Is this a good way to simulate the sound of tubes??

Hi,

I've just designed this schematic, and I'm wondering if this is a good method to simulate the warm sound of tube amps. I've designed it for someone who has an electric guitar but no tube amp, so I ask you guys if this is a good method to make a "tubebox" before I'll deliver it to him.

with R3 you can adjust the "tube-distortion" and with R5 you can adjust the gain.

Thanks,
HB.
 

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Hi,

I don't think you'll be satisfied with the sound of this schematic..

The output of a typical guitar will be around 200mV transient for humbuckers and about 100mV for single coils, so the diodes won't reach conductive state...

I can recommend the www.harmonycentral.com site! look for effects and then look for schematics... and a world of schematic information will be at your disposal..

personally I believe that the compression and bias shifting in tube amp is a key-factor of their signature sound.. the best way to accomplish that is by using single ended capacitive coupled stage without feedback ... try a cascade of N-JFET's, biased sub-optimal... reduce low-frequency transfer (to reduce intermodulation) to about -40dB at 1Khz and cut highs after the stage from about 1KHz ... for a nice sustained sound you'll need something like 40dB gain beyond headroom...

goodluck!

gr,
Thijs
 

MRehorst

Member
2002-05-17 8:48 pm
Try simulating this circuit in SwCADIII and dump the output to a .wav file. Then you can listen to it and know what it will sound like.

If you want to go all-out, get any .wav file (rip it off a CD) of a guitar and use it as input to your circuit. Then you can see exactly what the circuit will do to the sound of a guitar.

MR
 
Hey thanks guys,

Nelson, I was wrong: the schematic is not to make it sound like tubes, but to simulate a tube amp running into saturation (you'd better not try this with transistor amps huh, unless you are a squary heavy metal freak or something ;) )

tschrama, yes I know that the schematic won't work with that low voltages, I've forgotten two resistors in the input stage, it must be an amp with gain x20 instead of a buffer.

thanks for SwCADIII tip, I'll first give it a try with that.

Best regards,


HB...
 
hmmm, tubes and fets, unfortunately I've never experienced with them before so I think that it will be something for the future.
That's the negative point about my education: I'm studying electronics but we've never seen anything about tubes and almost nothing about fets etc... But we've done a lot of researche and experiments with transistors and opamps, so schematics with transistors won't be a huge problem to me.

I'm going to get myself a good book of the local library here and start reading articles about fets and tubes, and then I'll try some liitle experiments to get started with them.

I remember something about fets: is it true that you may never touch the gate, otherwise it will be damaged because of it's very high input impedance or something like that. Is it true that a little wire is connected between the pins of fets to protect them in case that they aren't placed on a pcb?
Can anybody confirm this because I'm not sure about it...

HB.
 

MRehorst

Member
2002-05-17 8:48 pm
That is MOSFETs that are particularly sensitive to ESD. JFETs aren't quite such a problem, but they and all other semiconductors should be handled with care.

Wait until you find out out depletion modes FETs. You must apply gate voltage before drain voltage otherwise the thing will blow up (all right, burn up) (well, OK, self destruct).

Why are all the terms for excessive current conduction leading to the permanent destruction of the device so violent? Its really such a peaceful, quiet process...

MR
 

MRehorst

Member
2002-05-17 8:48 pm
SwCAD is a good free tool, and as far as I know, the only one with .wav I/O.

I use PCAD2001 for schematic capture/board layout otherwise.

I also have an old copy of Electronics Workbench. Easiest user interface I have ever seen, but wires tend to place themselves in odd places, so it also has a "neatness" of the schematic problem.

Use the tool that fits the job...

MR
 
I'd use JFETs for the compression. They're field effect devices like tubes, so they'll probably be a better stand-in. Two connected as current limiting diodes placed head to head will be transparent to small signals, and only the source resistors will be seen by the op-amp. As the voltage increases the FETs will pinchoff the curren. The input impedance increases and the signal is compressed.

Now let's see if I can attach an image...
 

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hugobross said:
thanks, MRehorst.

another question: I've just downloaded SWCADIII and tried some things, do you really like this program??, it takes a long time to draw a neat schematic in comparison with the program I use (circuitmaker - www.circuitmaker.com). I think you should have a try on this program, you'll like it :).

best regards,

HB.

Circuitmaker's simulations are in many cases highly inaccurate; that is if it even manages to run thru without some error. It's not bad for drawing schematics (but believe me, there are better things available) but I would never trust it with a simulation.
 

MRehorst

Member
2002-05-17 8:48 pm
I have not used circuit maker, but all spice simulation results are only as good as the models and the experience/knowledge of the user. Some software companies may loosen up some of the default tolerances to make the simulator converge better on nonlinear circuits- I'm sure the marketing department doesn't like to hear complaints about simulations not converging;), but generally, spice is spice (they are ALL based on the source code from UC Berkeley). If you know how to use it properly and know what its limitations are you should be able to get reasonable results.

Attn. spice beginners: do you want to make an easy test of some of the limitations of spice? Make a circuit that consists of a 1 Hz sine wave voltage source and a resistor. Run the simulation for 2 or 3 cycles and force the maximum time step to be 1e-3, 1e-4, 1e-5, 1e-6, 1e-7. Do an FFT on the result and look at the output spectrum. What you will see is that there will be distortion products whose level decreases as the time step decreases. Of course, a real sine wave has no distortion products- the ones you see are artifacts of the "noise" in the math used to generate the sine wave and display its spectrum. This is one simple test that illustrates one basic principle- the longer you simulate (on a per cycle basis), the better your results will be- i.e. there is no substitute for long simulation time!

MR
 

SteveG

Account Disabled
2002-01-07 7:20 pm
Newton Falls, Ohio
I definitely have to agree with Nelson on this one- it's just not possible to get "tube" sound out of a transistor. You may be able to come close to it with a Jfet design- could be a good compromise. I take it this is for a stomp-box type effect? If you have more room, why not try real tubes... it's about as easy as can be to set up a couple cascaded 12ax7 stages. Or, another option could be a transistor gain stage (keep it clean!) driving a tube running at low voltage (starvation mode) for the distortion characteristic. The only problem with that is you get more odd harmonics, and less of a "real" tube sound. This is desireable in some circumstances- it gives a crunchier sound. As far as op-amps go- I have found them to be somewhat noisy- kind of a bad match for passive pickups. Jfets would be a better way to go, imho (next to tubes). Just my .02
SteveG- (btw, I play guitar)
 
Hi,

I reply because the discussion about good guitar tone tends the focus on the harmonic content of the signal. I do not think the secret of good quality tone lies in the distribution of harmonics. You can easely mix 2nd harmonic into your signal with a 'octave-pedal' but it will never sound like a overdriven tube amp. I have seen no evidence at all that 3rd harmonic should be avoided or 4th ... and I know that some amount of higher order distortion gives you that high gain 'wall-of-guitar' effect which can be highly desirable.

Good quality tone has more to do with compression/sustaining effects, quite possible from acoustic feedback that automatic comes in to play when you crank a tube amp and speaker-non-lineairiies than harmonic-content.

gr,

Thijs