Modding a ‘68 Princeton RI to ‘65 specs...

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jellybellyjellybelly Frets: 734
edited December 2018 in Making & Modding
Hi knowledgeable fretboarders, just wanted to check my thinking before starting to mod my perfectly good amp!!

i have a ‘68 Princeton which is great, but a bit mid-heavy and bassy with humbuckers (and even my Jazzmaster with A2 pickups). I know this is essentially a modified ‘65 so hoping it’s a quick job to ‘reverse’ some of the mods and make the two states switchable. 

From TDPR I’ve got the following info on the differences between the ‘68 and ‘65. 
  1. NFB resistor (R10) is upped from 2.7K to 5.6K
  2. The ‘bottom’ cap (C23) in the tone stack goes down from .047 to .022
  3. They add an 18K resistor (R34) coming off lug 3 on bass pot
  4. Reverb return resistor (R30) goes up from 470K to 1M
(http://www.tdpri.com/threads/’68-custom-pr-schematic-compared-to-’65-prri-and-aa1164.639609/)

I’m ok with reduced NFB and slightly dampened reverb, but I reckon (2) and (3) are easy to do - an extra .022 in parallel with the one already present to make .044 for (2) and a simple short for (3), and short them to ground with a switch to go ‘stock’. 

Thoughts??? Since it’s all preamp I’m guessing I can use pretty much any components that do the job without worrying about power handling etc. 

Cheers!!
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Comments

  • LebarqueLebarque Frets: 3755
    Sell it and buy a 65
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  • ICBMICBM Frets: 71959
    I would add a switch for the NFB values first.

    There's also the speaker, which might be a more important difference.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • munckeemunckee Frets: 12255
    ICBM said:
    I would add a switch for the NFB values first.

    There's also the speaker, which might be a more important difference.
    I’ve thought about a Jensen alnico p10 for the 68 to make it More american sounding. I don’t want it louder though.
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  • ICBMICBM Frets: 71959
    munckee said:

    I’ve thought about a Jensen alnico p10 for the 68 to make it More american sounding. I don’t want it louder though.
    It won't be - if anything probably a bit quieter than the Celestion.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • munckeemunckee Frets: 12255
    ICBM said:
    munckee said:

    I’ve thought about a Jensen alnico p10 for the 68 to make it More american sounding. I don’t want it louder though.
    It won't be - if anything probably a bit quieter than the Celestion.
    Would it be the best choice do you think, sorry for thread invading!
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  • jellybellyjellybelly Frets: 734
    edited December 2018
    Now you mention it @ICBM @munckee I just went on a bit of a speaker trawl. I’ve ended up with a Jensen P10r RI for now. 

    To help:
    WGS G10C - too much bass, very hollow sound and quite ‘hard’ sounding - everything I don’t like about small fenders (but I get why other like it!!)
    Emi C Rex 10 - not as ‘full range’ as the 12 and therefore a bit too dark. Would probably break in and be clearer but it took an age with the 12 I had...
    Celestion G10 Vintage - I’m certain this would be fantastic in a ‘65, amazing gain tones and super tight bass. Too many mids for the ‘68 though
    Celestion Ten 30 (stock) - actually very decent, well balanced and not too much of anything. Biggest complaint is thin bridge pickup tone. Pretty vanilla - I certainly don’t understand the haters. 
    Jensen P10r RI - has the alnico sweetness, reigned in bass frequencies and a slight scoop to the mids which suites the ‘68 really well for low volume playing at least! I’m happy with it for now anyway...


    But it I digress. 
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  • @icbm are you saying he negative feedback will change how the amp responds in the mids and low-end to take it closer to the 65 sound, at least more than the other mods I suggested??
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  • munckeemunckee Frets: 12255
    Now you mention it @ICBM @munckee I just went on a bit of a speaker trawl. I’ve ended up with a Jensen P10r RI for now. 

    To help:
    WGS G10C - too much bass, very hollow sound and quite ‘hard’ sounding - everything I don’t like about small fenders (but I get why other like it!!)
    Emi C Rex 10 - not as ‘full range’ as the 12 and therefore a bit too dark. Would probably break in and be clearer but it took an age with the 12 I had...
    Celestion G10 Vintage - I’m certain this would be fantastic in a ‘65, amazing gain tones and super tight bass. Too many mids for the ‘68 though
    Celestion Ten 30 (stock) - actually very decent, well balanced and not too much of anything. Biggest complaint is thin bridge pickup tone. Pretty vanilla - I certainly don’t understand the haters. 
    Jensen P10r RI - has the alnico sweetness, reigned in bass frequencies and a slight scoop to the mids which suites the ‘68 really well for low volume playing at least! I’m happy with it for now anyway...


    But it I digress. 
    Sounds like the Jensen might suit me. It's difficult because I don't want to lose the good stuff. Thanks for the run down. 
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  • ICBMICBM Frets: 71959
    @icbm are you saying he negative feedback will change how the amp responds in the mids and low-end to take it closer to the 65 sound, at least more than the other mods I suggested??
    Probably. It’s certainly the easiest switchable mod to do, so worth trying first.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • crunchmancrunchman Frets: 11413
    On the speaker front, another option is the WGS Veteran 10.  I've put one in mine.  Definite improvement over the stock one, and a more "American" sound that you would expect in a Fender amp.


    Cheaper than most of the other options as well.

    There was a Weber 12F150 for sale in the classifieds here for a good while.  Might be worth searching for that and checking if it's sold.  That would be a very good speaker.
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  • I wasn't sure whether to not use the search function and start a new thread or use the search function and resurrect an old thread! But it's relevant to the above so here I am.

    I just opened up my 68 Princeton so see what I can do in the there. I can add some wire to skip R34, I'm going to do that now. I'm tempted to try and removed the front panel to have a look at C23. 

    The main board looks far too much of a pain to remove. Above it's mentioned to add another resistor to R10 in parallel - does that mean I can buy another 5.6k and attach it's legs to the legs of the existing? I think so but I know some of you guys can help clarify.

    Thanks.

    Pic of my board:

    https://imgur.com/a/tbq2uNM
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  • adam_farmeradam_farmer Frets: 30
    edited February 2023
    Got the front panel out. Not sure about my soldering skills to replace C23! Looks like quick delicate work required.

    https://imgur.com/a/uuXOX4T

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  • Modulus_AmpsModulus_Amps Frets: 2569
    tFB Trader
    Got the front panel out. Not sure about my soldering skills to replace C23! Looks like quick delicate work required.

    https://imgur.com/a/uuXOX4T

    What is C23 in your amp? you need a high voltage cap replacement
    0.022uf has more low mids than 0.047uf.  You might find it great to fatten up single coils, but can make the amp a bit boomy and muddy. Super Reverb uses 0.022uf but then cuts bass before the phase invertor.
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  • Hi, thanks for the reply!

    C23 is .022. The blackface re-issue version has .047. My amp is a bit boomy and muddy - exactly as you describe. I've got my info from here: https://guitarampblog.wordpress.com/2017/01/26/fender-65-princeton-reverb-schematic-vs-68/

    It's tempting to have a go at replacing it. 
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