Eric said:
Cool stuff. Thanks for posting that.
When I was reading your intro, the point I figured you would end up making is something like this:
If you're looking for clean headroom, perhaps tube and SS would be of comparable volume. However, the entire spectrum of clean into clipping is considered "good" on a tube amp, whereas when a SS amp cilps, you're pretty much done with its volume range. Therefore, an 18W tube seems to have more volume/power than a 50W SS.
But then you blew my plan to pieces by saying something else...
So my question to you: is there anything to the stuff I wrote above, or is it just me being completely incorrect?
Not at all Eric. You are on the right track for sure. If we introduce a measurement of distortion into the discussion it should be clearer.
Firstly lets remove "compression" from the equation. I'll come back to it in a sec.
A tube amp producing 50W RMS at 0.1% THD (Total Harmonic Distortion) should sound exactly as loud as a SS amp producing 50W at 0.1% THD.
However, whilst its dead simple to design a SS amp with that kind of performance, it's a lot harder to do so with tubes, so in reality you'd never see a 50W tube amp and a 50W SS amp with the same THD rating.
Side note. Whilst researching my reply here I've decided I want to smack THD (the attenuator people) up side of the head. I was looking for THD, as in -distortion- specs for common tube amps, but all i can find are thousands of mentions of the damn hotplates....grrrrrrr!
I wanted to give you some THD figures for a few Fenders and Marshall's, but it seems they don't list them. I'd assume thats because a. they are enourmous, and b. largely irrelevant because the distortion sounds good.
Anyway, back to what I was saying..
Yes you're correct. Your 18W would be distorting its brains out at full noise and and that tube distortion is interpreted by the ear as an increase in volume as well as an increase in distortion.
There is even the chance that an 18W tube amp could have been rated as (for arguments sake, I have no data on this) 18W @ 1% THD meaning that it'll belt out far more than 18W before it distorts so badly it's no longer musical.
So yes, "clean power" Vs "clean power" should be close to the same.
I hope that makes sense regarding distortion
OK now compression.
Tube amps also "compress" a signal and this is significant also.
I think perhaps the simplest way of explaining it is that where a transistor will "slice" the peaks of a wave (hard clipping) and sound horrible. Tubes "squish" the peaks of a wave down into the available "space" (Space being the maximum amplitude determined by the B+ of the amp).
This "squish" is known as compression and if I've explained it O.K. you should be able to envisage that a compressed signal still carries all the original signal, just distorted somewhat by the "squishing", where a clipped signal just loses all the extra bits above and below the clipping point where the transistors slice it off.
The result to your ears is that the compressed signal sounds louder (and often more musical because of increases harmonic content and apparent sustain).
As Marnold said, the best place to observe this if you don't own a compressor is simply to watch TV.
I think its fair to say most people think the TV stations turn up the volume for ads. I know thats certainly how it -sounds-
In reality though, there are restrictions for that kind of thing. I think the FCC regulates this in the USA actually. What they actually do so sneak past any volume level regulations is compress the guts out of the ads audio.
That increase in volume as a result of highly compressed audio in TV ad's is the same thing your hearing when your ears tell you a tube amp is louder than an equivalent SS amp. (It's also why built in volume limiters don't work well, if at all on TV's)
Does that help?
