high gain amp design and questions

[vanslycken]

I've searched topic titles for "high gain", but couldn't find a lot of info on designing high gain amps.
Since I'm not just copying circuits but trying to make something sound right by myself, I was wondering what experience you guys have in designing high gain amps. I'm looking for good sounding preamp distortion.

What are your experiences with diode clipping or multiple gain stages, I saw the peavey 5150 has 6 or 7 gain stages and the jcm900 only 3.

How do you design such a thing, add gain stages one at a time and get them distorting just right?

How do you decide where to put the EQ?

Is there a real sound difference in the marshall way of putting the eq in the very last place after a cathode follower? I mean, loading a gain stage down a little isn't so bad, is it?

I know some of the theory but what is the real difference in sound between a 100k or 220k anode resistor, and a somewhat hotter or colder 12ax7 (i.e. a larger or smaller Rk)?

[tubeswell]

Well in my experiments with up to 5 gain stages in front of the splitter (if you count the splitter driver- otherwise its four stages and a driver-splitter ), is that the more overdriven the pre-amp is, the more bass you have to get out of it for it to sound any good (smaller coupling caps and cathode bypass caps - Marshally values). Also the more unstable the amp becomes in terms of weird oscillations etc. This can be overcome by lots in inter-stage padding (voltage dividers and tone shaping RC networks). Check out some of the AX84 lead pre-amps. But its a fine line you walk and to design it properly you really need to work out all the variables (such as how you are biasing the stages, the miller capacitance of each stage, the input and output impedances of each stage, etc) in theory first. So go and read up on some articles about the basics of gain stages - at Aiken, or the Valve Wizard, and others.

[tedmich]

How do you design such a thing, add gain stages one at a time and get them distorting just right?

How do you decide where to put the EQ?

Tubeswell is right; there are many good places to begin to understand these complex issues, the only thing I can add is that this begins to be so complex and empirical testing is very time consuming so the most common thread in the design is....borrowing! You will begin to see how there is a borrowing line from Fender->Marshall->Mesa/Soldano etc.

so (steal->test->tweak) repeat as needed!

[daz]

I second that. Tubeswell's post that is. there really is a very fine balance once you find the goods. And i gotta say, it's not an easy task. I've spent close to a year i'd say designing the preamp for my high gain amp and every now and then you find certain things that just work and nothing seems to change that. But it's very tricky because once you find that magical sound theres usually going to be some baggage that comes along with it. Tubeswell mentioned bass, and that was a big issue for me till just recently. And what i have found is that this fine balance IMO seems to be no coupling cap values or tone stack tweaks. Those things work when you nailed the tone and now just want to fine tune. But when there are issues it seems to be in the psu and the distortion stages and how they are biased and all that. I had a all 100k anodes till several months ago when i tried a 220k on the first stage, and i thought it was the $hit ! Then after a few months, and i'm talking just weeks ago, i tried a 100k again and left it there thinking it was better. i did not hear what i lost till i went back to a 220k just today. It was like my amp just came alive again.

But anyways, it really is a fine balance, and you have a long battle ahead of you if you do what i did and pretty much just design it yourself loosely based on a typical hi gain pre. On thing i would suggest is if you run into ocialtion issues and blocking distortion, all those things that happen when you're trying to get a lot of gain out of a preamp, try what i did....instead of using voltage dividers, try just using some small grid leak resistors on a couple stages. As low as 68 to 100k. That really helped me. In fact, that was the turning point between where the amp was full of problems and all of a sudden was great. It's improved even a lot more since then, but that was a huge step forward. May work for you, may not. But keep it in mind if you have a hard time getting the gain w/o the issues. Also, adjust the last dropping resistor that sets the voltage for the 1st tube. I found the tone and feel can be greatly adjusted with that.

Just a few thoughts from someone who is no tech at all, but has been thru a lot trying to do exactly what you're talking about doing. And i have an amp now thats better to my ear than any of the many marshalls i gigged with for many years. Hope this helps.

[Chuck H]

Why do you want to avoid other peoples designs? And how can you learn how to do it if you don't study whats gone before, and ultimately end up using it. I think there is very little "new" under the sun with respect to tubes. Trying to create an "all new" circuit is probably more trouble than it's worth. And the reason there's no rote info on high gain preamp design is because they are very sensitive circuits and the deisgns that work are worked out on an individual basis. Not by any 'high gain preamp' formulas.

I started with amps by doing mods to my Marshall some 17 years ago. One mod led to another, etc. A year later I had a pretty decent sounding high gain preamp. A YEAR! and I spent most of my spare time on it.

I don't mean to discourage building your own design. But you shouldn;t take it to the extreme of intentionally trying to be different because that may be contrary to your goal.

You should study the designs that have gone before. Much of the circuitry in a high gain preamp is there to avoid or cure inherant problems with these circuits. If you look at a circuit and you can't tell WHY the designer built it that way then you may want to figure it out before trying to do your own designs. That way you'll know at least one way that will work, and why. In a circuit as sensitive as a high gain preamp it's good to know WHY things work because you'll be able to better manage all the little gremlins that pop up. High gain preamps have evolved. There are many improved circuits to deal with the dificulties. Trying to build without the benefit of learning these circuits puts you twenty years behind the curve.

Chuck

[defaced]

This forum is definitely not ripe with high gain builders/discussion. Not that that's a bad thing, but I have found the SLO Clone forum to offer more information for study regarding high gain amps. You will have to register to view it, but there's discussion/schems on many high gain amps to help give you a background on the topic. For example, there is a thread where someone listed each part in the SLO preamp and what it does, and what happens when you tweak its value.

[tbryanh]

SLO Clone forum . . . there is a thread where someone listed each part in the SLO preamp and what it does, and what happens when you tweak its value.

Is there a link to the thread or a topic name for the thread? I was browsing the forum and did not see the thread.

[vanslycken]

thanks for the replies everyone.

The reason I want to design it myself is that I've tried copying existing circuits (m/b mkII, some marshall things) and they don't sound right in my setup - i.e. I have my homebuilt speaker cab, a power amp which is a bit different (2x6l6 and 2xel34) and so on. So I thought, why copy when I can find out for myself.

The grid leak resistors, that's a good idea, I never tried those. And of course, the patience and going forwards and backwards from time to time... that's the way I'm working, but I didn't seem to get it right.

Thanks, and further remarks on how to get great lead tone are welcome!

[Steve Conner]

If you're looking for high-gain ideas, I built this many years ago, and it still sounds pretty badass!
Toaster top page

It has three gain knobs that between them cover a range from completely clean to screaming madness. One of the gain knobs is a three-position switch that adds in extra triode stages, one is a variable cathode resistor (used with an undersized Ck, boosts gain while cutting bass) and one is a regular "volume pot" between stages.

There is no channel switching, just a footswitch input that has the effect of turning two of the three gain knobs up full when stomped. Using this with the guitar volume, you can clean it up quite well.

Finally it has an active EQ that lets you scoop the mids or boost them.

Of course "badass" is my own personal opinion and I don't guarantee you will share it. This is another reason to rip off a Mesa or SLO: at least you know what you're going to get.

[tubeswell]

Freaky lookin' tesla coils on your site there Steve. a hazardous robbie the robot out of space family robinson kind of springs to mind - they must require tons of power to get working. I simply must congratulate you on your truly mad science.

[defaced]

Is there a link to the thread or a topic name for the thread? I was browsing the forum and did not see the thread.

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[Steve Conner]

YouTube - Tesla Guitar demonstration A demonstration of my latest "high gain" amp

edit: Awesome, I didn't realise that linking to a Youtube video made it appear right here in the forum. You might want to skip the first half or so of the vid, unless you're interested in the theory behind connecting an electric guitar to a Tesla coil. Either way, 250,000V out for 100mV in counts as pretty high gain.

[Chuck H]

Excellent. Was that a bang on your door at the end?

Shake 'em up Steve.

Chuck

[vanslycken]

now that is mighty cool! Gives a new meaning to "sounding like rolling thunder"

[tedmich]

If you're looking for high-gain ideas, I built this many years ago, and it still sounds pretty badass!
Toaster top page

Any clips of this one ?


PS the tesla guitar was great, sell one to buckethead!