IIC+ lead channel sounds muted - missing treble response

[chipaudette]

Hi George,

Is this a new phenomenon with your amp? Or, has it always sounded this way? Asked another way, are you thinking that your amp has developed a problem?

If you feel that it has always lacked some high treble on the lead channel, I would suspect that it's the 1000pF snubber cap on the 2nd stage of the lead circuit. My IIC(+) (formerly your IIC) came back from Mesa with the 1000pF cap. Yes, it makes for a very smooth overdrive...but it could also be described as dull and lacking in treble.

If you back it off to 500 pF (the value used in the schematic of the Mark IV), you may like it better. Of course, you may also feel that it is now "too fizzy" sounding.

If my understanding is correct, many Mark III's have no cap there at all to smooth things out. Hence their reputation for a more "raw" sound.

I've got mine on a switch to go between 1000pF and 350pF. 350pF is a little too low (too much fizz). I find that I leave it on 1000pF most of the time.

Of course, if the lack of treble is a recent development, then your amp probably has a problem and should be taken in for proper debugging.

Chip

 

[chipaudette]

Oh, and I think that I'm remembering that you run your Presence on zero or something, right? I find that to be too low for my IIC(+). Too dark.

I run mine at 5 or so. You might consider giving it a try.

Chip

 

[chipaudette]

It's the same one discussed in this thread from a few months ago...

http://forum.grailtone.com/viewtopic.php?f=27&t=42290&hilit=bright&start=15

It's the circular ceramic one labeled #2 in the pic in the thread. This thread talked about clipping it out of the circuit to give maximum sizzle. That's too much IMHO. I recommend replacing it with 500pF.

If you're nervous about soldering on this board, you could also snip one of the legs and insert another 1000pF in series with the existing cap. The two 1000pF caps in series make for an effective 500pF value.

Again, you might not like the character of the high treble that this adds. I would definitely consider trying different presence settings first.

Chip

 

[Boogiebabies]

Hi George,

The lead channel alone has four caps that bleed treble to ground. Most of them are to prevent oscillation from the higher
gain levels, but doing so does effect the treble frequencies specifically. There is a 250pf cap on the lead output signal that is there to
help add some more highs off the lead drive. If you add the 1000pf cap to the mix on the lead channels second gain stage it's even more treble filtering.
With a 60/100W, you would barely if at all notice it with the tonal properties inherent with the 6L6. With a Simul it's a different issue with the darker,
softer sounding outer pair's triode wiring. In both cases I have always noticed the difference when not using the Treble pull shift. The high end just does not have
the same cut and needs to compensated by the presence, Volume 1 pull bright and lead pull bright to get the dull blanketed feeling off.
The Treble shifts effect on the midrange in my opinion helps as well. I feel it interacts with the bass frequencies much better allowing
the treble to cut through in a clearer way.

On every IIC+ I have ever owned or played I have exclusively used the Vol. 1 pull bright, Lead Pull bright and presence from 3-4.
With all things considered, I don't know what type or brand of caps Mesa used during the upgrade. They may all be ceramic, but
they do not all sound the same. This notion, I absolutely GUARANTEE. I have tested about five different brands and to my ears they were
all different depending on what stage they were used in the circuit. Although your amp is an upgraded IIC with the RP/SP-10, it should sonically
be no different from any other C+ you own. There is always a + or - 20% in the components though. If you send some gut shot's I would be happy
to look over the work. If your amp is really muffled, we may be able to see something else that's awry.

 

[tony777]

Don't go into the trap without checking your preamp tubes first. :wink:

 

[cremona]

I dont know if this is eny help but my upgraded simual c+ (upgraded in 2006) with the 100 pt tranny sounds enything but dull and muted, it sounds bright yet refined and analog , basicly the sound i hear from those orginal c+ amps i have heard.

I mostly have the precence at 2 , above that it sounds very angry witch i like sometimes , but never have it above 3.5.
Treble is at 7 , even with the pull bright pushed it dossent sound dull at all.

 

[Boogiebabies]

Don't go into the trap without checking your preamp tubes first. :wink:

Whatever, Export Boy !!!! :mrgreen: