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clipping. Ive read a hundred times that drive can be controlled by the amount of signal a particular gain stage feeds into the next. What are some gain stage designs that tame distortion, etc voltage dividers, coupling caps...
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The ubiquitous and fairly universal gain stage is the typical Fender first stage: a 12AX7 with a 100K plate resistor, 1.5K cathode resistor, 1Meg grid resistor and 22uF bypass cap. Then there is the more gainy Marshall design that uses a 220K plate resistor, 2.7K cathode resistor and a .68uF bypass cap. The Fender-style stage sounds more open and clean. The Marshall stage, because of the higher gain, sounds more aggressive and cutting, also due to the low-frequency rolloff of the smaller bypass cap.
Cathode bypass caps do NOT have to be used. Their removal will reduce the gain of the circuit via degenerative feedback across the cathode resistor, and the stage response will be fairly flat.
A voltage divider across the output of the stage, either fixed or as a potentiometer, can be used to control the gain into and hence, the distortion factor, of the subsequent stage. The process of designing preamps for "best" distortion (clipping) is empirical. You need to experiment and try different configurations. Typically, multiple stages will generate the smoothest distortion, but it involves throwing gain away and signal shaping in the process. Attempting to overload a single stage too much can result in blocking distortion, which is one type you do NOT want, because it is ratty and nasty.
If you want to learn about preamp design, you should just go to a website like Schematicheaven.com and see what has been done already. Despite many amp companies and boutique builders touting their own secret and special designs, most of what is out there are just variations on existing designs, and really, do you need to reinvent the wheel?
Also remember that it isn't all in the preamp, and getting the power amp and preamp to clip at the proper times is what really makes an amp design spin. -
hey man thanks for the response. Im aware of most of the things you mentioned, as I have built/worked on a few fenders, marshalls and vox, so Im not necessarily looking for a tutorial
I mean, I can look at schematics all day long and wire them up as shown, Im just trying to wrap my brain around what exactly is going on, ya know? Reason I posted this thread is because I am getting some fairly unpleasant distortion out of my preamp. Its basically a vox ac30, but I moved the master volume to just after the tone controls and used the original volume control as 'gain'. There are a lot of factors Im trying to take into consideration however. First, maybe this circuit wasnt intended for smooth distortion with the preamp cranked all the way up. I just got a new guitar( 1970 Univox Deluxe-really sweet) but maybe the pickups are affecting this as well. Im pretty sure either my speaker is torn or just plain sucks. luckily, my vintage 30 is coming in today. So maybe my problem is elsewhere. However, with the gain about halfway, which is clean for the most part, it sounds really good. anyway, thanks for the reply
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On the subject of cathode caps, what do you think abou using 330uF caps? I know Fender did it in some earlier amps, but the Ck/Rk combination were shared between two channels, where as my amp is only single channeled. Its not too much is it?Comment
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Didn't mean to sound condescending. It's hard to get a feel for level of experience on a forum sometimes.
The AC-30 overdrive sound depends heavily on power amp saturation. You are attempting to increase preamp distortion with your pre/post preamp volume controls, and there just isn't a ton of gain to be had here. I am suspecting a nasal, buzzy sound is what you are getting. Now, if you were to add another gain stage AFTER the tone controls, and tailor the frequency response a bit, you might just have what you are looking to achieve.
For me, 330uF is just too large in value unless it is a bass amp, and even then, anything above 25uF is really not doing much for low end. 47uF would be the absolute highest value I'd use, personally.Comment
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The Fender, tweed Bassman use a 250uF cap across the common 820 ohm Rk but I can't think of one that used 330uF.
With respect to an electric guitar (tuned correctly) and good sounding guitar amps, ... once you get up to 10uF, even across a lower value Rk such as 820 ohms, there is nearly no additional guitar bass response to be found.Comment
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Well, the only reason I am using a 330uF( and bruce, i was referring to that 250uF cap you mentioned) is because its what I had lying around. Not necessarily trying to increase bass response. and jrfrond, you described it exactly! Buzzy and nasaly. My poweramp is basically a Fender clone with
2x6L6, long-tailed PI, no negative feedback. You mentioned adding another stage after the tone/volume controls...just so Im clear would you set this up like any other stage, with respect to grid resistance, cathode combinations, etc?Comment
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