intermodulation distortion... OT

[vdm]

hey guys,
sorry this is a bit OT but i've been really trying to get some good distorted tone out of my marshall avt100 (which is impossible by the way), but every kind of tone no matter how much gain i use has this terrible intermodulation distortion. if you so much as let a string ring just a little, it gives low fuzzy bad sounding distortion and makes everything im playing impossible to hear.
i was wondering if i could change this with a different tone stack or what?
i'm almost finished my mxr dist+ with a few tweaks, and i'm unsure if that will help at all. me and my dad are also working on a matchless hotbox with extra tone controls and the such, but that will be a while coming.
does anyone know what kind of distortion produces this intermodulation or what pedals are good for a high gain (eric johnson-esque) tone, but allowing some semi-chordal stuff to be played?
thanks guys,
trent

[vdm]

i was thinking maybe something like ansil's alternate tone-control box, that i saw the other day.
thanks
trent

[Mark F]

I believe Eric Johnson uses a Fuzz Face

[petemoore]

I read some reviews of your amp at HC, and they were quite informative.
 I couldn't tell if it's a tube amp or not.
 I've got a DIST + with an AMZ tone control on it [not exactly 'loud', gets a little over unity at some settings] but would have to say relly helps dial in tne tone and dial out some nasties...

[Boofhead]

The AVT are basically solid state with a pre tube.

I haven't played through one of these but I have played through similar designs.  I think the problem you are referring to is the sound is indistinct and woofy, totally lacking any character, when you dial-up the treble is just sound harsh and it doesn't clean-up at all.  Yes?

The distortion+ might help a bit.

[Paul Marossy]

You could try doubling the size of the power filter caps. What you may be hearing is 120Hz intermodulation ripple distortion from the power supply. This can be heard more easily in transistor amps.

This would be my guess, judging from your post, it sounds like you have this problem no matter how you have it EQ'd.

[vdm]

yes, the problem is occuring at all gain and eq settings, though with very high gain the distortion is worst. i'll have to open it up, and see how easy it will be to replace the filter caps.
trent

[Paul Marossy]

Are you sure you know what intermodulation distortion sounds like? It sounds pretty terible, similar to playing a big fat power chord thru an octave pedal. It sounds to me like 120Hz ripple may be the problem. But, that may be only part of the problem. Perhaps the power supply could have been designed a little better, maybe not. Are you on 60Hz or 50Hz power?

The other thing is how the amp creates the distortion in the first place. If it's mainly by using clipping diodes, maybe you could mess with different types of diodes to try to get a better sounding distortion tone. If not clipping diodes, then maybe lowering the gain on the transistors in the preamp section a little bit might help to improve the tone. Sometimes too much gain can make eveything sound like crap.

What kind of guitar are you using with the amp? The type of pickups can have a large affect on the distortion, too. High output humbuckers can overdrive the preamp section too much, and help things get out of hand. But a Strat might sound great...

[Nasse]

:oops: I have a hole in real knowledge about different distortion species and their main causes. But I remember having read that clipping diodes, they may be in the feedback loop or shunted to ground, indeed really introduces intermodulation distortion when clipping. And that is considered as "unmusical" distortion as it may make your chords and playing-more-notes-than-one-at-a-time muddy or even worse. Sometimes that is desirable for guitar. Clipping is used in speech processing sometimes, and intermodulation distortion reduces the intelligibility of words. Rolling off lows before clipping reduces intermodulation distortion. Is that done in famous Tubescreamer pedal because of this?

But if your problem was in power supply ac hum intermodulating with lows, thats another story.

[gez]

A couple of years ago I read a Marshall special in one of the UK guitar mags and they seemed to be concentrating on diode clipping for their distortion sound.  Perhaps this is the problem?

In my experience, apart from cranking a vintage valve amp, the best distortions with the lowest intermodulation are CMOS designs - double note bends no problem!  

[vdm]

firstly im on 50hz, so it's possible the power supply was built for a 60hz supply, even though it is an australian model. i don't know how the preamp distorts because i haven't been able to find a schematic for the amp, or any in the recent valvestate series. i'll crack it open tonight, and see if i can draw up a schematic (simplified as it may be).
the distortion seems to only occur when i play higher up the fretboard and only with more than one note, and i'm using a gibson les paul studio, so it does have high output feeding the amp.
anyways, i better get off to opening this amp up.
trent

[Ansil]

hey i just went and looked up what your amp was.. and you might want to contact marshall as the music store in my aread that carries marshall says that they sent some back for this reason.  i didnt' think about that until i saw the amp.. i played a couple bout a month ago and i couldn't get a decent clean tone or a distortion tone otu of it.

[Paul Marossy]

If what Ansil says is true, then it may be a design flaw with that particular amp. IMO, diode clippers do not sound very good in a guitar amplifier. This seems to be the way a lot of the Marshall amps are designed from what I hear.  

[vdm]

hey,
i've taken this amp back to the store once before complaining of the overall quietness, or innability to get really loud (the thing is quieter than my fender squire 15) only for them to tell me it was too bad and "that's what you'd expect from this kind of amp". hence i don't like or find very informative the people at the place where i bought the amp.
surprise surprise i don't like the amp very much at all, i always just thought i needed a *bigger* amp... i wish i'd known what i was doing back then and realised i could have gotten a second hand tube amp for the same price.
but anyway, what should i do with the amp? i took a look inside last night to find the transformer is meant for a mains of 230V (i have 240) and it splits to 18v and 35v, each with their own filter caps.
18v has 2  2200uf 35v caps
35v has 2  1000uf 63v caps
should i try replacing these with higher values or is this not worth the trouble?
anywho - im going to cry in the corner ... and start building a cmos distortion pedal (i just finished the dist+ and i'm hooked )
trent

[Paul Perry (Frostwave)]

Don't sweat over the 230/240v issue, that's less than 5%. If you live in Collingwood, the mains probably fluctuate by more than that daily.
And I don't see those caps being a problem.
I guess it is too late now, but a good place to buy (or sell) secondhand gear in Melbourne is the Music Swop Shop 147 Elgin St. Carlton.

[Paul Marossy]

Yeah, those are pretty big caps, so they are probably fine. But, are you sure that they are all good? One of them may possibly be faulty. Are any of those caps bulging or smell funny?  Also, physically inspect all the components in that filter section. You might find something going on there.

If you can determine that everything in that area is good, then I would definitely look at the clipping diodes as being the source of the unpleasant distortion. Do you know what the clipping diodes are? I would guess that they are some 1N4000-somethings.

Intermodulation ripple distortion is most noticeable on single notes above 500Hz - like the 1st string, 7th fret. It will sound buzzy and unpleasant. You may also be hearing some crossover distortion, too, which doesn't sound pleasant, either. It's kind of hard to tell without looking at things with a scope....

[Boofhead]

If the problem exists with the master volume down to practice amp levels it's probably not the power amp, because under these conditions distortion from the power amp is unlikely.

Before getting off the power amp:  I would have suspected that a bad supply cap you give you a noticeable humm - perhaps you could check the soldering on the power caps. On the other hand if you are getting " low fuzzy bad sounding distortion" even on clean then that could be a power amp bias issue.   Yet another possible problem is oscillation of the power amplifer - that's a design issue.

If you have a  spare tube around perhaps swap the tube out.

One way to isolate the distortion preamp/power amp issue is to take the preamp out and feed it into a speaker sim and listen to it on headphones.  If the *same* undesitable distortion is present then it's in the preamp.

[vdm]

thanks paul,
i think i might have to resort to the old 'sell it and get a new one' before i go off and start altering the amp in ways that might not work.
maybe they'll have some tube stuff down there at an affordable price. and i did get the amp heavily discounted so my losses shouldnt be too bad.
thanks for all the input
trent