finally.....

[daz]

......i nailed the gain staging perfectly in my EL34 amp. For those who've followed my saga you probably know that when it comes to building amplifiers i am a sub moronic immiscible. I never quite could get the gain right, but i finally was able to not only remove any negitive artifacts that incorrect staging was causing, but it actually has as much gain as it ever has had ! I simply used 2 gain pots with voltage dividers after each one like in the AX84 octane amp. It's absolutely bitchen. (gave away my age didn't I? )

anyways, the amp is now better than ever, but it still has ONE tweak i did a while back which is a bandaid for one of the amp's original issues. the bandaid works beautifully, but being the anal inquisitive person that I am, i just cannot stand knowing there is one issue left that isn't right, even if the amp sounds fine. That issue is what i want to question you all about. A modern marshall especially had a smooth transparent tone that is like rubber. Squishy and transparent in that unlike say a fender, the notes seem elastic and compressed. But without the tweak the amp has that harder tone that has a feel and attack like a clean tone even tho i'm using a lot of preamp drive. The tweak was simply to put a 100k across the outer lugs of the treble pot. Some have said it simply changes the pot's value, but somehow it does more than that. It changes the tone dramatically into what a gainy marshall typically sounds like as i described.

The question is, i have tried everything imaginable with this amp. And nothing changes that hardness in the tone except that tweak. So i'm asking if anyone has any ideas as to what else it could be that causes that. Hopefully the fact that putting a 100k across the treble pot changed it dramatically to a compressed feeling/sounding tone will be the hint that might help one of you gurus (and to ME you all are) I can tell you that after all this time and tweaking, my feeling is that somehow there is way too much hi mid getting thru the tone stack that shouldn't be there and that resistor somehow removes it. But i'm using a totally marhall stack and all the components have at one time or another been swapped with no change in it. So it can't be a bac part or design issue that i can see unless it's happening somewhere else in the amp and doing that in the stack just cuts back a frequency that either has been over accentuated somewhere else either before or after the stack. What doth thou think?

By the way, this is more for peace of mind and curiosity more than making the amp right because it sounds quite fab as is.

[OverDriven]

Do you have a schematic you can post?

[daz]

Not now since i made a lot of changes. I'll have one eventually, but it may not be real soon. I'll post on when i do tho.

[Wakculloch]

You've moved the mid scoop up from 1 to 2KHz, and the treble hump now centres on 25K instead of 10KHz.
This will as you say attenuate the high mids more.
Are you using this amp into a 4x12?

[daz]

You've moved the mid scoop up from 1 to 2KHz, and the treble hump now centres on 25K instead of 10KHz.
This will as you say attenuate the high mids more.
Are you using this amp into a 4x12?

What i was saying is that it's always had that excessive mid. What are you saying i did to cause that? It's always been there from the beginning, and the only thing that helps it is that resistor across the treble pot. If thats what you mean, it didn't cause it, it helped remove it. Maybe i'm missing something in your post? And no, it's a 1-12 combo with a EV. Same as all my marshalls were.

[Wakculloch]

Sorry if my post seemed ambiguous, i'm not saying you did anything, i was just showing you the way that your resistor mod will move the frequencies in the tone stack.
Thought this might come in handy if you need to document your mods for future builds.

[daz]

Thats what it sounded like you were saying, but thats why i made the point that it's actually the opposite. Or at least the opposite in that the harsh pervasive mids i'm talking about go away with that resistor in place, not HAPPEN when i put it there. But i think the confusion is your idea verses my idea of what hi-mids are. i don't consider 2k the kind of mids that i have been trying to remove. I consider 1k more like it. So you may be right in your theory as far as actual frequency change, but you thought 2k was the kind of range i meant. Moving the center from 1 to 2k would help with the issue. Not a lot it would seem to me going by those figures, but it DID indeed help a lot. i'd have though going from a 1 to a 2k center wouldn't have made such a huge difference.

Anyways, i'm still looking for answers as to why this issue was there in the first place considering the amp from the PI onward is set up just like a marshall except for the PSU. And the pre has been thru so many changes it can't be there. With thet resistor in place the amp goes from this hard mid sound as i described to a typical marshall sound when i add it. Makes little sense to me that i have to move AWAY form a typical marshall design to something unusual for any amp in order to have a marshall sound ! Crazy stuff.

[Wakculloch]

See graphs:-

[Steve Conner]

I thought you said you used an unusual value of treble pot in the first place? Maybe the resistor helps to counteract that.

I also noticed you said you're using an EV speaker. I think the EVM12L has a somewhat different voicing to the speakers that people usually play Marshalls through. I have one of the reissue EVM12L Classics, and its bottom end is more Lance Armstrong than J-Lo. The upper mids and highs are kind of dull too. That could well make the lower mids more prominent. Using less NFB might help with this. I like mine best with the amp set for a big presence peak around 4k, whereas with previous speakers, I always preferred a lower mid scoop.

But then again, if you finally got it sounding the way you like, who cares why?

[daz]

See graphs:-

That show what i would say is barely even a fraction of the difference it makes. It isn't subtle at all.

I thought you said you used an unusual value of treble pot in the first place? Maybe the resistor helps to counteract that.

No, i once said i TRIED a 100k pot and it didn't do what this resistor does. It spent 5 minutes in the amp. But i've always had a 250k treble pot. All values are typical marshall jcm.

I also noticed you said you're using an EV speaker. I think the EVM12L has a somewhat different voicing to the speakers that people usually play Marshalls through

Trust me, it's not that. I've tried no less than 4 other speakers and every marshall i ever owned was a 1 or 2x12" combo which i equipped with the very EV in this amp. I use EV in ALL amps regardless of what brand/model they are. Have for many years.