Help understanding tube harmonic content

[dakotadave]

Ok, I am trying to understand what I am hearing, and how to replicate it, adjust it, enhance it, play with it etc. When I crank up a large tube guitar amp i.e Marshall, Fender etc, something is happening in the power amp side that I love the sound of but cannot seem to achive by any other method. Is it rectifier sag? Is there some weird electromagnetic interaction between the transformer and the tubes? Does anyone one know what I am talking about? It is a large dark sound, very smooth but not the creamy heavy compressed sound of cranked pre-amp tubes. It just has a body and depth and a rich thick spanky kinda quality. No it is not speaker cone break up that I understand ! Those lil amps that use pre-amp tubes don't have it, EL84's seem unable to produce it. 5881's seem prone to it as do some 6550's. Is it the large beam in some tubes? Anyway what I want to know is.....How do I get it at low volumes? No I dont want to use a power soak or thd hot plate or even a marshall power brake, cause all of them seem to reduce this strange thing I hear and love so much ! Can't I just build something that would be rich in that sound? What is "that" sound ?
Anyone ?

[Cal Weldon]

Hi Dave and welcome to the forums. I can't help you with the amp question but there are many on board who can.

I would suggest you post this in another forum as not everyone looks here in the Introductions. Hope you get an answer.

[poobah]

When you say spanky... that implies power supply sag.. and that does occur more as you hike the volume.

When you say dark... that implies low frequencies... you might be appreciating the lower frequencies more at higher volume because you are hearing them in greater proportion. Check out Fletcher-Munson for a greater understanding. Don't underestimate this effect... it is very real... try increasing the bass as you reduce volume.

In general, all tube amps show greater nonlinearity with larger signals... this means harmonics.

You might also be enjoying the effects of output transformer overload.

Guitar amps are designed with the highest volume for the lowest dollar. So, high volume is when all things start to squirm at the same time... the power supply sag is greatest, the tube distortion peaks, and the output tranny starts to saturate.

Mind you, if you can't get this effect with a power soak, then it is likely that it is either F-M or speaker distortion you are hearing.

[dakotadave]

Thanks! I have moved my thread to tubes so if you would like to follow along it can be found there !