The way Dumble's overdrive...

[Tdale]

As far as I understand, the first half of the OD tube distorts one half of the signal and the second half of the OD tube distort the other half.

I suppose it's a good idea to adjust the amp so that they distort as similar as possible.

But what is it that makes it work this way? Why is only one half distorted? Is it the bias point of the tube that is set in such a way that only one half gets clipped?



Tommy

[heisthl]

Without getting into gain and current here's the simple explanation: A tube is just a variable resistor imitating what you feed it on it's grid. Unfortunatedly if you feed it a bigger signal than the bias it cannot imitate and flattens 1/2 of the wave, and if it's WAAAAY too big it will distort both the positive and negative halves of the wave(Disclaimer: not a completely accurate statement because the bias changes with conduction). If you try to counteract this by raising the bias it quits imitating part of the wave until the input raises the grid enough to overcome the cutoff. A well biased tube in a proper design should have sufficient headroom to not distort at all, unless you put a little bit too big of signal on the grid - when you do one half will distort first.

[llemtt]

Tommy

unfortunately you are not right, take 12AX7 plate curves then draw the dynamic load line (different from static one!!!), then consider bias shifting due to bypassed cathode and you will see that clipping occurs sligthly asymmetrical only during initial few milliseconds then, due to bias shifting, the output becomes clipped symmetrically and the duty cycle becomes slightly asymmetrical.

BTW it's easier to use a spice simulation...

teo

[Bob-I]

I'm to blame for this thread as I noticed this behavior on the scope and told Tommy about it.

It was very clear on the scope that OD1 clipped the upper side of the wave, then OD2 clipped the top again, of course inverting the signal so that both halves were now clipped. Adjusting the trim allowed me to clip both halves equally.

[llemtt]

Hi Bob

mmm... very strange, signal on my scope is almost identical to simulation (as expected) and it's a little different (see above)

how do you take these measurements? do you inject a sine-wave at the entrance of the OD section? where do you take the output signals, after the coupling caps?

when you say "clipped the upper side of the wave" do you refer to the input or output signal? (they are 180°!!)

Referring to the input signal sounds ok because with "usual" plate/cathode resistors ratio (66.666...) the tube is biased more towards saturation (upper side of input signal) than cutoff, but the other way round sounds really strange.

Anyway static bias asymmetry isn't huge because is just 2 volts towards saturation and 2.5-3 volts towards cutoff, then with an input signal of at least 10v pk (20v pk-pk) at the entrance of the tube (before grid resitor) you should clearly see your bias shifting towards cutoff: can you measure this at the cathode? [there's another source of bias shifting due to grid current like in power amps but it's difficult to measure]

Clipping only one half of the signal can occur only with a very limited input signal, more than the bias voltage (say 2v pk as above) and less than cutoff voltage (2.5-3v pk), at usual settings (i.e. drive at 5) you should have at least 50v pk at the entrance of OD2 without OD1 clipping!

teo

[Bob-I]

how do you take these measurements? do you inject a sine-wave at the entrance of the OD section? where do you take the output signals, after the coupling caps?

After the coupling caps

when you say "clipped the upper side of the wave" do you refer to the input or output signal? (they are 180°!!)

Good question, I honestly can't remember but I believe it was the lower side after the gain stage, so I said "upper" refering to the input.

Clipping only one half of the signal can occur only with a very limited input signal, more than the bias voltage (say 2v pk as above) and less than cutoff voltage (2.5-3v pk), at usual settings (i.e. drive at 5) you should have at least 50v pk at the entrance of OD2 without OD1 clipping!

teo

I ran about a 100mv sine input at 400HZ. My goal was to balance the clipping on both sides. There was an article I read quite awhile ago that made the statement that 1/2 of the wave was clipped with each stage, and showed scope traces that supported it. I adjusted the input until I saw this and tweaked the trim until I saw a balance between the 2 sides with the drive at 50%.

I'll have to open up that amp again and check it out.

[glasman]

I keep thinking in the back of my mind that it would be interesting to run a sine wave into the OD just to the point of clipping and thing look at the frequency spectrum for the harmonics. It may be possible to correlate the harmonic response to ones "ears" .

I think I will try this next time I have an amp open.

Gary

[llemtt]

... and thing look at the frequency spectrum for the harmonics...

Yes good point I tried this in the beginning, it's a good starting point to understand lot of things, unfortunately it can't give all the answers because a guitar signal is very different from a sine wave.

Initially I thought, like many, that even harmonics is good and odd is bad but I soon find out it wasn't so, a correct balance of the harmonic spectrum is a better view but my final recipe for a good and smooth overdrive is another: avoid higher order harmonics and absolutely avoid "non-harmonic" distortion such as the ones usually called Intermodulation Distortion.

To achieve that you essentially need at least one thing: the signal before getting into hard clipping has to be already "rounded", no steep waveforms or spikes. How to do that?
Easy solution: cut all the highs before clipping -> result very smooth but...
Good solutions: up to amp builders invention and taste
Final solution: still to be found

teo