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Fender '68 Custom Princeton Reverb R34 location?

Discussion in 'Amp Tech Center' started by deadbeat son, Aug 20, 2015.

  1. deadbeat son

    deadbeat son Tele-Afflicted

    Apr 5, 2010
    Evergreen, CO
    This weekend I plan to make a slight change to my '68 CPR and short R34. I find it clearly labeled on the schematic, but cannot locate it anywhere on the layout. Am I overlooking it, does anyone else see it on the layout? If I'm not overlooking it, does anyone know where R34 is physically located inside the amp?

    Thanks!

    Schematic/layout:
     

  2. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    IT is connected to one outside lug of the bass tone control. I dont' see it on the layout either, but it has to be there. You will have to pull that front panel PCB, I am thinking. That is where the tone sections are located on the BF/SF Ri amps, ime.
    What are your plans there??
     

  3. deadbeat son

    deadbeat son Tele-Afflicted

    Apr 5, 2010
    Evergreen, CO
    Thanks Wally. I just wanted to know a general location before pulling the chassis and giving it a thorough look.

    As for my plans, they're pretty simple. This resistor doesn't exist in the '65 PRRI, so I plan to jumper it to see if I prefer the bass roll off without it. I based my information on this thread: http://www.tdpri.com/forum/amp-central-station/548514-65-princeton-vs-68-custom-tone-plots.html.
     

  4. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    LOL....I just took another look at that schematic....there is a broken line surrounding that tone section. AT the bottom of that broken line box, it reads....'on front PCB'.
    FYI, that is not the o n ly thing that is different about that tone section in comparison to the PRRI....or the vintage PrinRev amps. There is a reason...and that reason is why Fender built that SF Custom PRRI the way they did. They were trying to 'tweedify' the BF circuit a bit. They didn't go far enough, imho....
    but, jumping across that 18K won't take that circuit back to the PRRI thing, either.Taht said, you might like what it does??? Let us know what goes on.
     

  5. deadbeat son

    deadbeat son Tele-Afflicted

    Apr 5, 2010
    Evergreen, CO
    Thanks again for the response Willy. I don't want to go back to a '65 PRRI circuit, I just want to be able to turn down the bass lower than I currently can. With the bass control set to 0, the '68 still has too much bass with my LP on the neck or middle settings. I just hope to be able to roll off a little more bass. If that works, I have achieved my objective.
     

  6. deadbeat son

    deadbeat son Tele-Afflicted

    Apr 5, 2010
    Evergreen, CO
    Oh, and those other items in the box that reads "on front PCB", R46, R52, C19, are located on the front PCB on the layout. But R34 isn't, so I didn't have 100% faith in the note on the schematic. I have a feeling they just forgot to add it when the adapted the '65 layout. Thanks again. I'll post up when I get a chance to make the change.
     

  7. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    IF you want to change that bass thing, change the bass tone cap to a .033mfd or .022mfd while you are in there. That is the trick. Fender didn't quite get there with this mod, imho.
     

  8. deadbeat son

    deadbeat son Tele-Afflicted

    Apr 5, 2010
    Evergreen, CO
    Ha! I'm kind of embarrassed for even asking this question. Imagine my surprise when I opened up my amp and saw this:
     

    Attached Files:


  9. Wyatt

    Wyatt Tele-Holic

    998
    Nov 3, 2004
    (this is wrong: see below) Removing that 18k resistor will probably have litte effect on the low end, it's there so lessen the impact of tone stack (and, admittedly, diminish the effectiveness of the Treble and Bass controls) for more midrange.

    I agree you'll get a bigger impact from decreasing C24 to .022ug -.047uf. This shunts low end to ground...the snallee the cap the higher the cutoff point.
     
    Last edited: Aug 22, 2015

  10. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    deadbeatson, thanks for the pic. IT is interesting to see Fender's modification to that PCB.....looks just like something that would be done out here in the rest of the world. IF you pull that front PCb, you will see the tone section that has that .1mfd bass cap. That is where your change needs to be made, imho.
    Wyatt, if one were to increase the resistance there at R34, would the midrange be increased? OR....would one do better to increase the value of that 6800ohm resistor, which I take to be the midrange control resistor??
     

  11. deadbeat son

    deadbeat son Tele-Afflicted

    Apr 5, 2010
    Evergreen, CO
    Gentlemen, thanks for the help. I added a jumper to the resistor, and when I turned down the bass knob, there was a major difference. I could finally roll off the bass to a very diminished level, whereas stock it still has a TON of bass when rolled completely off.

    However, the bass wasn't all that seemed to be affected to my ears. The amp sounded tinnier overall, even when I brought bass levels up. It's hard to explain, but it sounded plinky and thin. I removed the jumper and returned things to normal.

    For practicality sake, I hoped I could get where I wanted with a simple quick mod. But in reality, I would probably have to begin experimenting with the tone stack to get where I am ultimately satisfied, and that's a rabbit hole I don't care to go down right now.

    Thanks again for the advice guys.
     

  12. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Deadbeat,, the mod you need to do is really not that complicated. That front panel PCb comes out for handy work, ime. The danger to damaging the PCb is about the same as you encountered in doing what you did to that easily accessed area, perhaps. IT is a guaranteed deal that changing the bass tone cap...to anything smaller than the .1mfd...will change that bass. Hmmmm..are there two inputs on that front panel? I think so. One might be tempter to use that inside jack's hole for a switch and access more than one option in that tone cap position??? I doubt if there is room there for a rotary with 4 options? Put it on the back panel....this is just a RI...who cares if you put holes in the chassis? :)^) Now, deadbeat!! Get to work. That's a job worth doing! You have me dreaming stuff up, and it is stuff that someone needs to do. LOL.....had to say it. Me?? Nah....I'm going to ride the bicycle home in this bit of heat we have today. I'm the deadbeat in the heat now. Have a good one....
     

  13. deadbeat son

    deadbeat son Tele-Afflicted

    Apr 5, 2010
    Evergreen, CO
    Ha! Thanks Wally. It's not that I'm afraid of the work, I just don't have the time to invest right now with tinkering, listening, and tinkering again until I get it right. Maybe someday when I don't have a 2-year-old kid and a shift-working-wife!

    By the way, what's this heat you speak of? It's a balmy 68* here now at my cozy home at 7500'!
     
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  14. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    I hear you on all issues. First things first. It sounds like your schedule is set.
    Cool mountain air.....I need to make a ride up to Littleton....haven't been in Colorado for awhile.
    Have a good one.
     

  15. Wyatt

    Wyatt Tele-Holic

    998
    Nov 3, 2004
    Actually, having taken the time to rethink this...I was wrong. Pretty much what the 18K resistor does is making it so that even with the Bass on 0 on the '68 Custom., it's going to have the same resistance as the '65 PRRI on (roughly) "4" (the stock Fender 250KA pot for Bass should be around 25K when on "5")

    The 6.8K resistor is the Middle preset resistor. I've heard they put that up to 10K on the '68 Custom DR "Custom" channel*, but haven't seen the schematic. Dumbles move to a 100K linear pot and Allen uses a 250KA poit there to create the RAW control. I've used a 50K bias trim pot in place of that resistor to preset the traditional Fender normal channel to taste for more mids/less tone stack.
     

  16. deadbeat son

    deadbeat son Tele-Afflicted

    Apr 5, 2010
    Evergreen, CO
    That's what I understood from the tone stack plots I linked to earlier in the thread, but for some reason the amp sounded a bit lifeless without that 18k resistor on the bass pot. Maybe some day when I have time I'll add another 18k in parallel to see if that is a good happy medium.
     

  17. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Wyatt, thanks for the reply. I also have used different values for the mid resistor...and I have read about increasing the mid control pot's value to increase mids.
    Deadbeat....I'm still pushing the change to a smaller value for that bass tone cap if and when you go back into that amp. That is where your change is gone come. (;^)
     

  18. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    I just wanted to confirm the purpose of the added 68 Custom Deluxe Reverb 18k tone stack resistor that is located between the Bass pot and intersection of the Mid cap and Mid resistor is there to simulate the 5F6A Bassman's 25k pot set to 6.8k (6.8k + 18k = 25k).

    But the effect of the 18k resistor is to set a minimum bass level. You can see its effect with the Duncan Tone Stack Calculator. It keeps the bass from falling off a cliff as you approach minimum bass on the Bass pot. If you want less bass at the minimum setting then reduce or jumper around the 18k resistor.
     
    Last edited: Jun 8, 2016

  19. caspersvapors

    caspersvapors Tele-Meister

    158
    Nov 28, 2008
    CA
    Deadbeat, did you ever end up doing the mod to the bass cap/tone stack? Results?
     

  20. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    The 68 Custom Deluxe Reverb has the added 18k bass resistor too.
     

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