carl martin plexitone mods and clipping info

[harrisxr650]

Hey guys,
a friend of mine asked me to build him a plexitone ( the one with the 3 switches) after playing with my own. After some slight modification i did based on the madbean pedals boneyard th pedal just sounds awesome.
but i've noticed that the clipping leds inside dont light up all three the same to be more specific D2 on the schem lights more the D2 and D3, i'm not sure this is an issue, the pedal sound awesome ad as it should be. so i opened mine and noticed the same thing but this time D1 was more bright than the other 2.
schem link
<a href="http://s272.photobucket.com/user/harrisxr650/media/marshall%20preamp.jpg.html" target="_blank"><img src="http://i272.photobucket.com/albums/jj162/harrisxr650/marshall%20preamp.jpg" border="0" alt=" photo marshall preamp.jpg"/></a>

also i would like to ask if there is a mod for a bass-mids-treble eq for that schem
thanks in advance
Harris

[GibsonGM]

Hi Harris,

Good observation.  They don't need to light up at the same time; it's normal.   LEDs need a certain forward voltage to turn on - one may be taking more than the next, thus lights up more, but they will both be 'doing their job'.   If not, the string wound not turn on at all.   

What you see there (the 2 in series one way, and 1 in the other direction) is 'assymetrical clipping', which intentionally clips one half the signal more than the other (sounds good).  So that section isn't balanced.   And they are working on peaks, of course, not all the time (unless you dime something, hit them hard all the time).   

Don't know of any TMB mods for this, but the third opamp section could probably be replaced with something like the tone stack from a Marshall Guvnor and tweaked to taste! 

[knopflergx]

hi guys...any ideas about the match of the volumes?

when i change from crunch to hi gain there is an increasing of volume...how can i solve?

thanks a lot!

[slacker]

You could try this to reduce the difference in volume between crunch and hi-gain.
Replace the crunch/hi-gain switch with a DPDT switch.
Cut the trace between pin 14 of IC1D and lug 3 of the volume pot, and put a resistor in there instead. Connect lug 1 of the spare half of the DPDT switch to one end of the resistor and lug 2 to the other, it doesn't matter which way round.

Now in Crunch mode the switch shorts out the resistor and the circuit is exactly the same as before, when you switch to hi-gain the resistor is switched in series with the volume pot effectively turning it down. You'll need to experiment with the resistor value to get something that works and it will probably always be a compromise because the volume difference will be different depending on the settings of the crunch and hi-gain knobs.
Instead of doing that if you don't mind an extra pot you could add a separate volume control for hi-gain. Connect a second pot with lugs 1 and 3 going to lugs 1 and 3 of the volume pot. Then use the other half of the DPDT crunch/hi-gain switch to switch the left hand side of R8 between two wipers.

[knopflergx]

thanks a lot for now.
i'll try this night and i reply here!

[knopflergx]

great, now is so much better!
but...i've nociced that in HiGain channel there is an increasing of mid/bass frequences...
that obviously i still notice now, even if the volumes are matched.

what do you think about?
what can i do?
any ideas!

thanks a lot!

[samhay]

Remove C4 - or at least make it smaller.

[knopflergx]

C4 is indicated on pcb?

how much is the value of C4 cap?

thanks!

[samhay]

Don't know, never seen the PCB.
In the schematics you posted, C4 is 330p. I don't see any other caps that size, so should be easy enough to find.

C4 works with the gain pots to set the high frequency roll off. The larger the cap, and/or the larger the resistance (crunch or gain pot) the lower the corner frequency.
It is usually a good idea to have some low pass filtering to reduce fizz, but if you don't find this objectionable remove it. Otherwise, try reducing the value until you find a happy compromise.

[knopflergx]

hi guys!
at the end i changed everything!

i've noticed that the difference between the volumes of the 2 channels is determinated by the different types of mid/bass frequences when the second channel is pushed on...

so i have putted inside a 3pdt to change a capacitor (the 2.2uF capacitor) in base of what channel is on...

original capacitor for crunch mode (2.2uF) and NO CAPACITOR when HiGain is on.

my 2 cents.

thanks everybody!

[dimgoutz]

Any mods for the high gain channel? I could use a liiiiittle more for heavier stuff  ...

[lioparis36]

Hi you all !
I am new on this forum and I really need your help:
I have an old Plexitone that I like very much, and I would like to know if there is a way to tame the overbearing highs on this excellent pedal?
I run it with the tone at 0 and it is still too bright...
Thank you for your help !