Hi...i have plexi 50 lead with the PPIMV mod...i've owned it from new(2 months) and i've tried everthing i know to maybe improve the overall tone...
           1st, tubes, mullard, JJ's,Seimens,Brimar & winged C's which are in there now along with brimar pre's...best so far!!!
           2nd, Speakers/cabs, Celestion g12h's Originals 2X12, G1275 2X12 and an original 68 100 watt plexi cab, plug a donkey in this and it sounds great, but not the ceriatone!!!!
           3rd,Guitars vintage les paul & custom shop strats...
           4th, pedals F/tone,analogman,Foxrox etc etc all good stuff..pedals help of course but i don't want to have to use pedals to get good tone!!!! but even when useing say, an OCD, when i lower the guitars volume the tone gets real dull/muddy???sounds like crap!!!even with the fenders single coils....
       
      i'm alittle lost guys, is there somthing wrong with the amp??? to me it just has NO ZING!!! Like the guitars volume is around 6-7 or the tones down to around 5-6...i'v had so many marshalls 60's onwards and still have a 70's 2203 which has more zing even with the pre turned down below half mast....i know old marshalls and they sound VERY REAL and have never been able to have the presence & treble above say 6...with this ceriatone i have both the Treble & presence at full tilt and i need more!!! alot more!!!

      i'm hopeing there's a problem if i'm honest, it can then be put right....i realy want this to work as it sure looks the part, build quality is outstanding...there has to be something wrong here.....

      any help at all would be great guys, i know i'm in the right place....

      thankyou for your time.....

Have you contacted Nik? He's always willing to help out as much as he can and make it right. Sounds like you may have a bad cap or even a bad preamp tube. The good thing is these amps are easy to service and make right again because of the hand wiring.

hi buddy......i'm getting 433 plate voltage..... please understand, i know very little about electronics, i've messed with pedals and changed out faulty parts on amps inc a transformer once but thats just a direct replacement........i use a Weber Bias Rite and it's from this i get the readings...useing their methods i'm getting 28mA low & 48mA high and been setting the bias between 36-40mA...
thanks......

I am not sure what the Weber "method" is, but 36-40 MA is what most people would say is just about right.  Do you have a DMM?  Nik installs a 1 ohm resistor in his amps... cathode to ground.  It would be interesting to see what numbers you get there.  Just set your DMM to VDC and take a reading from ground to pin 8.  It will be a direct conversion to milli amps,  .038 would be read as 38 milli amps.  Needless to say, you take this reading without your Weber installed.
If you do not have a DMM... well never mind then.

hi buddy.......no, not yet!! i hoped it was something i was doing wrong, and i could fix it...i'll now drop nik an email as i'm sure it's no fault of mine.....do you have any idea's, the presence control seems to make no difference wheather at 0 or maxed out on 10...
thanks for your time....

So you are seeing a difference of 6 milli amps between the two tubes.  That is nothing to worry about, just means that your tubes are not matched real close.  I would just set bias by the higher reading of 40 MA.  So you are right at about the 70% mark now.  I would say you are just about perfect where you are. 
It does seem that something is not right with your Presence control, but it is so hard to READ about what other people HEAR.  The other pots should also be giving you noticeable adjustment.  I do not really know how to respond to that.  Maybe some of the other guys will have a suggestion for you.  Are you the original owner of this amp?  Or has somebody been inside of it before you?