# Why do tubes compress?

Discussion in 'Amps and Cabs [BG]' started by Texx, Jan 8, 2005.

1. ### Texx

Sep 10, 2004
Germany
In my sansamp BDDI manual it says when push the it to saturation
it will act like a compressor and limit transient peaks just like real tubes would do.
So is a tube only compressing when its overdriven? And overdrive =compressing ?

2. ### nonsqtrThe emperor has no clothes!

Aug 29, 2003
Burbank CA USA
Good question, but it sounds like you need some theoretical background before the answer will make sense.

Compression is basically a "reduction in volume", but it s a specific "kind" of reduction, and it doesn't equate with overdrive. But, compression can be an "artifact" of overdrive, especially with tube amps.

Yes, a tube should only be compressing when it's overdriven, and

No, overdrive does not equal compression.

Overdrive is typically compression "plus" a lot of other stuff, like changes in the harmonic structure and so on.

"Clean" compression is just a reduction of gain through the amp, which is something very different from overdrive.

Suggest you check out the Rane site, there are some white papers that explain these concepts in detail.

3. ### _Unregistered_

Nov 3, 2004
The sudden demands placed on the tube by a large transient can cause a drop in voltage in the tube. The tube is then unable to deliver the transient at full peak resolution. It thus effectively "softens" the output of the transient, creating a form of compression.

4. ### Thunderfunk

Mar 27, 2004
McHenry, IL
A preamp tube has a resistor from it's cathode to ground. When current flows through this resistor (even with no inpout signal) the cathode becomes more positive which is the same as making the grid more negative. This is the equivalent of the "negative fixed bias" voltage applied to the power tubes. As an input signal raises the grid voltage more positive, more current flows through the cathode resistor and therefore the cathode becomes more positive. In effect it's tracking the level of the input signal. As the cathode becomes more positive the grid in effect becomes more negative, lowering the input signal's effective voltage on the grid. Thus a naturally occurring form of compression. The same thing happens with transistors.

5. ### _Unregistered_

Nov 3, 2004
Yeah, what he said. I knew someone here had to know the technical reason behind the voltage drop...

6. ### Tim__x

Aug 13, 2002
Isn't that just local negative feedback? Not compression. Compression by definition is a reduction of dynamic range, reduced gain on high level signals normal gain on low level ones.

Overdrive does compress, but as nonsqtr said, it also does alot of other things.
Compression is also more than just a "reduction in volume", simply put, it makes loud sounds quieter and quite sounds louder.

7. ### Thunderfunk

Mar 27, 2004
McHenry, IL
Yes it is local negative feedback, and negative feedback reduces gain, but it's compression because it has more gain reduction as the signal gets louder. And yes, it tracks the signal, which then also makes the quiet passages sound louder. Don't get confused thinking that someone turns the volume down on all the signal when it gets loud. It tracks the signal and varies the gain as it passes through which "normalizes" the signal level. It's compression, and not limiting.

8. ### Tim__x

Aug 13, 2002
But the amount of gain reduction is not level dependant. Yes, the voltage across the cathode resistor does go up as signal (and cathode current) goes up, but the ratio between signal and the voltage across the cathode resistor stays the same so long as the tube is in it's linear region.

What you describe is simply distortion, a nonlinear transfer function. If things worked exactly as you say they do, it would be impossible to make a low distortion tube stage without global feedback, but it is possible.

9. ### Texx

Sep 10, 2004
Germany
Wow, thats becoming highly scientific. Now it´s a little clearer to me. Thank you guys, this forum´s so much better than the crappy
bass forums in germany.

10. ### Thunderfunk

Mar 27, 2004
McHenry, IL
I can see what you mean. It's just a lower gain being produced linearly. So then the "compression" being referred to is soft knee clipping? And the clipping comes from where?

11. ### notanaggieGuest

Sep 30, 2003
Tubes aren't very linear. They actually produce a reasonable amount of distortion, and it is more at larger signals than at smaller, typically.

The distortion is related to the gain being less at large signals than at small. So if the tube has a 3 volt signal, it will have a given amount of distortion, whatever it is. At 10 volts, a bit more. At 20 volts a bit more again.

So, if the signal is 30 volts rms, it starts at zero, passes through 3, 10, 20 and 30 volts on its way to the peak voltage of a bit over 40 volts, and then back down thru 30, 20, 10, and 3.

The gain varies as the voltage varies. At 3 volts, the gain is larger, and distortion at that moment is smaller. At 10 volts, the gain has decreased a bit, and distortion is larger. At 20 volts, same thing...etc.

The effect is to "squash" the peak voltage a little compared to what it "should" be.

It is essentially a variable gain and increasing distortion as the signal waveform goes up in voltage, and the reverse as if comes back towards its "resting point" (where it is with no signal).

Notice, I mean this occurs on each cycle of the signal. That is why it acts like compression, or really a "soft" peak limiter to a certain degree.

You can design a circuit to do this more, or to do it less. Depends on what you want.

Tube distortion is this same effect, only with the tube driven to the limit, so that it gets the signal as squashed and distorted as it can before it runs out of power supply voltage, and is actually clipped. Distortion amount and harmonic number gradually increase with level up to clipping and even after clipping.

The equivalent SS amp uses feedback to stay linear, and is very linear until clipping, and then instantly gets to be very distorted. There is no gradual increase of harmonics, and of harmonic number. First very low distortion, then suddenly high distortion including the 10th and 20th and even higher harmonics....not very musical.

Aug 13, 2002