The fact that no knob tweakage was altering the sound indicates for 99% sure that the preamp is not at fault.
So that leaves us with anything after the master volume, starting with the effect send connector, which turns off the internal bypass when something is plugged in. Hence the jumper test.
The mp3 test would be a good idea also, because it would indicate if the problem lies in anything after the return effect plug (but as the amp now works fine, it is difficult to test that until the next occurence of your problem).

Was this amp stored and not played for a long period of time?
Because even if I may not be able to explain exactly why, but I suspect dry dust to be a electric isolation material for weak current, whereas it might be burnt and turned conductive (more or less permanently) by a stronger signal, which is consistant whith what I observed on amps that where not played for a long time and with your symptoms.

So what are your options now?
Play this amp often, and when you rehearse with your band, always bring with you a spare patch cord to shortcut the effect loop if this happens again. Anyway some carefull visual inspection might be a good idea but I fear the faulty contact is tiny enough for not being noticed this way. Maybe ohm testing every connexion past the master volume knob would help, but I'm not even sure you'll find anything and that can prove an harassment to do so.

Most of "vintage" speakers do have a huge boost (sometimes 3 to 6db more than the overall ratting) in high mids (in the region of 3KHz to 6KHz). I do think you should really be carefull when selecting a speaker based on efficiency. Try to find response curves and see if most of this efficiency is located in frequencies you don't want to push too far. Overwize you will need to filter them out somehow and obtain an overall efficiency of the speaker + filters that is not what the overall spl indicates.
E.g. if your tone is more in the 200 to 2000 Hz frequency range, a speaker with overall 100 db/W.m with a speep boost at 4KHz won't be more efficient for you that a 98db/W.m one which have a boost precisely in the frequency range you seek.

Yep, a lot of you guys obviously know what you are talking about. You all have been of a great help!

I finally bought a graphic EQ. Although the 10 band MXR looked nice, I settled with the Boss 7 bands for various reasons (one of them being the availability and a discount on this product, which was already cheaper at list price).
I made some quick tests yesterday, and I think I can confidently say that an EQ in front is probably one of the most versatile stompbox anyone can in front of a 2 chanel SS amp.
First thing: the Boss is not true bypass. As far as I can tell, this is actually a good thing: the buffer really adds some fullness to the sound, even if it is the only effect in the chain. Not sure about why it does so, but ihe amp already sound more dynamic with it.
Then the possibilities of the EQ itself + the level adjustment... wow!
I just fiddled with various positions of the slidders and so far I have been able to:
- slightly clean boost (which as Mexi pointed out adds some grit to the clean chanel of the Peavey). My personal sweet spot is somewhere around a +6/+7db boost, but I run humbuckers
- reduce the part I didn't like in the sound of the attack (in my case, just cutting 3 to 6db out of the 800Hz band is very helpfull with both my electrics, but I would guess it is player/strings/pick/guitar/amp dependant)
- dial a super soft overdrive from the drive chanel (just cut 200Hz and under a bit on the pedal, boost back lows/low mids on the amp tonestack and dial in the level of gain you like)
- dial a super high gain tone without parasitic noises (100Hz +/-0db, 200Hz +5db, 400Hz +2db, 800Hz -5db, 1600Hz 0db, 3200Hz -3db, 6400Hz 0 db + amp's tone stack not that far from flat)
- use the pedal as a dirty oldschool fuzz (drastically boost 100 and 200hz, and push the level a bit)
-...
I still have to gain experience using this, in particular at reheasal volumes (I only tried at home at low volume as of now), but I am pretty confident this EQ will be a key part of my rig in the future.

Mind you, some people say this particular unit produces some hiss, but what I found is that it's untrue unless you boost a lot in both the higher frequencies and the overall level. I would even say that it is a helpfull tool to reduce noises you don't want.

Fear you didn't understand me fully, or my limited english striked again and I didn't explained properlly. I am not trying to say I cannot obtain the tone and dynamic response I want from my gear, which happens to be a US made Peavey Envoy (or from any tranny I had a chance to test for that matter), whatever guitar might be plugged in (and this includes a nylon accoustic powered with a simple piezzo, a high output level Yamaha RGX and a more common Les Paul type electric).
What I am saying is that when testing some tube amps, I often have to drastically refrain my playing dynamics in order for the amp not to exaggerate it. I am also often very happy with how pure accoustic instruments respond to the variation of string attack I make.

Sure I didn't try the Fender Blues Junior long enough to fully adapt my playing, but I really chocked me how any attack just a bit stronger than usual resulted in a sound both considerably louder than what I expected and with a lot of unpleasant harmonics. I know how to EQ out these harmonics, but then softer parts would just be hidden behind the mix. I surely don't have these problems to that extent with a lot of other gears I tried and that includes trannies, but also oversized tube combos.
I am just curious what could explain such a difference and thought this dampening thing might have something to do with it.

Edit:
I just had the opportunity to test the BJ longer. Interestingly enough, some tweaking of the Volume and Master knobs enable to lessen the booming effect and dynamics were far more controlable. I guess a lot of components/features are interacting to influence the end-result. Still on my quest to understand what happens.

Thanks for this explanation. Not so sure I understood all of this, but it's an interesting point of view.
I'm just wondering if these non linearities that are enabled either by a tube/output transformer or by a SS amp with CF are in certain cases what causes the dynamics of the playing to be exagerated.
Wouldn't say for sure I have a comprehensive understanding of what "linear response" to dynamic playing should hear like, but I'm usually pretty good at producing the sound (both tone and volume) I want with accoustic guitars and at controling the distortion level of a crunching preamp with just how hard I attack the strings. Fact is, on some tube combos pushed in there usually prefered volume range, I often struggle to control these dynamics, generating barely audible sound when plaing soft and ice-pick-through-the-ears far too loud notes when picking just a bit harder. Just in the interest of better understanding, could you tell me if the dampening factor might play a part in this?

As pointed out by others, trying to replicate tone strategies that were used with tube amps is not likely to yield convincing results. Using a "magical" tube preamp is likely to not do the trick either. So what are your options?
- first thing is to realise that SS amps are a lot better amplifiers in a strict meaning. They will do a far better job at replicating (louder) whatever you feed them with. So work on your playing technique and consider plugging a guitar that has a natural tendency to sound "warm" rather than inserting in your chain units that were design to push a tube amp doing what a tube amp is supposed to do (that is adding some warmth where there wasn't).
- second thing is chose carefully your clipping stage. Try to find some "soft clipping" circuit. It will probably be better than a hard clipping one for your needs. You can also go the asymetric way if you like that, although it should be noted that push pull tube amps do not clip asymetrically (single ended tube preamps do that though).
- third, don't underestimate the power of filtering. The good way to do it is to significantly filter some of the frequencies before the clipping stage and to boost them back later. That enables you to actually chose what part of the signal is clipped. That's of a great help to avoid generating overtones that will be either close but not strictly equal to some harmonics of other notes you might play at the same time (generating intermodulation distortion) or out of key (generating something not pleasing either)

I'm not really good at understanding schematics, but maybe the RSO presented here does just that (second and third)?

Quote from: Rutger on July 06, 2012, 02:59:56 AM
My conclusion now about this design is that I don't like the heavy distortion because of a lack of high-frequency shaping. I think this is done because the amp is specificly designed for nice cleans.

Think about it this way: pure square clipping will add odd harmonics. The more you clip, the more of them you get. A third harmonic is orginal frequency multiplied by 3, a 5th is original * 5, etc...
If there is no specific tone shaping circuitry to boost back the lows (or rather smouthing the highs out), then your signal has a lot more high frequency content after the clipping. As the pathfinder is a monochanel, the EQ is shared. As you like the clean tone, you probably EQ for the clean tone.
Depending what type of switch you use to activate the boost, you could also use it to filter some of these harmonics out of your signal, but going this way has the potential to open a whole new can of worms I believe.

Actually, if we put feedback aside, the only way for a distortion unit to increase percieved sustain is by having enough gain to amplify the dying vibrations of the strings after having severly limitted the voltage produced by the initially loud vibration.
It will do nothing helpfull to the sustain, if there is no vibration left on the string.

What causes the string vibrations to attenuate with time are some resistance from the air, but also characteristic of the guitar wood and design (does it resonate with the string or does it fight the vibrations), and of course your fingers (do they actually let them ring/excite the string with some light vibrato or cushion it?).
The height of the strings in relation to other frets may play a part too: if the strings are too low, the initial peak of vibration will be attenuated by intermittant contacts with other frets (probably the next one to the one you play). I don't buy explainations about the height of pickups, as magnets should both fasten and accelerate the strings in cycle, resulting in an overall neutral influence.

Question: do you feel you have more sustain when playing an open string versus playing a fretted note?

If it's the case, then you could benefit highly upgrading your playing technique and tuning your bridge to have the strings a bit higher.
As said by stormbringer, some hard string attack (favoured by a heavy gauge pick) will certainly help the strings resonate longer.

Another aspect of the problem, is the ratio between the compression percieved as "clean" and the "dirt". Here, the type of harmonics produced by your distortion unit may help (odd harmonics are percieved as more dirty than even harmonics) and also the pre-dist EQ>dist>post-dist EQ plays a big part: the pre-dist EQ will set what part of the signal is actually clipped, while the post-dist EQ will act after the clipping to reequilibrate the frequencies. Carefull optimisation of these EQ settings may result in a sound that doesn't feel "that" distorted but is still actually really compresses, hence giving a lot of percieved sustain without sounding too much aggressive. The other way around, you can dial a tone that is really aggressive without being too much sustained, which can be nice for certain songs.

Question: did you ever played with an EQ before your amp? If you don't have the relevant material, did you at least try to ply with the tone knobs/pickup configuration of your guitar?

There certainly is some potential here.

OK, do you have an idea of how many db gain your clean boost has? Just to sort out what type of gain I would need to achieve this type of result.

Remark: If I want a preamp centric type of distortion, I'm pretty sure some work on the EQ of the drive chanel (which is a pretty decent one on the Envoy - just not that easy to set up for a "two humbuckers in parallel" guitar signal) will do the trick, but you know how it is: you can work at home at low volume, and you still have to rework your EQ once in the rehearsal room with the drummer (and bass player, and the other guitarist, and even a saxophone in my case) doing his thing. It's pretty hard to say to them all: "OK guys, sorry but I have to work on my EQ" when everyone just wants to work on songs... especially when they all think your tone is not that bad already  .

Roly: surprisingly enough, I had some compliments for the tone of this piezzo directly put into the clean chanel of the Peavey. It's certainly not "thin" and it is even a great deal better than the accoustic sound of the guitar captured with a chip mic such as the ones you can find in cameras or webcams.
Only thing is with same volume knob position on the same amp, it's something like 20dB less loud than an electric.

Mexy, do you mean clean boosting between the guitar and amp, or rather using a slight overdrive?

So I tested the Avenger at rehearsal volume. Although it can be heard, the tone is no longer there. That probably had to be expected from a combo with a shitty 6' HP.  :duh
Surprisingly enough, all this process of trying things with an open mind led me to a wonderfull tone with the Peavey alone on the clean chanel. Basic settup process is
- using both (humbucker) pickups of the Benett, all pots of the Peavey's EQ at 12 o'clock, tone of the neck PU at 9, tone of the bridge at 7, volume of each PU at around 6
- dial in the bass content and the high harmonic content only by fine tuning the volume of each PU
- then dial the Amp EQ to had or reduce mids
It does work wonders, but I still have to work on a saturated tone. Options are:
1° to EQ the drive chanel of the amp (separated EQ) to work with the same PU configuration. Nice once the chanel is EQed properly, but bth pickups sort of put out of phase certain mid frequencies, with can be good but produces mainly a nice slightly overdriven tone - not suited to highly overdriven/distorted tones
2° assume that I will switch to only one PU when saturated and EQ the drive chanel to a good compromise work with only the bridge and with only the neck PU with tones respectively on 9 and 7. Better suited to lead tones I guess, unfortunately, that's not the same EQ as the one which works with my "both PU" conf.
3° assume that I will boost either one of the PU when using drive chanel, but staying on both (i.e: turn the volume knob of one of the PU to reach 10). This would be nice as it sort of brings back all the controls on the guitar, but I still have to work on EQing this chanel first.

This setup is cool, but I could have use of something to either boost and fatten the clean chanel and/or adds just that bit of more punch to the drive one. I've found on the net some reference to a, affordable pedal and would like your opinion about it (or it's concept at least): Ibanez LF7. It's basically a passive low cut with cut frequency tunable from about 100Hz to 1000Hz, a Hich cut with cut frequency tunable from 1000hz to 10000hz, combine with a clean boost and a pre gain, that is supposed to crunch a bit when pushed.
Although some use that for dealing a "through telephone line" tone with extreme knob positions, Ibanez claims that this can also be used to add some warmth to a solid state amp with more moderate ones. I guess that put in the signal chain between the guitar and the preamp, it could be interesting to cut those unneeded lows and highs, and why not to add a bit of some dirty old pre gain to my clean chanel. At least on paper, this could also prove usable (although not as good as a real EQ with clean boost) with my nylon accoustic with a non-preamped piezzo. Any thoughts about this approach?

Spud, if you search for a graphic EQ with 2 outputs to drive either 2 different sound chains or 2 amps, maybe have a look at MXR m108 Kerry King. It's 10 bands, have a pre gain and post overall level, have 2 outputs...
Sounds promizing, even if a bit pricey.
What's interesting is that you have the 2 most bass bands outside of typical guitar range. Might look unneaded, but it's an opportunity to cut completely (well, not completely but -12db is pretty significant I would say) these unneeded frequencies and therefor help your amp dealing with only what really matters. Same in the high range with 8 and 16 Khz. Use the rest of the bands to fine tune how your signal will be presented to your drive.
In my recent testings, I found that there is a lot to do with pre-dist EQ, not in terms of general EQ, but in terms of what part of the sound your dist will clip/compress.
I'm currently simulating some clipping on complexe signals made of several harmonics (guitar signals are NOT sinusoids, even when playing single notes, and obviously more so when struming chords). I'm not at the point where I can present some results, but it's really interesting how filtering some of the harmonics and frequencies produced by the guitar before clipping (I'm using a square hard clipping for simulation purpose) does affect a lot the wave form of the resulting signal.
Clean boosting has its merrits, but if you can combine it with some alteration of the EQ in the same time, you're golden.
Another thing to try would be to insert a soft clipping slight overdrive adjusted to just barely clip the heaviest strummed chord you can make before your amp. This retains a lot of dynamic variations. Then you set your amp dist to clip a bit before that. You know have access, just with the power of your attack on the strings to:
- clean and clear tone when strumming really gently
- various amounts of distortion, in proportion with your attack
- a really fat tone with lots of sustain when both stages clip
This is really subtle to adjust, but if your overdrive as a different colour to what your amp does and you are able to control your attack, you will be able to have a lot of variety in how each note sounds. It's also super usable for "let ring" arpegios: the attack might be clipped, but as the note decreases, it becomes cleaner and cleaner so you have both sustain and clarity. Combine this with intellingent EQing before your clipping stages (actually, even the tone pots of a guitar are a really basic sort of pre-dist EQ) and you will find that your amp might produce impressive tones. Well, this might cost you a lot of time to experiment, but I'm pretty sure the results will be worth it.

I'm actually in a process of thinking about what to do with my current gear and what would help me use it in an efficient way.
Next test will be to bring both my amps to our next rehearsal and actually try the avenger alone and/or the avenger chained with the Peavey at rehearsal volumes. Last time I did try the avenger with a drummer was with a too loud drummer (you know the typical metal-fan drummer who slams the stick as strong as humanly possible). My current drummers are a bit easier on the sticks I believe so it could work.
I'll have the opportunity to hear if it still retains the warm "on the verge of crunching" tone I like.
Then I can think of a couple of configuration:
1° a line selector with adjustable loop level (e.g. Boss LS2 stompbox unit) to insert the avenger in its loop. That could put some warmth in the Peavey clean chanel when needed and give a boost to the overdriven chanel, effectively turning the settup into a 4 chanel system with two foot switches (Peavey clean alone - Peavey clean turned warmer by the Avenger - Peavey Overdriven - Peavey overdriven with the Avenger pushing it). By the tests I made, all 4 tones are definitely usable. It should be noted that a unit such as a LS2 can mix dry signal to loop signal, so the possibilities do look insane.
2° only if the Avenger alone can be heard in the band mix, why not try to run both my amps in parallel (stereo). This could either be run with the help of a standard AB box I think, but in this case I might also try to insert an EQ with adjustable overall gain (or replace both with a Kerry King MXR 108 which is an EQ with 2 outputs). I think it might be interesting to run both amps as I believe a clean amp in parallel with a slightly crunching crunching one does have somme merit and the dynamic of the playing could also benefit runing a slightly crunching amp in parallel of a more overdriven one.

Anyway, to test things without spending too much money, I might consider your gismo idea, but would probably try to make it a "Y" gismo (one input, two outputs).

Actually I just tried to use the jack of my footswitch to present a stereo side to the headphone out of the avenger and a mono side to the intup of the Peavey. It does work, and the sound is pretty much what it is when just using the Avenger alone. I have to be very light on the master of the avenger to not overdrive the Peavey's preamp though. Usage of the volume of the Avenger gives me the typicall drive I was looking at.
Using this with the drive chanel of the Peavey is amazing, but requires serious tweaking of the avenger's EQ.
I think I will need some time to really appreciate the potential it has and decide what to do with the info.