Author Topic: Preamp and Poweramp distortion (Read 2986 times)

BloodMountain · « **on:** April 10, 2007, 07:14:54 PM »

hi
i have been thinking about this for while now:

what is each of these and how would you obtain it?

and i'm presuming it only works for valve amps?

thanks dudes :twisted:

Pierre · « **Reply #1 on:** April 10, 2007, 07:31:05 PM »

You get preamp valve distortion by simply running them hot. So basically... with one dual triode standard valve like a 12AX7, one triode into the other produces very little distortion. Hence why early amps had few gain stages.

Cascading gain amps use a multitude of 12AX7s, each with two gain stages. So amps like a JCM800 which have I believe 4 gain stages already become way more overdriven in the preamp stage.

To overdrive your power valves, you need to send them a strong signal. So in older, high headroom Fender amps, you had to run the volume all the way up. The 'volume' control is what we would now call gain, our 'volume' being placed usually a little after along the signal path.

So you'd get your clean sound louder and louder, introducing only negligible preamp distortion. After a while your power valves will distort.
Power valve distortion is less thick and more harmonically rich. It preserves note clarity. Think AC/DC. Against say, Metallica ;)

In THEORY it works with transistor amps as well.

For more modern amps... The power stage is usually designed to be pretty neutral and offer as high headroom as possible. So it's harder to overdrive the power valves. And obviously you need your amp WAY louder. But it does brings out a little more gain. But those amps are built so that the distortion comes from the preamp.

Hope this helps :)

BloodMountain · « **Reply #2 on:** April 10, 2007, 07:35:26 PM »

what do you mean by running them hot?

and do you get poweramp distortion by turning the gain knob up??

i'm confused.

dave_mc · « **Reply #3 on:** April 10, 2007, 07:42:53 PM »

simple (probably over-simplified) answer:

preamp distortion by turning up the gain knob.

power amp distortion by turning up the volume.

BloodMountain · « **Reply #4 on:** April 10, 2007, 07:48:22 PM »

Quote from: dave_mc

simple (probably over-simplified) answer:

preamp distortion by turning up the gain knob.

power amp distortion by turning up the volume.

thanks for clearing things up!

and thanks for the original post, Pierre, a re-read almost helped me understand :lol:

so high volumes on my MG/Valvestate 8100 wouldn't improve the tone?

indysmith · « **Reply #5 on:** April 10, 2007, 07:50:43 PM »

Quote from: BLOODMOUNTAIN

Quote from: dave_mc
simple (probably over-simplified) answer:

preamp distortion by turning up the gain knob.

power amp distortion by turning up the volume.

thanks for clearing things up!

and thanks for the original post, Pierre, a re-read almost helped me understand :lol:

so high volumes on my MG/Valvestate 8100 wouldn't improve the tone?

well at first it will, as it will get the speakers moving nicely as they are designed to, but after that it will get worse, as transistors do not distort in a pleasing way (a lot harsher than the smooth breakup of a valve)

BloodMountain · « **Reply #6 on:** April 10, 2007, 08:10:09 PM »

ok thanks dude!

Pierre · « **Reply #7 on:** April 10, 2007, 10:09:13 PM »

Yes he's right on the money. If transistors ODed the same as valves, valves wouldn't exist anymore. Unfortunately indeed it's not the case. And now I do realize my explanation was maybe a little too technical hehe sorry about that :lol:

But yes he's right. Essentially a pot is a variable resistance.

So the more open it is, the more it lets through.

The more it lets through, the hotter the signal is. The harder the following valve is pushed.

A triode is a valve that works with plate/grid/cathode. Input, output and bias. Bias selects how hot it runs. Input voltage also does that. Usually the higher voltage a valve gets, the more headroom you have before it may start distorting.
12AX7s are dual triodes. This means that each 12AX7s is in fact, TWO valves. So that's 6 pins. The remaining 3 are heaters: Like a lightbulb, you gotta heat the cathode in order to provoke the flow of electron and after that, the amplification itself.

BloodMountain · « **Reply #8 on:** April 10, 2007, 10:41:25 PM »

ah, i see! thanks!

i have an 12AX7 in the preamp of the distortion channel of my valvestate.... would turning up the volume of this channel give this effect (just less obvious)?

Pierre · « **Reply #9 on:** April 10, 2007, 10:55:46 PM »

No. This valve is basically an expensive lightbulb. It does nothing for the circuit but act as 2 of many gain stages provided by transistor. I doubt it has any noticeable effect on the sound at all. What you hear is probably 80% trannies.