Help! Stokes mod in process!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by willhutch, Jun 17, 2006.


  1. willhutch

    willhutch Member

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    OK tech gurus! Here's one for you guys:

    I am ready to attempt the stokes mod on my newly acquired SFPR. I've done my research and printed out the diagrams. I pulled out the chassis and, guess what, my chassis doesn't look like the diagram I have!!

    Please follow this link to a shot of my chassis:
    http://pg.photos.yahoo.com/ph/willhutch/album?.dir=7609re2&.src=ph&store=&prodid=&.done=http%3a//pg.photos.yahoo.com/ph//my_photos

    The white arrows show the the endpoints of a red wire running from a resistor connected to the filter cap can to a solder point located in the reverb circuitry (a wire from the reverb transformer connects to this same point). My amp is a late 70's model with the pull-boost volume control. Is this red wire is part of the boost circuit?

    How does this affect what I do for the Stokes Mod? Can I ignore this wire and proceed as usual? Can I get rid of it (assuming it IS part of the pull boost circuit, which I won't be using)?

    Any help would be appreciated. Please let me know if the link isn't working or the photo isn't clear enough.

    Oh, and FYI, here's the link to the diagram I'm using to help with the mod: http://people.cornell.edu/pages/mt24/Amp/mm/PRtweak.html
     
  2. BP-Plickner

    BP-Plickner Silver Supporting Member

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    If That wire connects to the red wire over by the reverb transformer it is the B+ connection for the reverb tranny.
    You can see in the picture at the end of this link http://mysite.verizon.net/dogtone/Preverb-Boost/79-PR_0190.JPG
    the orange wire that comes from the Cap Can connected to the red wire fromthe reverb transformer from above. That wire goes under the blue cap that is part of the boost circiut.
    Keep in mind that the stokes mod is typically described for the non reverb Princton. That wire is one of the differences
     
  3. willhutch

    willhutch Member

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    Thanks Plickner!

    Yes, you are right. On my amp, the B+ voltage for the reverb tranny is supplied via the 320vdc terminal from the can cap - which I was going to use to supply the PI's plate voltage.

    So, now what?

    Can I leave that part of the circuit alone and perform the stokes mod per the normal procedure? That would mean both the reverb tranny and the PI would be connected to the 320 terminal via the same eyelet. Is this acceptable?

    Any other thoughts?

    But I guess I should just ask - can I do the stokes mod on a Princeton Reverb??
     
  4. BP-Plickner

    BP-Plickner Silver Supporting Member

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    If you look at fender schematics for the AB763 circuits on you will see that the PI and the reverb transformer share the same filter cap. You should be Ok. Remember that cap can knock you on your butt if it isn't properly discharged.
    Here's A pic Of A Power Supply Mod I did on a Pull Boost. I didn't have a 18k OHM resistor so I put a 10k and a 8.2k in series. You can see I took the reverb B+ off the GRID and pre amp tune B+ and left the PI on it's own. I guess that gives the PI a bit more voltage
    Hope this Helps
    http://mysite.verizon.net/dogtone/Preverb-Boost/PR-MOD.JPG
     
  5. willhutch

    willhutch Member

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    OK. Here's the status...
    I moved the reverb to the higher B+ voltage. I tested the amp and everything worked perfectly.

    Then, I did the stokes mod as described on the various links on the internet (and as Plickner's photos showed). When I fired up the amp, the fuse blew.

    What diagnostic work do I need to do now??????
     
  6. willhutch

    willhutch Member

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    Problem solved. Guess what - I put the rectifier in wrong. I didn't know that I could make it go in wrong, so I didn't suspect this.

    I don't know if I should be proud because my wiring was good, or stupid because I put the rectifier in wrong!!!

    I guess I'm be proud my soldering worked and will consider myself wiser for learning from a stupid mistake!

    Tone report: in all honesty, I can't say I notice much of a difference after doing the mod. :messedup

    I'm going to play the amp the way it is for a while (did I mention I have a Weber 10F125 in it?). If I feel a need to do any more tweaks, I'll try the old Paul C mod. I'll probably check back with you guys then!:)

    I will say I learned a lot from going thru this process. Assuming I haven't done any permanent damage, it was a worthwhile undertaking - even though I dodn't notice a great difference in the performance of the amp.

    Thanks you guys for your advice!!
     
  7. BP-Plickner

    BP-Plickner Silver Supporting Member

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    Good Stuff:BEER

    Half the fun is doing the work and the other 80% is figuring out what went wrong. :crazy
     

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