The "compression" generated by the power amp section of a a REAL TUBE AMP is from the power tubes being driven to their max and, when they can't deliver more juice, they...don't. They don't get louder, instead, they "clip", and their harmonic structure gets richer and more complex. This is usually a good thing. When a transistor gets pushed too hard, it can't deliver any more juice, either. But a transistors' clipping is usually nasty and gritty, very non-musical tone.
"Power sag" you referred to is the inability (or ability) to instantly deliver voltage to the output power tubes. This is handled by the rectifier. In some amps the rectifier is a tube, in others it is a solid-state diode affair.
A rectifier tube is not as reliable, and is the source of the "sag" in that it's not as "quick" to deliver the needed voltage to the output tubes. The aural effects between the 2 types is fairly subtle. Some players want the "fast" response of the diode for a "snappier", "punchier" attack, others don't.
In any case, there are no "attack" or "release" parameters that the manufacturer has programmed into the amp. It's a bit more organic than that.
Why do you want to use an outboard compressor in the first place? Not arguing , just curious if you know what they really do. Some players use one to make their playing dynamics a bit more consistent (fewer "hot" notes), others use one to add a bit of "zing" to their solo parts, and others
use one to sustain a note for a couple of days. For any of those instances, a stompbox type of unit is usually a better fit. Placing the compressor AHEAD of any other electronics in your signal chain yields less noise, because a compressor is basically making soft volume levels louder and loud levels softer. The attack time set determines how quickly the level-controlling responds to your picking. There are some very good boxes out there for guitar. These include the Keely (one of my personal faves), Analogman makes a couple of nice ones, including a clone of the old Dan Armstrong Orange Squeezer (have an original one of those-great!), and the Visual Sound Route 66 (updated version) is pretty good, too. These all work better on a guitar signal than a rack unit, IMHO. The key, however, is to experiment with them and find the one that works with your guitar's pick-ups and playing style.

Actually the compression effect of a tube amp is more from the rectification circuit, yes you can have tube rectifiers.

The internal resistance of the tube rectifiers, increases the loses (sometimes called droop) to the supply voltage (The power tubes rails).

As you drive the amp harder, with say a large input signal (you hit the power chord real hard), this extra impedance pulls more current from the power supply, the voltage rail to the power amp tube is lowered. With the lowered rail the output signal of the amp is reduced.

Thus a compression like effect is created.

moonbaby wrote: In any case, there are no "attack" or "release" parameters that the manufacturer has programmed into the amp.

According to VacuumVoodoo "the PA will behave like a rudimentary compressor, its attack faster with smaller filter caps and release faster with higher power transformers current capability i.e. how much current it can supply to recharge filter caps once the signal envelope attack phase is over." (http://www.powerscaling.com/community/index.php?topic=226.0)

So it appears there are attack and release parameters built in to the amp by the manufacturer whether inentionally or not.

As you mentioned, sag is affected by the type of rectifier, and some manufatures offer variable sag controls. Island Amps offers "a variable sag circuit that can take you from solid-state stiff all the way up to the most saggy tube rectifier tones with a turn of a knob." (http://www.islandamps.com/mods.htm)

The stomp boxes you mentioned give the specs, so that is helpful.

I understand that you say to put the compressor between the guitar and amp. I want to be able to do that also, but I want to be able to put it between the preamp and the power amp. If I avoid running the power amp hard, the power amp will not compress. Then I will have full control over the compression parameters with the compressor, and compression will still occur where it typically does in a guitar amp, after the preamp.

NO, if you DON'T run it hard, it won't compress!!!!!! And it DOES depend on whether this is a REAL tube amp or some crappy little solid-state amp
with a plate-starved tube in the front end. Sorry, the A/C broke down yesterday, it's a sweltering 97 degrees in the shade here, and the repairman is still "on his way"...

I actually read what you said about the power tubes clipping, and that transistors sound bad when clipping.

I was thinking that the PA tubes sound bad too when they clip, but since they are driving a transformer, maybe the transformer makes the clipping sound good.

Preamp tubes circuits are usually class A. Class A has soft distortion (even order harmonics--desirable) before it has hard distortion (odd order harmonics--undesirable).

Power amp tubes circuits are usually push pull class AB. They lose the ability to produce soft clipping and can only clip hard like transistors. If PA clipping sounds good, then the output transformer must affect the clipping somehow; otherwise, it should sound bad just like a transistor do when they clip.

Yes, the output transformer on a tube amp does contribute to its' sound, mainly acting as a giant lowpass filter. There are many factors that determine an amps sound. You asked about "power compression" and "power sag". As was stated earlier by both myself and Link, this is a factor determined by the rectifier design. An audio compressor inserted into the signal path of an amp is not going to generate the same results.
BTW, Bryan, your questions and statements smell suspiciously like...
LIQUIDSTUDIO, aka Aqualand666

I just joined this forum. I am a member of other forums too. I am not LIQUIDSTUDIO, aka Aqualand666 in disquise.

Sheesh, you guys work too hard!

Bryan, just buy one of these and you'll be happier than a pig in shyte:

[[url=http://[/URL]="http://www.fmraudio…"]RNC Compressor @ FMR Audio[/]="http://www.fmraudio…"]RNC Compressor @ FMR Audio[/]

Just buy it and use it. You'll know what to do once you figure-out what the controls do. It's not rocket science.

Overdriven power tubes are not for everyone. Didn't Dimebag Darell, a metal guitarist use solid state amps? Makes sense, cause you need note definition in fast played note runs. Oversaturated tubes won't cut it in most cases.

Hey Moonbaby, don't forget some of the greatest overdriven and compressed tone comes from germanium transistors in our most coveted pedals.
Using a big clean Blackface for tubesque soundstage effect, and a transistor pedal of choice gets the ultra high quality sound.

On the other side of the camp, small tube amps cranked and mic'd up is a standard way to go for some. Ever notice all those empty fake backline amps with nothing but a glowing pilot light in the box ? Gotta make you laugh-ZZ Top playing Crate-yeah right!

Compressors need to be used tastefully, and for me, they sound better in front of a clean amp than a dirty one. Don't dirty up good dirt with bad dirt.

Electro-Harmonix Blackfinger compressor is a beautiful tone machine. Very subtle. It has typical EH cheezy construction, but the tones are sweet and worth dealing with the cheese. It has a setting to overdrive its tubes. And EH fixes any pedal of thiers for 15 USD. I've sent my abused Blackfingers out to them, and got them back quick enough. The EH Blackfinger works great in a Blackface type amp circuit but not a Bassman type. Just loses too much clarity and note definition in a Bassman, not enough clean headroom. If you are strictly a blues player, the Bassman would be ok.

We are discussing compression here though, and with overdriven tube amps that does unfortunatley goes into a discussion of distortion.

As was mentioned, cleaner sag from the tube rectifier(if your tube amp has one) something to try....but then of course you're stuck with it. If you get an urge to flick your reflexes, you will be dissapointed. Some boutique amps will let you switch between rectifiers, then you're not stuck.