signal until grossly distorted, and then the attenuator drops the signal level down, just so the next stage can distort it some more - all in the interest of creating more complicated tones.
Construction Of A Guitar Amplifier
It's all about touch sensitivity or whatever term is in vogue today. The trick is to get multiple stages to be at the verge of distortion at the same time. This way the amp can be relatively clean and with a twist of the volume knob on the guitar, or even just playing the notes a little harder the amp transitions into distortion, with each stage contributing its own characteristic sound. The guitar player can then play the amp by altering his picking style.
Multiple volume or gain controls (3 in this case counting the one on the guitar) allow more flexibility. Crank up the guitar and the input gain and back down the master gain to get lots of distortion at low volume, or crank the master and back down the other two to clean up the act.
The high valued grid stoppers are common in high gain amps. They affect the frequency response, stability, and yes throw away some excess gain. Find the schematic for a Soldano SLO-100 for a similar design.
Build Report: Tube Preamp For Bass (or Guitar)
I have built a 6 stage high gain preamp for screaming lead guitar. The issue with 6 stages of gain can be hum, noise and microphonics. You may have to try a bunch of 12AX7's for the first tube to find a quiet one. Even the second tube can contribute some microphonics. I have several hundred 12AX7's and I wound up using a military spec 5751 for the first stage. It has a slightly lower gain than a 12AX7 and is built like a tank for less microphonics. I used DC heaters to kill hum. Some of my grid stoppers were 270K carbon comps.
OK, boys. I have a friend, Carlitos who once explain the question of deliverately distort the signal, and the theme of acoustically couple the speaker to the guitar to create Larsen effect as a musical resource, but I have no idea how do you use it, and how to help you get it. Then, this thread isn´t for me. Thanks for the time and patience dispensed to me.
My goal has always been touch sensitivity, and I do a lot of palm muted chugging for rhythms. This takes a metric ton of gain to sound good.
Connecting Guitar Tube Amp To External Transformer For 70 Volt Line Audio System
Tubelab.com said: I have built a 6 stage high gain preamp for screaming lead guitar. The issue with 6 stages of gain can be hum, noise and microphonics. You may have to try a bunch of 12AX7's for the first tube to find a quiet one. Even the second tube can contribute some microphonics. I have several hundred 12AX7's and I wound up using a military spec 5751 for the first stage. It has a slightly lower gain than a 12AX7 and is built like a tank for less microphonics. I used DC heaters to kill hum. Some of my grid stoppers were 270K carbon comps. Click to expand...
My current amp (L3 designed by Brian S. at AX84) has four stages feeding into a cathodyne EL84 output stage. Works very well for me, but I want more
Thanks for the tip on the 5751. I know one of Sovtek's 12AX7's is built to the old 5751 specs; I'll be sure to try them. I'm planning on DC regulated heaters for the preamp, but I really haven't had any problems with heater induced hum in my L3 with it's elevated AC heaters and tight wiring.
Guitar Preamp Circuit Based Fet » Circuitszone.com
Stormrider said: Thanks for the tip on the 5751. I know one of Sovtek's 12AX7's is built to the old 5751 specs; I'll be sure to try them. Click to expand...
Found it... It's the Sovtek 12AX7LPS. It has the spiral wound filament and larger plates like the 5751, with the higher gain of a typical 12AX7.
I would add some on-board screw-pots for those resistors, maybe even some front-panel inter-stage volume controls. You know how it works, so in-use you'll have no problem intuitively making adjustments.
Electronic Diagram For Ampeg J 12t Vacuum Tube Amplifier, Schematic
Perhaps my favorite preamp is two Fender channels in series, with a volume between and a volume after them both. I like your input almost directly to the first grid, and you may find playing with the resistors in that front-end can change how it works (and the amount of treble) with the inductance of your pickups (unless you're using powered pickups). Those first resistors in series and parallel also affect what happens when you turn down the volume on a Fender versus Gibson guitar and whether the hum comes up when you turn down a Fender a bit; those resistors determine the input impedance that affects whether a strat's higher-resistance volume control lets the hot line get too far from ground. You might consider a trm pot to ground useful to adjust to match your guitar better if the gain is really high. Really good move not using some two-input-jack resistor network that throws away half your gain. Also, I really like to boost treble early, then roll off the treble later; so I would put a very fender-ish bright switch and cap across that first volume pot (or a bright pot in series with a cap both in parallel with the volume). Its effectiveness obvioulsy changes with the volume setting, but that's OK in-use (at full volume a fender bright switch does nothing). Main use for distortion modes is that the guitar signal gets treble boosted early and then reduced later; but the distortion generated later only gets its treble reduced, making for
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