So for a while now, I've been kicking around the idea of designing my own tremolo pedal.

Yes I know, good designs and schematics already exist on the net and I know that function generator ICs are pretty inexpensive and plentiful but I like to do things the hard way because I like the satisfaction of building stuff that's completely my own and, I guess, because I'm a sucker for punishment. So humour me on this one.

So the idea is to have 3 different waveforms to choose from: square, triangle and sine (and possibly sawtooth). There are probably more than one ways to do that but the plan is illustrated by this block diagram:



It starts with a square wave oscillator (probably a relaxation oscillator or possibly a 555) feeding into an integrator which should yield a triangle wave, feeding into another integrator which should give a sine wave (right?). Sounds pretty simple, except how do you design the integrators? More precisely, how do you choose the values for R1, C1 and R2? All the tutorials I came across need a university level math comprehension to understand - and I took (and passed) integral calculus in university! That was a couple years ago, though so it's all clear as mud now.



The only useful (to me) tidbit I could find was this:

QuoteTo produce a linear ramp on the output triangular waveform, the CR time constant of the integrator circuit should be similar to, or longer than half the periodic time of the input wave. In the case illustrated in Fig. 6.6.9, a time constant R1 x C1 (10exp3 x 22exp-9) = 220µs converts a 1kHz square wave with a periodic time of 1/2exp3Hz = 500µs/2 = 250µs into a reasonably linear triangular wave.

http://www.learnabout-electronics.org/Amplifiers/amplifiers66.php

There were some example circuits out there (didn't save the links unfortunately) for quick-and-dirty signal generators based on the same idea but they all work at high frequency. What if you want to integrate a very low frequency tremolo wave? Say 1 - 15 Hz? Do the values of R and C become so large as to be impractical?

Playing around with LTSPICE, I couldn't get a triangle wave out of a square wave even with stupid large values of R and C.

Any ideas or am I just barking up the wrong tree with this one?

Hey everyone,

So back in September I completed my first tube amp and I've been in love with it ever since <3) I put about a year and a half of work into designing and building it and to my surprise and delight I got sound out of it the first time I plugged a guitar into it!  :dbtu: The tone is pretty decent for a first try and I've been playing it and tweaking it ever since. Once I got I finished tweaking I planned to make a youtube video to show it off but instead I started having problems with it in December.  :grr

After a longer session I noticed the sound faded right out once or twice. I thought it might just be blocking distortion (although it didn't sound farty) but as I couldn't find a problem with it, I continued playing it. Then one day I switched it on and the mains fuse immediately blew.

So I pulled the power tubes, replaced the fuse and tried it again. Again the fuse blew.
So I pulled all the tubes, blew another fuse.

After hours of probing with a multimeter and not finding any shorts I started to worry about my power transformer. So I desoldered all the secondaries and terminated them (except for the HT which I can disconnect with standby switch) and tried it. The fuse survived.

So I resoldered everything, double checking for shorts and this time ran the amp through a dim bulb current limiter. It powered up. I plugged a guitar in and it sounded just like it used to.

Then I tried it straight into the mains without the dim bulb and immediately blew the fuse.

I don't know what's going on. I've gone through half a dozen fuses now trying to test with only one winding. It seems like it blows it with any one winding connected. Is my power transformer screwed? I have a slight temptation to pop in a larger fuse and see if anything gets crispy but I can't afford to fry that transformer as there's no way I'll get another one like that.

Here is a link to the schematics: https://dl.dropboxusercontent.com/u/3421497/miniplex.pdf

Sorry but they're hand drawn. I haven't found a program I like enough to make a more formal schematic. It's F1 that keeps blowing btw. It's worth noting that I never blew any fuses on the secondaries, always the primary fuse.

So I'm working on a tube amp project and I bought some E-Ten 1222 DPST switches on eBay for the power and standby switches. According to the website, they're supposed to have a contact resistance of < 10 milliohm but when I measure my switches (I bought 4) with my multimeter I get much more. Most are in the ballpark of 10 - 30 ohms or so and one of them measured more than 100 ohms! That's going to make for some significant losses when I put 120v across them.

To make sure I wasn't going nuts I tested some other switches in my parts bin. None of them tested over 50 milliohms in the on position.

So are these switches junk or are they usable?

And yes, I know that you get what you pay for with cheap Chinese parts but switches are fairly simple devices and I figured it would be hard to screw up a switch. Maybe not...

Hi,

I asked about heavily overdriving a BJT a few days ago and now I have a (probably dumb) question about heavily overdriving a JFET. So I'm simulating this JFET common source amplifier with a voltage gain of 10 (picture below) and I'm driving pretty hard (10v p-p). The green signal is the input waveform, the blue waveform is taken on the gate of the JFET (after the 500k resistor) and the red waveform is the output, taken at the 1 MEG load resistor.



You can see the signal is clipped on the output and partly on the INPUT as well because the voltage forward biases the JFET junction when the signal swings upward (unless I'm mistaken).

Is this a problem? I've been wanting to design a little practice amp for myself and I've been trying to decide whether to use JFETs or BJTs for the preamp and I can't decide whether or not I have a problem with this or not. I basically lose almost the whole top half of the signal. And I've read asymmetrical clipping causes a strong 2nd harmonic and I don't know if that's what I want.

I was hoping to design the preamp by stringing together a bunch of gain stages like this one (common source or common emitter) - like a tube preamp. I realize I could just use an op-amp and clipping diodes and not have to concern myself with this stuff but (for me, anyway) that's no fun. I know it sounds dumb but I like to do things the "old fashioned" way, I guess.

Hey,

I'm a real electronics noob. I was really into it a couple years ago and I sort of put it on the back burner and now I've been slowly getting back into it - starting with the basics: simple amplifier circuits. I have some NPN BJTs and I want to use them to amplify a guitar signal and I've been reading tutorials and simulating some basic circuits in LTSpice and I have some questions.

Fig 1:

So I have this common emitter amplifier pictured (fig 1) and it works just as expected. The quiescent collector voltage is 6v (1/2 of my Vcc) it has a voltage gain of 5. I'm feeding it a sine wave with amplitude 1v p-p and I get a sine wave 5v p-p at the collector. So far so good.

Fig 2:

So I increase the input signal to 2v p-p (fig 2) and it starts to clip asymetrically. I thought transistors were supposed to clip symmetrically. The bottom half of the signal clips at around 2v but the top half is undistorted.

Fig 3:

When I increase the input signal amplitude a lot, to 10v p-p (fig 3) the output starts to get REALLY ugly. The top half of the signal clips at 12v just like I would expect but the bottom half of the signal gets these "humps". I've circled them in red. What is going on here? I can't imagine it would sound good. Also the "width" of the waveform seems to be distended, if that makes any sense.

Fig 4:

Now if I bypass the emitter resistor (fig 4), the "humps" go away (and the voltage gain shoots way up as expected) and the shape of the waveform changes even more. It looks a square wave with a very large duty cycle. I guess that would make sense if it was clipping very assymetrically but my bias point doesn't change. It stays centre biased regardless of the amplitude.

I was hoping to design a simple amplifier with a few cascaded BJT common emitter stages to get plenty of gain and distortion but I didn't expect the transistors to behave this way. I actually built a small amplifier with JFETS (1 stage common source unbypassed feeding into a JFET mu-amp) and I didn't get this effect. It clipped nicely and I was actually surprised at how good it sounded. Am I doing something wrong here? I admit I'm a newb and I've had a harder time wrapping my head around how a BJT works.

Thanks in advance for your help.

I'm fairly new to electronics and I've been building and fooling around with basic circuits. Mostly common source amplifier circuits using JFETs to create distortion but the sound isn't all that pleasant. Kind of like the song Revolution by The Beatles. My best guess as to why is that I didn't really design the circuit to filter out any frequencies pre-distortion so what I'm hearing is ugly harmonics from the lower notes but that's just a guess. I don't have a scope to test it.

So my question is: How do you get a pleasing tone out of a clipping gain stage. I'm aware that's pretty subjective but generally speaking.

And also, how do the commercial amp manufacturers usually do it? I've looked at schematics but my knowledge of filters so far is still pretty basic. Most solid state amps seem to use active filters with complex networks of components in the feedback loop and I can't really wrap my head around what they would do to the signal let alone work out the cutoff frequencies. I realize that different amps have different tones and all but electronically they still use variations of the same "building blocks".

Any help would be most appreciated!  :tu:

Hey everyone. I'm still pretty new to electronics but I think I'm starting to get it. I designed and
built myself a little 9v battery powered practice amp as a first project and I'm having some trouble
with oscillation, I guess. I'm hoping someone here can help me out. It sounds pretty good a low to medium gain but as I turn it up more it develops this fairly high pitched (still easily in the audible range though) noise which sounds kind of like a ring modulator effect which just kills the tone.

I don't have a scope so I can't tell what the frequency is or exactly where it orginates. It doesn't happen in LTSpice but real life is often different from a simulator as per Murphy's law of :p . I think it comes from the second gain stage as turning the tone pot to the bass side reduces it significantly. Actuall just touching the tone or gain pots changes the frequency of the ring. If I touch the input jack or even the surface (not the traces) of the PCB the ring stops but the gain is also much reduced which I've never seen happen before.

I built it on a fairly generic piece of prototype PCB and I'm just bench testing it right now. I did try putting a metal container over it to shield it but it didn't change the sound. I've attached a schematic. Hopefully someone can provide some insight.

Any help is much appreciated!