Thanks a lot guys.
How could I be dumb enough to not have thought of the mono jack used?  :duh
Well, only stereo 1/4 cable I have currently is the amplifier side of my footswitch cable. Not ideal as it splits it into 2 distinct cables with mono jacks at each end, but that should do the trick for testing purpose.

Roly, where the "dirt" comes from is actually a usefull information for me as I feared that the poweramp/speaker part of the avenger could have played a big part in the overall tone of this amp, effectivelly putting a stop in any project to re-direct it's preamp to a better poweramp and speaker.

If mono cable proves to be what causes the very weak signal out of the headphone and a stereo one enables to correct this and "drive the peavey crazy", then it may have a potential to turn my system into something interesting (even it could still benefit some mods, but I'll see that later).

OK, I tested chaining the Avenger into the Peavey through the headphone out of the Avenger. I don't know if the preamp of the Avenger has an overall gain of less than 1 or if there is actually an impedance mismatch (I've read that instrument inputs have a fairly high impedance (like 1K or so)while headphones are supposed to be anywhere between 16 and 64 ohms, but this headphone out of the Avenger is clearly very bad at driving the Peavey's instrument input. With maximum settings for the headphone and volume at 10 on the Peavey, the sound in the Peavey speaker is not any louder than the guitar direct in it with volume at 1.

As far as tone is concerned, it's difficult in these conditions to know for sure, but the sound is pretty similar to what can be heard with the Avenger alone: dirt and warmth come in really early and sort of keeps it's texture.

Now that I know for sure that it's the preamp (or is it? maybe the headphone out has its own mini power amp, but in this case they did a pretty good job at reproducing the exact same behaviour as the power amp and speaker in it) of the Avenger that introduces these dirt and warmth, I think I'll try to open it in order to see what it's made of. And if it's accessible, I might try to put a resistor in serie with the volume pot to have more usable range on it...

or put some stompbox with a good "level" setting in my to-buy list.
I think I would be interested in some sort of EQ with the following specs:
- a low pass with Fc tweakable from 2Khz to 6Khz or a bit more (would be nice if it's a high order one I guess, but it's not that critical as the tone pots of the guitar add their own low pass filter anyway)
- a high pass with Fc tweakable from 60kz to 500hz (for the double purpose of preventing the amp to amplify frequencies that should not be there in the first place and to remove muddiness if needed)
- optionally some usable mid cut (not a big fan of mid cuts, but can help to shape the tone, especially if there is a distortion stage after it)
- a gain stage that should be tweakable from -15db (maybe the passive filters  can do this alone) to at least +15db (-15 should help me using the Avenger in a usefull range of tone, +15 or more could be fun with the Peavey and definitely usable with my piezzoed Nylon). A gain stage that amplifies the whole signal is prefered to individual band-specific amplification that could be found in some graphic equalizer.

I realise that this looks a lot like a PhaBB Tone, but I don't think I have the skills and patience to build one. So if you know about a stompbox commercially available that can do similar functions, that would be great.

And what are the benefits of such a design?
Reliability?
Ensuring that the power amp just deals with power, thus leaving all the "tone making" function to the preamp (and the speaker, of course)?
Any thoughts?

My limited english as striken here...
What I was (eroneously) refering too as a "coil tap circuit", is actually a redirection of the coil tap itself to a pot with one end shorted to ground. So this pot have the capability of partially shorting one of the coil, enabling a continuous variation from single to humbucker. Some internet research proved that it has been done in various forms. One can be found pages 13 and 14 of the chapter 5 doc in this page:
http://www.peavey.com/support/technotes/
Actually, if any beginner wants to learn one thing or two about concepts of sound treatment to emulate a "tube sound" the chapter about transtube is also a pretty good read - although highly biased to try and sell some Transtube stuff (which are good and versatile amps by the way, just not able to replicate one precise type of sound I would like to have in addition to what the envoy110 is giving to me).
Anyway, I've got some testings to do with my current stuff. Maybe I will be back to you after this week end.

Thanks a lot for your answer Roly.
Yes, exactly this model. The audiofanzine review is pretty much the only reference to this amp I have been able to find, and as you could read, it's pretty much biased. My ears tell me that the "on-the-verge of crunching" sound is as good as it gets (plenty of warmth, dynamic response etc...) but that's pretty much all about this amp.
So I guess I will test the headphone-to-low-input trick before going any further in my reflexion.

Mexi: wow, these are some unexpected suggestions that make a lot of sense. I have 2 electric guitars: One is an old Yamaha RGX 110, which has just an humbucker and a volume control. This pickup-although a passive one- is already lowered to the max and even like this, still requires to use the -6db pad on the Peavey... With this guitar, the Avenger is super difficult to use and never clean. The other one is a Bennett AV6 ltd (some sort of nice LP copy). Don't want to mod this axe as it is great as is and it is a birthday present from my wife. Modding the Yam is a consideration, and to tell you the truth, I am considering using a splittable HB with a coil tap circuit. Buying another electric is not an option yet (next one will be a nylon electro-accoustic, and it won't be cheap if it has all the features I want).
Anyway, the compressor/clean boost thing might be a good bet. I still have to test some units, but I might use it even with the Peavey or with my current nylon guitar (which has just an aftermarket piezzo and no preamp built in) so that could be a good multipurpose buy.

Thanks again guys, this forum is really a fantastic ressource for us who are tired of the "you-should-by-a-15W-tube-combo-instead" type of answer.

Hi all.
I'm new here and would like to ask some questions you may have answers to:
I've got an old 15W mono chanel SS combo. The brand name is Avenger and the model name is FA15. I've found no schematics or even serious review of this amp, nor any info on the brand.
Features are simplistic: A gain knob, a Volume knob, a master know, and a 3 band EQ. There is a 6' small speaker in it (closed back).
I also own a Peavey Envoy 110 Sheffield, which I consider a great sounding amp, but cannot obtain with it the beautifull crunch I am able to hear with the avenger. I've read numerous times that SS amps are supposedly unable to produce a good sounding clean, but I swear this little low end thing sounds just as good and dynamic as a Fender Blues Junior as far as light crunch is what you want to hear. There are a few problems though:
1° the gain know is unusable: Adds a lot of harshness. More harshness than distortion I think, so this amp is limittated to various slight overdrive tones. My feeling is that this works as designed (poorly, but as designed).
2° the volume know is good for settings from clean to average overdrive. Only trouble is that the range on the pot is not optimal: volume at 0: no sound, volume at 0,5: clean, volume at 1: clean with a tiny bit of crunch when striking the strings heavily, 1.5 to 2: nice slight crunch to light overdrive, 2 to 3 nice overdriven sound. Past 3 or a bit more, it becomes too fuzzy for my taste.
Mind you, I have humbuckers on both my axes, but even with reduced volume on the guitar pot, I kind of regret not being able to use the full range of the volume pot even when the gain one is on 0.
3. I'm affraid it lacks a bit power when playing with my drummer. No external speaker output, and no real preamp output to direct it to my peavey, which as no power amp input or effect loop return for that matter. So chaining both amps would use the Peavey preamp in the signal chain.

I realize my knowledge is limitted, but:
1° Do you think I can make some testing directing the headphone out of the Avenger to the instrument Input of the Peavey without damaging any of these amps? If so any recommandation on the settings of both amps?
2° In case this test is not possible, is there any way to anticipate what part of the Avenger produces this nice tone and dynamics? I would like to know if there is some probability the nice crunch and dynamic response may be lost if I find a way to direct the préamp of the avenger into a poweramp and speaker (probably guitar type ones).
3° do you think there would be some mod possibilities to have the gain knob dropping some highs when rising the gain and some other mods to change the range of the volume knob?

Not a question, but might be the goal after all:
If tests indicate that there is a possibility that this preamp + external power amp and speaker retains the very specialized tone I have, I could be tempted to use its preamp as an alternative to the one built in the Peavey. That would obvioulsy call for some work (a power amp input for the Peavey, a true preamp out for the Avenger, some A/B switch, etc...), but if the result has a chance of being good, why not?