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Topics - Koreth

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I figured this was the best section for this post. If it's not, I apologize and can repost it where it is appropriate.

This is something that's been occupying my thoughts for a while. Off and on, I've been looking at datasheets, jotting down notes, doing bits of math and generally mulling things over. I figured it would help to put my thoughts down in writing where people smarter and more experienced than myself can off their input and comment if they want. The point of this isn't try to prove how the valve sound or solid state sound is inherently superior or more desirable, but focusing more on why they are different and assuming one wants to accurately replicate a valve sound with solid state parts, how that might be achieved. I know this has been discussed before, if not here, then plenty times elsewhere. It probably sounds like I'm about to start beating a dead horse and parroting the same out-of-context half-truths that abound on the Internets. For that, I apologize. I'm only posting this because I've made a few observations I believe haven't been discussed to death. Though I warn I may still ramble a bit.

So there's two tube types commonly used in guitar amps, triodes and pentodes. Pentodes are commonly used in the output stage for their greater capabilities. Triodes are commonly used in the preamp. There's exceptions, but that's the norm. Both are voltage-controlled -  no appreciable amount of current in the control grid during normal operation. In fact, current on the control grid is a bad thing unless you like a hard, sudden clipping on the positive swing of your signal (Makes for a great crunchy distortion!  :)). You probably knew that already.

There's two main types of transistors, BJTs and FETs. FETs are like tubes in that they're voltage-controlled devices. I don't know that any appreciable current flows in the gate in a JFET during normal operation, and I can't see how current can flow at all in a MOSFT, what with that insulated gate and all. Bipolar transistors however work with current. It's the changes in the current from the emitter to base that causes changes in the current from emitter to collector. Again you knew probably already knew that.

Transistors and pentodes have similar output characteristics. Look at the graphs in the datasheet for your favorite BJT, FET and pentodes. The curves have a very similar shape. They all sprout up from 0V/0I, shoot up sharply, then bend sharply or softly after a small portion of the max voltage and swing almost horizontal.

Here's a FET...


...here's a BJT...


...and here's a pentode


So we have a SS equivilent of the pentode -- the FET. Cool. Of note is how the curves are mostly horizontal after a small fraction of the output voltage. After the knee of each curve, a change in input voltage is going to affect more change in output current than it will in output voltage -- a low internal output resitance. That's why a pentode makes for as such a great output tube, poor damping characteristics of a low output resistance notwithstanding.  A huge voltage swing is nice, but without current capability to back up that voltage swing, getting some decent output wattage and driving a loudspeaker is gonna be harder.

So high current capability is desirable on the output of the amp, but what about on the input of the amp, in the preamp? Here we want voltage gain and lots of it. The output signal of a guitar pickup is of so little current and voltage that it doesn't take much attenuation to kill it entirely. give a .1Vp guitar signal a gain of 100, and now we a have a voltage swing that's much easier to work with. We can play with its frequency content to make it sound prettier, compress it so it's levels are more even and easier to work with, clip it for some crunchy sounds, and bring the voltage swing up high enough that with some current behind it a la the power amp/output section, can drive a loudspeaker.

Yes, that's nice Koreth, congrats on finally getting guitar amps 101. Now go put your newfound understanding to use and go build a Noisy Cricket already. Do you have a point, or are you just rambling?

Yes there's a point. We need voltage gain in our guitar preamp. Enter the triode. The curves for the output characteristics of a triode stretch more vertical than they do horizontal. A change in input voltage gives more voltage swing than current swing. Yes, a pentode can be made with with far more gain than a triode (the upper limit for a triode is about 100µ, pentodes can get over 1000µ.) But a pentode also costs more, is more prone to noise and microphonics (more active elements to go wrong). and requires a more complex circuit. We don't need a gain of 1000+µ in our preamp, plus noise and microphonics are Bad Things™ in a preamp. So if we could use a tube that requires less external parts, gives us the gain we need, isn't as prone to noise and microphonics, and on top of all that, costs less to boot, why wouldn't we? I suspect this is why the triode is the common preamp tube. It could also be that everyone is simply copying Leo Fender, and Leo was simply copying the RCA handbook, but that's another discussion.

The point being is that unlike the pentode and his SS brother, the FET, there is no SS analogue of the triode. Maybe the Trioderizer counts. (http://radiomuseum.org/forum/the_trioderizer_a_solid_state_triode.html). Put some local negative feedback on a MOSFET and it starts to behave much more like a triode than a pentode (the SS verson of Ultralinear mode maybe?) But with only 10 hits on Google, either nobody cares, or there must be some fundamental problem with it that keeps it from seeing more widespread use in SS designs trying to cop a valve sound. So without a SS equivalent to the triode, where do we get our voltage gain for our preamps? I see opamps in a lot of schematics. Op Amps are great for the application. They have downright stupid amounts of gain available, so much we have to use local negative feedback to get the gain down to a useful level.

Problem being is that especially with all that negative feedback improving it's operating characteristic, an Op Amp is really linear across the range of audio frequencies, AFAIK. But linear is good! We want linear! Harmonic Distortion BAD! Yes, and all this linearity is probably why a not insignificant portion of the guitar playing populace prefer tube amps to SS amps. An SS amp can be too accurate. The mysticism and some sort of placebo effect surrounding tubes aside, their imperfections apparently can color and distort the sound of a guitar in a pleasing way for some people.

Great, it's two in the morning I'm tired and just realized I've just spent over a thousand words running in a giant circle without coming to a point. There's one, I promise. I'll post it later when I'm rested. It involves the nonlinearities of a triode.

People smarter than me are welcome to point and laugh at the rambling newb now.

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The Newcomer's Forum / Voltage supply rating on datasheets
« on: January 02, 2010, 01:12:51 AM »
I've been looking at datasheets for things like op amps and chip amps the past few days, and some of the specs in them confuse me a bit. Some say something like 18V, others say +-18V. Are these meaning the same thing or something different. I assume that 18V means it can have a total of 18V across it's power supply terminals before taking damage, and that +-18V would mean that the V+ terminal can be +18V, and the V- terminal can -18V, for  total of 36V across its power supply terminals, but we all know what happens when you assume. Am I on the right track here or am I off in la la land?

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Amplifier Discussion / Modifying a First Act MA104
« on: January 01, 2010, 04:01:01 AM »
Greetings all.

I have a First Act MA104 that I purchased from Wal-Mart years go when I first started playing guitar. I hated it's tone pretty quickly, as even with the gain knob all the way down, and the volume set very low, even vintage single coil pickups would send the amp into a nasty sounding breakup very quickly. When I found another amp (a Ross Systems 22) at a garage sale with a slightly better sounding distortion and the ability to clean up when the gain knob was turned down, the MA104 was shoved into a corner and forgotten about.

Fast forward to the present: The Ross 22 has since died and been cannibalized for parts and my main amp is a 100W Mesa/Boogie. The Mesa will remain my main performance and rehearsal amp for the foreseeable future, but a 100W all tube half-stack isn't very practical for bedroom practice, never mind the impracticality of lugging a big heavy head and cabinet across town twice a week for rehearsals. So the MA104 has been brought out of its corner and put to use again. Stock, it still sounds gross, but unlike years ago, I now own and know how to operate a soldering iron. :)

First Act wasn't forthcoming with a schematic, so I traced the circuit out and made my own in LTspice. It is attached. I'm pretty sure the diagram is complete, but I could have made some mistakes in tracing the circuit. So if anyone sees anything that doesn't make sense, please lemme know. Though the output chip is labeled "UTC2003", I'm pretty sure it is a TDA2003, which UTC makes. The power ratings and test circuit seem to be close to what's in the amp and First Act's claimed power output for this amp.

Thus far, I've made a few mods. I spliced a 1/4" plug onto the speaker out and used one of the 10" 40W 8Ohm speakers left over from the Ross 22. Overkill for a 4W amp, I know, but the 10" speaker sounds a lot better than the little 4" speaker it came stock with. With the help of the fellows at Music-Electronics-Forums, I lowered the gain in the 1st op amp stage. With the stock values, the gain of the 1st stage was over 300, enough for even a vintage single coil to overdrive the op amp hard against it's power rails. I changed R2 from 270Ohm to 1KOhm, and C4 from 10µ to 1µ. The gain knob has a much more usable range from clean to barely breaking up to full distortion now.

At this point I'm looking to achieve the following:
  • More clean headroom. Even when the gain knob is all the way down, when the volume knob is swept past 4, there starts to be a bit of breakup on the attack that I don't like. Past 4 on the volume knob, the amp stops getting louder as quickly and starts getting dirtier. Since the volume control is after the gain and tone controls, I'm guessing that the 2nd op amp stage is making the signal from the 1st stage too hot and it is overdriving the output chip. The voltage divider on the output of the 2nd stage barely attenuates the signal before it hits the output stage. I've got few ideas here:
    • Reduce the gain of the 2nd stage
    • change the voltage divider to attenuate the output of the 2nd stage more.
    • Reduce the gain of the output stage
    • A combination of all or some of the above

  • More highs. When I run the amp clean, I dime the tone control. When I turn up the distortion, I turn it down to tame some of the harshness that comes when the turning the gain up.That's all well and good, but when the tone control is dimed, it still isn't enough highs. The amount of highs with the tone control turned up is short of "pleasantly warm." I'd like it to be able to go all the way up to "sparkly bright", if possible. I wouldn't mind if it went a little bit into "piercingly bright", as I can always turn the tone knob down. I've got a few ideas here too:
    • Lower the value of or remove C3 in the 1st stage
    • Lower the value of C8 in the tone stack
    • Lower the value of or remove C13 in the 2nd stage
    • Some combination of the above

  • Tweak the distortion character of the amp. I want to do this after I have the above two addressed. I'd like to play with LEDs both as the hard clippers and in the negative feedback loop of the 1st stage as soft clippers, as I've discovered through modifying one of my pedals that I prefer the clipping sound made by LEDs to that of 1N4158s.

Hopefully all that isn't tl;dr. Your input on the above would be greatly appreciated.

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The Newcomer's Forum / Newb here
« on: December 07, 2009, 03:38:55 PM »
Greetings. I found this place via music-electronics-forum.com (Ampage?). I have some threads over there with some of my questions, but I figured this place would be a good place to cross-post with some of them since this place has a Solid state focus, and some of my music gear is purely solid state.

Should I throw my immediate questions into this thread or start another one in another section of the forums appropriate for it?

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