Preamp tube clipping. Top vs bottom of wave.

[JJman]

If a preamp tube clips asymmetrically, is there any difference in the sound of it saturating (clipping the top of the current wave) vs. reaching cutoff (clipping the bottom of the current wave?) Assuming the same input signal and %age of the wave being clipped.

I ask because I have a 3rd stage that is clipping at 75% volume (as I want it too) but it’s all via cutoff. I’m wondering if I should re-bias for more symmetric or perhaps saturation-only clipping.

 

[ClinchFX]

It's a really good question and I'd like to see an answer from one of our resident tube wizards. I may be wrong, but I'd expect saturation clipping to sound smoother than cutoff clipping.

 

[Blue Strat]

Assymetrical clipping produces even harmonics...the kind we tend to like. Doesn't matter which half of the wave is clipped, the results are the same.

Since you're asking, I recommend experimenting with every type of clipping to see which sounds best to you.

When you say "saturation", what exactly do you mean?

 

[ClinchFX]

When you say "saturation", what exactly do you mean?

Mike, I take saturation to mean that the tube is turned on as hard as possible. I've found that this doesn't clip as harshly as cutoff. I have just realized, however, that, in a basic common cathode stage, saturation will clip the bottom of the output (plate) waveform, because plate voltage has bottomed out and can't go any lower. When the tube stops conducting (cutoff), plate voltage rises to maximum and can't go any higher, causing the top to be clipped.

Having said all that, I agree that it doesn't matter which half of the waveform is clipped. What matters is whether the clipping is a hard square wave or so called "soft" clipping.

Peter.

 

[stevel]

Assymetrical clipping produces even harmonics...the kind we tend to like. Doesn't matter which half of the wave is clipped, the results are the same.

Since you're asking, I recommend experimenting with every type of clipping to see which sounds best to you.

When you say "saturation", what exactly do you mean?

How does one experiment with the different types?

Steve

 

[Wakarusa]

How does one experiment with the different types?

Steve

One employs an oscilloscope and, if available, a spectrum analyzer. I'm not sure that I agree that both cutoff and saturation produce the same results. For starts I think it depends a lot on the tube and the circuit where it's happening. You can get odd distortion products in the clipped output that differ between the two situations. Also, with saturation (again, depending on the circuit surrounding the tube) you can get grid conduction that leads in some cases to grid clamping which is a very different result than cutoff.

 

[VaughnC]

While I don't think the ear would be able to detect which half of a waveform is clipped, with asymetrical clipping (especially) the speaker polarity will affect the final outcome...so make sure you try the speaker polarity both ways and let your ear decide what's subjectively better and not you see on an oscilloscope. I use my scope as a guide to tell me what's going on in the circuit but my ear makes the final decision.

 

[Wakarusa]

I use my scope as a guide to tell me what's going on in the circuit but my ear makes the final decision.

Just to be clear, I wasn't suggesting for a moment to leave your ears out of the equation The o'scope and spectrum analyzer are to help understand what you hear.

 

[JJman]

Thanx for the comments. This amp distorts in the output and PI before the subject stage so this preamp stage’s cutoff clipping is late on the volume dial and cream on the coffee. It is definitely a “hard” square clip. The cathode uses a 10k and the plate a 100k. This seems to me to be designed to reach cutoff since it’s biased “cold.” I probably leave it as is. The preamp is an Express design.

Top wave is the 3rd stage’s plate voltage. Invert it for current.

 

[stevel]

Thanx for the comments. This amp distorts in the output and PI before the subject stage so this preamp stage’s cutoff clipping is late on the volume dial and cream on the coffee. It is definitely a “hard” square clip. The cathode uses a 10k and the plate a 100k. This seems to me to be designed to reach cutoff since it’s biased “cold.” I probably leave it as is. The preamp is an Express design.

Top wave is the 3rd stage’s plate voltage. Invert it for current.

So what am I seeing here JJ?

The top wave is an asymmetrically clipped sine wave, with the top half being clipped turning that into a "square wave" shape, right?

So this is telling me it's producing distortion, correct?

And are you saying that those 10k and 100k values for example can be changed to vary the way in which, or the point at which the wave gets clipped?

Curiously,
Steve

 

[CitizenCain]

stevel, here's a nice little website that has a lot of good info on designing tube amps. The first link, "Triode Gain Stage" has a downloadable PDF file that goes into good detail on picking plate/cathode resistors for biasing the stage, and how they affect whether it's cold or hot and moving to cutoff or saturation, etc, etc...

http://www.freewebs.com/valvewizard/index.html

 

[JJman]

So what am I seeing here JJ?

The top wave is an asymmetrically clipped sine wave, with the top half being clipped turning that into a "square wave" shape, right?

So this is telling me it's producing distortion, correct?

And are you saying that those 10k and 100k values for example can be changed to vary the way in which, or the point at which the wave gets clipped?

Curiously,
Steve

Yes, yes and yes. (AFAIK)

 

[stevel]

Yes, yes and yes. (AFAIK)

Thanks!

 

[stevel]

stevel, here's a nice little website that has a lot of good info on designing tube amps. The first link, "Triode Gain Stage" has a downloadable PDF file that goes into good detail on picking plate/cathode resistors for biasing the stage, and how they affect whether it's cold or hot and moving to cutoff or saturation, etc, etc...

http://www.freewebs.com/valvewizard/index.html

Thanks for the link - it will take me a while to digest but this will help me out a lot.

Preesh,
Steev

 

[scottl]

Thanks dude!! Excellent link.

stevel, here's a nice little website that has a lot of good info on designing tube amps. The first link, "Triode Gain Stage" has a downloadable PDF file that goes into good detail on picking plate/cathode resistors for biasing the stage, and how they affect whether it's cold or hot and moving to cutoff or saturation, etc, etc...

http://www.freewebs.com/valvewizard/index.html