Differences between old and new Plexi modules

[Dr.G]

Hi,
I'm having some issues with my (apperantly old) Plexi module. It sounds pretty good played through my neck pickup but turning to my bridge pickup which has a higher output it heavily distorts and looses a lot of the Plexi mojo.
Doing some research I found out that all older clean to mid-gain modules including my Plexi have this issue which can be fixed easily by changing R1 resistor from 470 to 150 Ohms.

http://www.grailtone.com/guitar-tone-articles/randall-modules-clean-modifications.html

So my question is was this the only change that was made during the Plexi "evolution" or is there more?
Does anybody know?
How can I find out?

I try to avoid sending it to a modder for changing 2 or 3 parts as I live in Europe.

Thanks for replies

 

[DonP]

Bump for you.

Also, how do you tell what is a new or old Plexi? Mine is a chrome faced one.

 

[webrthomson]

Also, how do you tell what is a new or old Plexi? Mine is a chrome faced one.

I believe that the stock face plates went like this (oldest first)

• All Chrome
  Chrome + Black stripe
  All Black

Not sure of the dates on these but all chrome are the oldest

 

[Dr.G]

Mine is a chrome+Black stripe. The reason I think it's an old one is, that it has the 470 Ohms R1 resistor. At least the link I posted tells that there where changes at this resistor position. :?:

 

[webrthomson]

Mine is a chrome+Black stripe. The reason I think it's an old one is, that it has the 470 Ohms R1 resistor. At least the link I posted tells that there where changes at this resistor position. :?:

I'd say do the swap if it's a 470 - I did this on my all chrome faced Deluxe and it did make a audible difference, much better afterwards

If you don't like it you can always swap back!

 

[Dr.G]

That's what I gonna do, I just was wondering if there where more changes. During christmas and new year I will have some time to do this changes and let you know about my results.

 

[webrthomson]

That's what I gonna do, I just was wondering if there where more changes. During christmas and new year I will have some time to do this changes and let you know about my results.

Go for it dude - love to hear the changes and the effect that they have!

This thread has some ideas on it too:

http://mtsforum.grailtone.com/viewtopic.php?t=16368

Hope this helps!

 

[Dr.G]

BTW, "Graham Pearson" is trying to put a component spreadsheet together with all the values that were changed in the different releases of the modules :!: . Very helpful, check this:

http://mtsforum.grailtone.com/viewtopic.php?t=13007&highlight=component+spreadsheet

 

[WienerDog]

I am going to be changing R1 in my old Plexi module to 150 ohms. To make it easy, I'm just going to connect a 220 ohm resistor to the existing 470 ohm resistor, in parallel.

 

[Dr.G]

So this is a rather dump question :roll: :
I konw that R1 is usually a 1/2 watt resistor. Does it make any tonal difference using a 1 watt resistor instead of 1/2 watt (regardless of its ohm-value)?

 

[GtrGeorge!]

fwiw:
I own 3 stock plexis....all very similar fairly low gain modules. No matter the logo no big difference in sounds..
just my exoereinces.



GtrGeorge

 

[Dr.G]

I checked all the values of my Plexi with the ComponentValues sheet of Graham. There are differences. Even the labeling of some of resistors was different. As soon as I figured it out I will give it a try and change them.
Here's what I know so far:
- R1 470 instead of 150
- R2 470 instead of 150
- R3 470 instead of 150
- R31 56K (respectively R25?) instead of 33K
- C14 220p? instead of 470 (silver face) or 250 (blackface)
- C18 Empty instead of 22?
- Tube Board C1 Empty instead of 100p

lets see if it sounds different

 

[Dr.G]

Changed a few things on my plexi module, here are the results:

R1, R2, R3 to 150 ohm
cleared the sound a lot! better sting articulation. got rid of a lot of mud.

R31/25 to 33k
different mid-response that harmonizes better with the overall sound.

module riser board cap C1 changed to 100p
fuller sound, less highs, more bass.

here's a intersting thread about it
http://mtsforum.grailtone.com/viewtopic.php?p=87246&sid=f3266f6cbc43c616f39ff9e86d1df6e3

so far I'm getting very happy with my Plexi module

 

[mikegyver]

Dr.G
Posted: Thu Dec 20, 2012 4:07 pm Post subject:
So this is a rather dump question :
I konw that R1 is usually a 1/2 watt resistor. Does it make any tonal difference using a 1 watt resistor instead of 1/2 watt (regardless of its ohm-value)?

Nope, it shouldn't make any difference. The material can make a difference (carbon comp vs metal film etc) but the wattage doesn't.

 

[GtrGeorge!]

maybe its my hearing....
GtrGeorge

 

[MarcoR]

Timely post... here is a black face plexi I just upgraded the caps to SoZo and added some carbon comp resistors. changed the value of R7 to 510k for more gain. Sounds outta site.

 

[Kapo_Polenton]

Nice, any rhyme or reason behind using carbon in those two spots vs say carbon film? More drift for sure maybe more mojo? I also replaced a few caps to sozos on my judge. I dig sozos in all my amps. I'd like to try a stock black faceplate plexi at some point. What is the amount of gain like on these compared to say a brahma or mr. Scary?

 

[BigBrewtus]

Timely post... here is a black face plexi I just upgraded the caps to SoZo and added some carbon comp resistors. changed the value of R7 to 510k for more gain. Sounds outta site.

Very cool MarcoR. My Peacemaker has the likes.

 

[MarcoR]

Nice, any rhyme or reason behind using carbon in those two spots vs say carbon film? More drift for sure maybe more mojo? ?

Yes, my intention was for more mojo but as it turns out R1/2/3 is not even in the audio path; they are used to set the bias for the switching circuitry (thanks Rob for clarifying). My plan now is to follow the traces from pin 1 & 6 of the 12ax7?s and determine the positions for the plate resistors and try the carbon comps there (no one tell me! I need to figure it out myself).

I'd like to try a stock black faceplate plexi at some point. What is the amount of gain like on these compared to say a brahma or mr. Scary?

It's very mid gain and I usually crank it all the way. Never tried the brahma or mr. Scary.