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Old 4th June 2012, 12:18 PM   #1
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Default High Gain Guitar Preamp

I'm working on the design for a high gain 50W head for modern metal. I've laid out my ideas for the preamp:

Click the image to open in full size.

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.

This pre will be feeding into a P-P 6L6GC power amp targeting about 50W in class AB1.

The power supply will based on an Antek toroid with a solid state rectifier, possibly with a cap multiplier for the preamp section.

Any suggestions are welcome!
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Old 4th June 2012, 03:39 PM   #2
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The circuit looks fine. But, I dont 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?
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Old 4th June 2012, 04:34 PM   #3
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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.
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Old 4th June 2012, 04:47 PM   #4
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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.

Im not musician, Im talking only based in electronic theory and practice.
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Old 4th June 2012, 05:13 PM   #5
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Quote:
Originally Posted by Osvaldo de Banfield View Post
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.

Im not musician, Im talking only based in electronic theory and practice.
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.
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Old 4th June 2012, 05:42 PM   #6
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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.
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Old 4th June 2012, 05:55 PM   #7
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Quote:
Originally Posted by Osvaldo de Banfield View Post
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.
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.
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Old 4th June 2012, 05:58 PM   #8
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Ah, OK, then if the idea is to distort, sorry, I cant help you no more. Im not a distortion expert
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Old 4th June 2012, 05:59 PM   #9
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Quote:
I know that musicians like you use distortion deliberately, but help me understand you.
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.

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.
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Old 4th June 2012, 06:21 PM   #10
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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 isnt for me. Thanks for the time and patience dispensed to me.

Good luck.
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