What's happens to the PT when I turn my amp up?

[leadfootdriver]

I have a 90 watt amp with a smallish PT compared to other amps I own. It sounds and feels way different when I turn it up loud the way it was intended to be used compared to a bedroom level. It's not just the Equal Loudness effect, the amp gets more juicy and seems to have more gain or is at least easier to play compared to dull and crusty at low levels,

I assume this is the PT adding something to the sound and is intended by the designer. I don't play it loud enough to get actual power clip I don't think, but I could be wrong. What is happening?

[Enzo]

I would tend to think not. A lot of amps react differently when turned up versus down.

And there is 90 watts and then there is 90 watts. Some amps can crank out 90 watts all day long, others can do it for a moment at best.

And for that matter speakers can also react differently to level.

[loudthud]

Tube preamps react to changes in B+. When an amp is played loud, the B+ sags (perhaps a 10% reduction) and the preamp reacts. If the preamp has it's own power supply, such as a rack mount preamp, there is very little reaction. These reactions are rather slow, in the range of 1/10 to several seconds. You don't hear them as much as you feel them, but there are subtle audible effects. The term "Touch Sensitivity" is used to loosely describe the effect. Sometimes it's called Mojo.

[Leo_Gnardo]

What loudthud said ^^^. Some players had a hard time when guitar "systems" were the style back in the 80's - separate preamps & power amps could make for a sterile sound. Harry Kolbe, a NYC mod meister popular at the time, used to put an entirely separate power supply for the preamp in Marshall heads, a costly affair. He'd charge his victims, uh, customers a grand for this plus a couple other dicey mods. Then they'd show up at my shop and I'd return the amp to its normal operation for a C-note. And they were always very relieved to have their "sound" back. Some players managed to make the "component" style guitar system work for them in spite of its lack of "Mojo." When you got it in your hands, you got it.

[loudthud]

Some players like Clapton use the effect to their advantage, faster speed metal players generally don't like it, or not as much of it. They describe the amps as mushy or spongy.

[leadfootdriver]

Thanks.

[Zozobra]

At full tilt lots of things happen that add very subtle and non-subtle distortions to the sound. The obvious ones are that the power supply will sag as the load increases, effecting preamp and poweramp. The screens will sag due to the SGR limiting the screen current causing compression and this is a large effect. If there is a NFB loop this will collapse at a certain point, drastically changing the sound. The bias excursion and recovery is a factor that is often overlooked and hard to quantify the effect of without scoping it.

This book has a lot of details on power amp design and what kind of wacky stuff occurs when overdriven: https://www.ampbooks.com/mobile/books/power-amps/

It's quite a challenging read. I should dig my copy out again and look over it.