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Messages - Koreth

Sorted into tables for easier browsing. A note, these descriptions come mainly from label on the drawer or a quick web search. I might have some of these wrong.


MJ10003NPN Darlington
MJ10004NPN Darlington
MJ10007NPN Darlington
MJ10008NPN Darlington
MJ10009NPN Darlington
MJ10022NPN Darlington
MJ10023NPN Darlington
MJ11011NPN Darlington
MJ11012NPN Darlington
MJ11017PNP Darlington
MJ11018PNP Darlington
MJ11028PNP Darlington
MJ410NPN Darlington
MJ490PNP Darlington
MJ4033NPN Darlington
MPSA13NPN Darlington

Field Effect Transistors

MPF6661N-channel TMOS
MTH50N05N-channel TMOS
MTP10N25N-channel TMOS
MTP25N05N-channel TMOS
MTP25N10N-channel (TMOS?)
MTP30P06P-channel TMOS
Quote from: phatt on February 27, 2023, 05:01:48 PMHello Koreth,
I admire your conviction but are you aware this would Definitely takes years of R&D to perfect and in the end may not be any better than what is already available.

It would take years of bench testing to perfect,, I should know as I've worn out 6  breadboards (and counting) and sent many perfectly healthy transistors to smoke heaven in the last 30 plus years.
I'm willing to risk this ultimately being a waste of time. There is no doubt in my mind that it will at least be a fun and educational waste of time.

Quote from: phatt on February 27, 2023, 05:01:48 PMRegards the Tx output using discrete components is likely no better that using current Feedback which is very commonly used in a lot of Guitar amps now.
I saw this mentioned in a large thread on I will restate my understanding from what I read, and welcome any corrections

To my understanding, driving a speaker from tube plates (whether triode or pentode), through a transformer causes the combined circuit to act more like a voltage source, which is not the ideal way to drive a speaker. This is because you end up with a poor output impedance, and the speaker's impedance curve gets reflected back at the tube plates, causing frequency dependent gain. In essence, the non-linearities of the speaker heavily color the final output sound as a result. Most guitar amps do use some negative voltage feedback to improve the output impedance of the output stage and cause it to behave more like a current source, thereby damping the speaker's non-linearities, but not very much. Thus, the the choice of speaker & cabinet to have a still large influence on the final sound, regardless of what the preamp is doing, especially on designs without NFB (e.g my Dual Recto employs no NFB, and sounds noticeably different driving an open back 1x12 vs a closed-back 4x12).

With transistors you have the opposite. Transistors are typically *much* better at acting as current sources when wired in a common emitter/source config, removing the need for an output transformer. The various solid-state output stages I've seen in schematics have typically had a complementary pair wired in a common emitter/source config, driving the speaker load directly. In this setup the final transistors act as current sources, providing the current gain needed to drive the speaker, once previous stages have provided the voltage gain needed for the +/- voltage swing required for the target output watts.

What I read was that providing current feedback in such a setup has the opposite effect of voltage feedback in a common-cathode->trafo->speaker setup: It increases the output impedance, causing the output stage to behave less like a current source and more like a voltage source. This in turn reduces the dampening a low output impedance would offer and allows the guitar speaker to more heavily color the output tone, much like it does in tube & transformer designs.

Do I have that correct? If so, I am willing to consider that idea should an output transformer prove unfeasible with the parts to hand or what I'm able to get for cheap/free.

I do still want to pursue the output transformer idea, as I understand the poor output impedance and resultant speaker coloration is just one part of the sounds of the tube amps that have caught my ear. As I understand, there's other bits to that characteristic tone, including, but not limited to:
  • the transformer's main and parasitic impedances interacting with the those of the output tubes and speakern forming some semi-complex filters
  • magnetic saturation during hard drive doing...I'm not sure honestly, some kind of high-pass effect, I think? I understand Fender deliberately undersized the transformers for their power output in early models deliberately to try to protect the speakers from being blown out by excessive bass.
  • inductive flyback voltages during crossover when the output actives are driven into clipping.
I could see the first couple items possibly replicable with judicious placement of reactive components in the output stage, though I might worry about phase margin when using negative feedback. Does the phase shift that comes with the various deliberate and parasitic filters inside the feedback loop give cause for concern about oscillations and stability with current feedback and transistors like it does with voltage feedback with tubes and a transformer?

I don't see how get or control the inductive flyback at crossover during clipping unless that's a characteristic of the speaker's inductance, not the transformer's (or FML if it's both).

Quote from: phatt on February 27, 2023, 05:01:48 PMIf you are chasing Tx output design ideas then find some of those Factory pa systems as a lot of those use Tx drive for line drive.
I have and old Inkel Pa with Tx output giving 4 or 8 Ohm as well as 70V and 100V out puts.
I used it a lot years back as it was 120W output driving a Quad box, gave me a massive sound.
I'll keep an eye out for such. Any other brands to look for besides Inkel and Factory?

Quote from: Tassieviking on February 27, 2023, 08:18:02 PMIs the 24v 40VA transformer centre tapped, or is it just one output of 24v?
If you have 12v + 12v it would make it easier I think.

The 24V 40VA transformer does not have a center tap on its secondary, unfortunately. I haven't seen a single 24V transformer intended for furnace or doorbell use that does. Rewinding is unlikely, the E and I laminations were welded together at the factory. In theory I could cut, re-wind and re-weld, but I am disinclined to try. Yes, I have access to a welder, but my welds make baby Jesus cry, and I don't know if the filler wires I have on hand are appropriate for magnetic applications anyway.

Looking at things, it think my best bet will be to make use of some voltage doublers, and stick the ground reference in the in the middle to get a bipolar power supply. It has already been pointed out to me that the votlage swings available from 24VAC are going to limit how much power I can practically get out of any design that uses the 120:16VAC doorbell transformer as an ouput transformer. I'd have to do my own center tap refernced to ground to get +/- 15VDC. A doubler in theory could get me roughly +/- 30VDC, a quadrupler roughly +/- 60. The latter would get me about 15W into the 8 Ohm load of the speaker when using the doorbell transformer as an output step-down, if my math is right.

Quote from: Tassieviking on February 27, 2023, 08:18:02 PMDo you have any Op-Amps or will it be all transistors?
I actually do, but I don't think they're fit for audio use. No 5532s, or 4558s, rather U741s and similar. I'll have them in the parts lists in my next post.

Quote from: Tassieviking on February 27, 2023, 08:18:02 PMThere are lots of old transistor amp schematics around that you could borrow sections from, maybe a proven pre-amp that sounds good already.

It all depends on what transistors you have, I would like to see just the part numbers, I don't need to see the specs for each one.

That's a relief. Hunting down datasheets for 227 different transistor types, some of them obsolete before I was born, is no small task. I'll have the full list in my next post.
I have finished transcribing the pictures of my hackerspace's electronics parts drawers. This is not the full set of components. I have deliberately omitted a good bulk of the collection for being 7400 and 4500 series logic chips -- which I don't see as likely to be useful unless I start getting into some multi-voice channel switching fanciness.

I could post tables of the transistors on hand now, but they would be just tables of name/type/polarity, and nothing of their specs At 230+ devices that's a lot of device model numbers that are probably obscure, and you'd need to look up their specs to know if they're useful or not.

Would you guys like to see basic specs for each transistor type like max voltage, max current, max wattage, amplification factor, etc, or just a list of the devices on hand? The latter I can post right away. If I am to post the former, I will be spending several days hunting down datasheets for over 230 transistor types.
(note for moderators: I have crossposted this thread to other forums on the web where it seemed it might be relevant. I did not find anything in the rules against this, but if this is against the rules, I apologize)

Greetings. Yes, the title is exactly what I intend to build, as silly as it might seem.

I want to build a moderate power guitar amplifier using solid state actives. Why obsolete and salvage parts?

There's a few reasons:
  • I have access to bunch of obsolete and salvage parts through my own collection of stuff, and membership at a hackerspace. At least some of my BoM will be cheap or free this way.
  • Being constrained by what I can get for extremely cheap or free will make this somewhat of a challenge and force me to get creative with the circuit design, and might even be fun.
  • And finally, these older and salvaged parts are probably not going to run as clean and linear as modern parts would. I see that as a potentially useful quality in a guitar amplifier. Exactly how and why the poor electrical performance of old tube circuits creates the je ne sais quoi sought by guitarists has been discussed extensively, and IMO, still hasn't been fully worked out. But in either case, if bad electrical behavior can be pressed into a musically useful role, that bad behavior is possibly desirable to design for.
Here's what I have on hand so far:
1 120V:24V 40VA power transformer from a furnace
1 120V:16V 15VA doorbell transformer.
1 40W 10" speaker left over from a long-dead practice amp.
1 reverb tank, left over from the same long-dead amp.
A few hundred different transistors, in varying quantities

I am willing to buy some parts, if for no other reason than I know I'll have to. I'm going to need various resistors and capacitors for filters, voltage dividers, setting operating points for the transistors, etc. I'm even willing to buy a few complementary transistors, as I saw almost no complementary pairs in the parts drawers. I may have to purchase some voltage regulators as well, depending on what power supply circuits I am able to come up with. Beyond that however, I really would prefer to limit myself to the transistors on hand for the design, because that's the essence of this game: How good of a sound can I make with this old stuff, even if I have to force it into behavior it was never designed for?

Design Outline
Here's the outline of the design that's been forming in my head over the past week:

  • Class AB push-pull output section based on a complementary pair using some of the higher power transistors selected from my hackerspace's parts drawers, running at high voltage, into the primary of the doorbell transformer, stepping the voltage down and the current up to drive the speaker. Yes, I know this is unnecessary, appropriate BJTs or MOSFETs should be able to push enough current to drive the speaker directly. However, I believe the interactions between the output devices, transformer, and speaker are an important part of the vibe of the old circuits that have caught my ear. Thus, I think it is worth at least trying to design the output stage to use a transformer.
  • A preamp section using a number of the small signal and not-so-high power transistors from the parts drawers, running at medium and lower voltages. Exactly how many gain stages, I'm not sure yet. I would like to span from nice clean tones, through edge-of-breakup, to a moderate crunch. Very high gain distortion is a maybe; we'll see what sounds I'm able to get out of the parts on hand.
  • A power supply section running the furnace transformer, with a voltage doubler or quadrupler to get up to the voltages needed to make use of the doorbell transformer as an output transformer. Other portions of the power supply section can offer lower voltages appropriate for the transistors in the preamp section.
  • Maybe use the reverb tank for a reverb circuit.
  • All of this into a single combo chassis to keep the amp readily portable.

Timeline / Steps

Here's how I see this going. I welcome any input and discussion as the project moves along through these phases. If this order of design is wrongheaded, I also welcome input on what might work better.

  • Transistor Selection
    I need to work out which transistors are going to appear in the output and preamp sections. This will determine the voltage and current requirements from the power supply. I am leaning towards using the few JFETs and MOSFETs in the pile. This is mainly because I understand those are more readily coerced into similar misbehavior as the high-mu triodes and power pentodes that came to define the voicing of many tube amp circuits. However if some of the BJTs, SCRs, UJTs, or other miscellaneous parts available in my hackerspace's parts drawers might also be useful, I am interested.
  • Power Supply Design
    I need to design a power supply to meet voltage and current needs of the transistors selected for preamp and power amp roles in the previous phase. I don't have any super firm ideas here yet. I like the designs I've seen from Walt Jung, but I don't know if those can be realized at both the high voltage I want in the output section, and the obsolete parts I'm limiting myself to for this project.
  • Power Amp Design
    Next, I will need to come back to the Power amp and work out its design in greater detail than the current vague idea of "high-voltage push-pull complementary pair across a doorbell transformer, because lol". This is where I expect a lot of the discussion on exactly how and why tube circuits misbehave and which of those misbehaviors are musically useful and worth attempting to replicate.
  • Preamp Design
    Moving on to the preamp section, I expect even more time will be spent on how to get the transistors in this section to misbehave in fun ways, and how that misbehavior can be leveraged towards producing the kinds of voicing and character I want out of the amp.
  • Testing and tweaking
    There should be some testing and tweaking of each of the above bits as they get built, but I expect there's going to be a lot more once start I connecting them together. I should anticipate lots of time playing riffs into the amp, critical listening, staring at bizarre oscilloscope traces with a "WTF?" look on my face, and coming back here with my observations to get help with the Why and How behind those WTFs.
  • Build the cabinet/enclosure
    Honestly, this one gets into woodworking and cabinetry, which feels out-of-scope for this forum. If I remember to take pictures, I'll be happy to share them, though.

That's all I have for now. I am still working on transcribing the photos of my hackerspace's parts drawers. I figured I'd get this initial post up so I could get input on the basic design outline and the intended design process/sequence. I will have parts lists in my next post. They will be long. I am tempted to put them into tables for easier readability.[/list]
Maybe part of the problem is that you're looking at toroidal transformers? An EI core transformer, while bulkier, is easier to make, and thus costs less. Yes, a toroidal transformer is smaller and leaks less EMF noise, but they also have higher inrush current, and thus all other things equal, are more likely to blow fuses on power up than an EI core tansformer.
It sounds to me like that would work for a quick and dirty but functional power supply. Connecting the -12V from one transformer to the +12V from another sounds like it should work for a virtual center tap/common/neutral/whatever the appropriate term is.  However since I'm still in the learning the theory stage of power supply design, I'd sooner take the advice or opinion of someone who has actually designed and built a power supply over my own.
When speaking of this 3/2 law or 1.5 law, are you referring to the spacing, or the slope/curvature of the grid curves in a triode, or both?
Do the wall warts put out 12VDC or 12VAC? 12VAC, after being put through a full wave rectifier and a couple smoothing caps is closer to 16 or 17VDC. 12VDC has already been rectified and is just that, 12VDC. You an run a TDA2030 on 12VDC, but you won't have as much headroom or power output than if you gave it a higher supply voltage.
If it is saturation effects in the output transformer that plays such a large role in the 'valve sound', than I can't help but wonder if one could reproduce some of these effects or similar effect by employing a transformer in an SS design. I know Teemu's book talks about some early SS designs that employed transformers, but that is largely unnecessary know, since SS devices have such an incredibly low output impedance. The impedance matching function an transformer serves in a valve design isn't needed to drive the speakers in a SS design. I wonder if one could work some transformer magic with an appropriately built 1:1 transformer between the output device(s) and the speaker. I'm pretty sure I'd want to be careful with impedance mismatches. Unless I'm misunderstanding data sheets for various SS devices, under-loading them is a great way to destroy them in a hurry.

Another thing to add to the (growing ever longer) list of things to research and experiment with, I guess.
Thank you. I have read Teemu's book and attempted to read it. It's a hefty read in some parts, and as such, I've mostly only skimmed it thus far, but I agree, it is a good book.

So I promised to make a point about the nonlinearities of a triode. Such non-linearities are present in all tubes used in guitar amps, really, and I think it is safe to assume, all tubes. The above graph of output characteristics given for a pentode was for a 6L6GC.

Here's a KT88, used in output stage of the Ampeg SVT, and a few other amps.

Here's an EL34, used in many popular Marshall designs.

Here's the EL84/6BQ5, used in the renowned VOX AC30, and many other designs.

And I couldn't quickly find a graph for the 6V6GT, used in many lower output Fenders, but I can assure you it follows the same pattern. I do realize that the 6L6GC and 6V6GT are considered beam tetrodes and only the EL84 and EL34 are "true" pentodes. Fair enough. The operation characteristics of a pentode and beam tetrode are slightly different and this may account for part of the difference in sound between a 6L6 amp and an EL34 amp, but functionally, they serve the same purpose and will be considered functionally the same for now.

One thing I want to point out is the spacing of the input voltage curves. Towards the bottom of the graph, the curves bunch up and get closer together. Towards the top of the graph they spread apart. FETs do this too, but it is not as pronounced, IMO. Neither a pentode or a FET are the perfect, ideal component that is easily described mathematically, but tubes are far less so.

The curves on a triode do the same thing, as seen in this plate characteristics graph for a 12AX7 triode, one of, if not the most commonly used tube in guitar preamps.

Well that's nice, but what does this mean? This means, that the gain of the tube, pentode or triode, changes during operation, depending on what point it is operating at at any one instant. When the signal inside a tube swings positive, gain goes up until the tube maxes out and clips, and when the signal swings negative, gain is reduced until the cuts off  and clips. Notwithstanding negative feedback or any other tricks applied to a tube gain stage to make it more linear, any signal fed into the tube, no matter how perfectly symmetrical and balanced will not come out as perfect. It's like we have a compressor on one side of the signal and an expander on the other, playing tug-of-war with the signal. Furthermore, this change in gain as the signal swings from one end of the graph to another in itself isn't linear. The closer the valve comes to clipping on the positive swing, the mroe quickly the gain ramps up. The more quickly the valve comes to clipping on the negative swing, the more quickly the gain goes down.

To demonstrate this, I took the plate characteristics graph from a 12AX7 datsheet and plotted a load line for a 100k plate resistor, a B+ of 300V, and "center biased" with the grid sitting idle at -1.5V -- an arrangement similar to that found in many Fender amps. I took note of what the plate voltage would be at each grid curve, figured the size of the out voltage swing for each .5V step of the grid and the resultant gain. The numbers are below.

Vg is voltage at the grid. Va is voltage at the plate. DeltaV is the change in Va from it's previous value.
       Vg     Va       DeltaV   Gain
Clip   0.00 092.0036.0072.00
       0.50 128.00   33.2566.5
       1.00 161.2531.2562.5

AFAIK, nobody has attempted to replicate this nonlinear variable gain characteristic of a triode in a SS preamp circuit. For all I know, this characteristic may be unimportant when it comes to trying to get SS components to mimic the distortion. If nothing else, I think it would make for an interesting experiment to make an SS design with a preamp which mimics this specific characteristic of a tube, and compare it to an otherwise identical SS design that did not, and see if there was an audible difference. Perhaps an actual valve preamp that the SS design was mimicking would be a good comparison also.
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...'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. ( 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.
Amplifier Discussion / Re: Modifying a First Act MA104
February 08, 2010, 01:01:47 AM
Let the record show that I am blind. My multimeter was sitting on the shelf where I last placed it all along. I just hadn't stared at the exact spot it was sitting hard enough.

I took a measurment of the gain pot. Let the record also show that I am an idiot. The taper in the simulation was way wrong.

An oscilloscope would still be useful to have.
Amplifier Discussion / Re: Modifying a First Act MA104
February 07, 2010, 11:20:11 PM
So I figured out a few more things with LTSpice, and even found enough models of parts used in the MA104 for a simulation of the whole amp to run. Yay!  :) Thus, I have spent most of my weekend making tweaks to circuit in the simulator and staring in fascination at pretty colored graphs of the resultant waveforms and voltages in various parts of the amp's circuit. Good times. I also redrew the old schematic, because it was a pain to try to follow. The new one is attached. I didn't draw out the output stage because I can't find a model for the TDA2003 and the arcane art of making models from scratch is still beyond my ken.

I made a few discoveries. Unless the 4558 model made by Texas Instruments is innaccurate, the 4558 can't swing all the way to it's voltage rails, it clips hard about 2V before them. I was tempted to blame the model first, but when I tried other opamp models that came with LTSPice, they did the same thing, clipping hard a volt or two before hitting the power rails. So I guess it is a behavior of op amps. I didn't know op amps did that, but it is a good thing to know.

However there's been some differences between the simulation results and the results of plugging a guitar in and playing the amp that are puzzling me. According to the simulation, with the gain, volume and tone knobs cranked, the highest voltage swing that could happen at the preamp's output is about 350mVpeak. The TDA2003's gain is presently set to 10, and it is getting about 13V from the power supply. So, unless I'm Doing It Wrong, a 700mV peak to peak times a gain of 10, means the TDA2003 should have a signal of about 7V peak to peak on it's output, well below it's power rails, and shouldn't be clipping, ever. However, when I plug in my guitar and strum as hard as I can, I can hear a bit of breakup on the attack with the gain knob set at 3.

Now, my big 100W tube amp has it's input stage biased at 1.5V. So the hottest signal my guitar could possibly put out without overdriving the input stage and giving me dirt on the clean channel (which it doesn't) is 1.5Vp. I doubt even the bridge pickup puts out that hot a signal. Now in my simulation, a 1.5v signal with the gain knob set at 3 doesn't even cause the opamp to clip. The clipper diodes have been lifted from the circuit for now, the 2nd opamp is set at unity gain, and the TDA2003 is set to a gain of 10. So where the heck is this distortion coming from?  >:(

Some thoughts. My simulation is wrong? (likely). If that's so, I'm not sure where to start looking. Maybe I have the taper of the gain pot set wrong? (i.e. What LTSPice thinks is 70% rotation is actually 30% on the pot itself). The TDA2003's gain is set too low and the chip is unstable? (maybe?) The chip gets warm enough to feel that it's warm when the amp is running, but it doesn't get HOT like I've read about in these forums when a chip oscillates. The datasheet specs a minimum closed loop gain of 92.3, but stock, gain was 32.1, I don't think it had any problems being set that low.

I wish I could find my multimeter and take some measurements, instead of making blind guesses like this. An oscilloscope would be nice too.

I suspect those of you who know what you're doing and know how to use SPICE are probably rolling your eyes about now. Fair enough. But your input would be appreciated.
The Newcomer's Forum / Re: More datasheet questions
February 04, 2010, 03:29:09 AM
I know it's not a voltage question, but it is a datsheet question and I figured better here than to start a new thread.

What about the closed loop gain rating seen on the datasheets for various chip amps? For the TDA20xx series, it's specified with a min, typical and max in dB. Is the minimum gain specified on the sheet mean that's as low as you can set the closed loop gain and any attempt to set it lower will result in some internal configuration in the chip taking over and keeping it at the max closed loop gain? Or is it more like the max voltage ratings, IOW, go too low and you risk damaging the chip, having stability problems, or other badness? Or is this a thing which can vary from manufacturer/chip family/lunar cycle to the next?
Amplifier Discussion / Re: Modifying a First Act MA104
January 11, 2010, 01:57:16 AM
Success.  :)

C8 is now .047µF, and C10 is .0022µF. According to the simulation, this produces a notch -14dB down at about 330hz and spreading two octaves from center with the tone knob on 10. Perhaps not ideal, but it definitely gives the tone knob a usable sweep. I ran some more simulations and found that with C8 at .1µF, C10 at .001µF, and R7 at 6.8K (which I think is a standard value) I could make the tone knob approximate a Fender tone stack with the mid knob set at 2. I'm not sure how much more I want to play with the tonestack without changing to a better speaker.

Either way, the amp sounds much better and useful now.