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Old 9th February 2011, 10:51 PM   #1
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Default 12AX7 high gain guitar amp

Hi all, i've been trying to design a fairly high gain amp, (4 gain stages and a cathode follower) im using a +HT voltage of about 400v and i was thinking, what would be the best way to do this? i was thinking of cold biasing so that one side of the wave is clipped in one stage, and the other side in the next, or should i just centre bias everything and get as much gain as possible? the 2nd option seems the best but that would mean the output signal from the preamp would be a bit under 400vp-p this seems to be quite high, or is this normal?
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Old 9th February 2011, 11:31 PM   #2
jjman is offline jjman  United States
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Output from preamp is not likely to be 400v p-p. You may be basing that on maximum hypothetical amplification factors instead of real world results. Best bet is to check out some common schematics for amps you like and go from there.
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Old 9th February 2011, 11:57 PM   #3
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I will add to that comment that so much gain is not desirable. Starting with, say, 100 mV of audio you can get all the distortion you can stand with a single gain stage. If you want overkill, put in two. But four? Might as well use a square wave generator.

And 400 V is way too much for those tubes. They run fine on 100V.
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Old 10th February 2011, 12:06 AM   #4
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Long tail pairs of 12AX7 in enhanced cathode coupled paraphase.
400 is not too much for those tubes, if we consider a tail voltage.
Paraphase Splitter Experiments

400V of output from the preamp is silly. As the output grids will
forward conduct long before that. Even cathode follower "driver"
grids will forward conduct and you will be in blocking distortion
(not the kind of distortion you are looking for)...

Last edited by kenpeter; 10th February 2011 at 12:22 AM.
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Old 10th February 2011, 01:32 AM   #5
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You should be able to get sufficient overdrive/saturation/sustain for most lead sounds from 3 stages BUT for the total shreader massively overdriven sound 4 stages is not that unusual. 4 stages are however difficult to design. You need interstage attenuators to make sure you are not overdriving the subsequent stages too much. you need big grid stoppers, not for their "stopping" function but to provide real HF roll off and limit grid current, you need to limit the amount of bass to prevent muddiness (small coupling caps and small cathode bypass caps) and noise becomes a serious issue. To control the noise you need to keep grid to 0v impedances low and the interstage attenuators can actually help with that.

Spend a little cash - Kevin O'Connors TUT6 (London Power, Canada) has the entire final chapter devoted to high gain preamps with lots of great information. The money you spend on the book will be saved in the first design.

I'm about to try something new - a jfet/triode cascode input stage (high gain, low noise) and just 2 gain stages following.


Last edited by gingertube; 10th February 2011 at 01:35 AM.
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Old 10th February 2011, 11:07 AM   #6
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thanks! i have been looking at a few schematics, mainly the mesa dual rectifier and peavey 6505, dropping to 3 gain stages does sound ok, but il keep room in the chasis incase i decide to add another. but what would be an ideal impedance for the inputs?
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Old 10th February 2011, 11:27 AM   #7
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I suggest - as low as possible. Noise is your enemy - in all its guises - you can't be too finicky. Maybe even make a list of all the tricks you aim to through at it - so that others can add in more.
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Old 10th February 2011, 11:45 AM   #8
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ok heres a schematic of a 3 gain stage i was thinking of. the 1MEG resistor after the first stage would be a 1M gain pot. then from the cathode follower il have an FMV tone stack and volume control. and the voltage would probably be 400 not 420.
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File Type: jpg high gain preamp.jpg (57.1 KB, 355 views)
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Old 10th February 2011, 01:22 PM   #9
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How to go about getting hi gain from a guitar preamp depends a lot on what kind of a sound you are after. If you're after a more modern sound, then go with lots of gain stages with attenuation in between them; for a classic rock sound, go with two, perhaps three (+ the PI), and some like that raw, supercompressed sound that you get with a very low number of stages (2 + SE output, think tweed champ).

I myself have found that the most useful and multi-purpose setup is the Trainwreck Express/Liverpool preamp. I've made a number of variations on this theme, and I feel they perform in a very wide range of music styles, from classic rock to 80's heavy metal. I've been pondering about changing the cathode RC's to diodes, and perhaps getting even a modern metal type of sound out of it - I will surely try this out at some point.

The Trainwreck has three gain stages + PI (in the purest form the output is the first to clip, but I haven't tried this one myself, as I live in an apartment and have no attenuator at the moment), each with a bigger cathode R than the previous. This leads to a very rich, harmonically complex sound.

Check out ampgarage.com if you're interested.
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Old 10th February 2011, 02:15 PM   #10
Cassiel is offline Cassiel  Libya
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Check out ampgarage.com if you're interested.

I like a pentode for the 1st stage (brutal approach), I have found out that by tweaking the screen voltage you can get a wide variety of tones. If you do it wrong it will sound harsh and thin. Needs experimenting.
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