Princeton 68 Reissue R34 Resistor Have Fender Removed this?

Discussion in 'Glowing Bottle Tube Amp Forum' started by woodnut, Jun 3, 2020.

  1. woodnut

    woodnut TDPRI Member

    Age:
    40
    Posts:
    8
    Joined:
    Jun 3, 2020
    Location:
    UK
    Hello Folks,

    A new Fender Princeton 68 Reissue owner here. I picked this up second hand but it is less than a year old.

    I have been reading a lot about this amp and the various mods. Last night I took a peek at the board, I have been considering adding a mid control.

    Inspecting the board one thing that has me puzzled is that the resistor R34 does not seem to be installed and further to this, the PCB trace that I understand Fender cuts in order to get R34 the 18K resistor in there, is not cut.

    I am happy with the bass control so it is not that I am looking to add R34 in there but I am curious to know why it is not installed and the trace isn't cut since this seems to be the standard spec for the 68 reissue.

    I thought perhaps there has been a revision with R34 now underneath the EQ control PCB and Fender no longer cut the CB trace. However, I measured resistance with a multi meter between the two points where R34 is usually installed I can see that it is not there.

    Does anyone know if Fender stopped cutting the trace from the bass EQ pot and adding R34 18K on the Princeton 68 Reissue?

    Or has it simply been forgotten when my amp was built?

    Many thanks.
     
    That Cal Webway likes this.
  2. Wally

    Wally Telefied Ad Free Member

    Posts:
    35,780
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    In your situation, I would be curious enough to pull that front panel PCB and see what tone stack caps for the bass and treble were used.
     
  3. Wally

    Wally Telefied Ad Free Member

    Posts:
    35,780
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    And...welcome.
     
    That Cal Webway likes this.
  4. woodnut

    woodnut TDPRI Member

    Age:
    40
    Posts:
    8
    Joined:
    Jun 3, 2020
    Location:
    UK
    Hi there thank you for your reply.

    I have got a 25K pot arriving tomorrow which I will hopefully install as a mid control. Yes, I figured I would have a look at that point and cross reference a few of the components against the schematic for the 68 custom reissue and also the blackface 65 reissue to see if there is anything else that isn't to the expected specification.
     
  5. Wally

    Wally Telefied Ad Free Member

    Posts:
    35,780
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    If I wanted to install a mid control pot, I would use a larger pot than that...50K at least. This will allow for a considerable increase in mids beyond that 25K limit.
     
  6. woodnut

    woodnut TDPRI Member

    Age:
    40
    Posts:
    8
    Joined:
    Jun 3, 2020
    Location:
    UK
    Thanks for the tip.

    25K seemed to be the most common one suggested when I did some looking on forums. I will see how I get on since I have already ordered that but if it doesn't give me enough of the mids I am looking for when fully engaged I can easily get a 50K in there and swap it out.
     
  7. Axegrinder77

    Axegrinder77 Tele-Holic

    Posts:
    869
    Joined:
    Mar 21, 2019
    Location:
    Springtown
    Not sure how relevant this is, but I soldered a wire around the 18k resistor and am happy with the result. Looking back, it makes no sense the way it was, as most find the bass unusable above around 2 with it stock.

    Mid control is interesting, although a bit ambitious for my level of electronics skill. I just use a tubescreamer! :)
     
    tah1962 likes this.
  8. Mad Kiwi

    Mad Kiwi Friend of Leo's

    Posts:
    2,834
    Joined:
    Nov 17, 2009
    Location:
    Land of the Long White Cloud
    Snap. I just recieved my near new 68 Princeton Custom Reverb last week.

    I have the same issues as most in that the bass is too much and it is a bit middy for the normal Fedner clean sound. From all my reading it seemed that despite turning up the bass and tone, it actively lowers the mids. I am not sure if this is true but it does sound like it (placebo?)

    I intend to put the R18 bypass on a switch.

    I am hoping to tame the bass slightly but not all the way to 65 PRRI levels, I REALLY love the amp except for this slight fine tuning.

    I am surprised to hear anyone wants to increase the mids, from all my reading that would be a first :)
     
  9. 1300 E Valencia

    1300 E Valencia Friend of Leo's

    Posts:
    2,631
    Joined:
    Dec 13, 2010
    Location:
    Fullerton CA
    Depends on what you consider "normal".
    The '68 Custom Princeton is not a blackface, it's based on the tweed sound.
     
  10. Mad Kiwi

    Mad Kiwi Friend of Leo's

    Posts:
    2,834
    Joined:
    Nov 17, 2009
    Location:
    Land of the Long White Cloud
    Its my understanding the amp IS a 65 board with mods to the tone stack to be more in line with a brownface sound. I'd like to split the difference a little between the 65 and the 68/brownface.

    I figure the r18 mod and possibly if further required try the same speaker the 65 comes with .....Jenson C10Q (from memory?)

    I suspect the R18 will be enough though. I think it will be a bit more flexible with a tad less bass. Especially if the less mids with more bass is true...
     
  11. woodnut

    woodnut TDPRI Member

    Age:
    40
    Posts:
    8
    Joined:
    Jun 3, 2020
    Location:
    UK
    I am mostly enjoying the scooped mids on my 68 custom princeton, but I am finding there are a number of times where I seem to be fighting to get some more openness in the mid range.
    I dug out a cheap Artec EQ pedal and put a slight boost on 500, 800, 1.4K. It worked really well for for those particular pieces I was playing. For me it removed the sense that a blanket was over the speaker. Its just certain styles and pieces that I play where I am missing those mids. I figure if I can get some flexibility by adding a mid control then that would be fantastic, it seems to be quite a common mod for these amps from what I have read.

    Although the EQ pedal worked quite well, it doesn't seem to make a lot of sense to boost the mids with an EQ pedal and then cut them in the tone stack.

    That said, I hadn't ever really considered using an EQ pedal in my electric rig before and it does add some flexibility. The Artec is quite noisy and it has peaked my interest in getting a higher quality EQ pedal for some subtle tone shaping.
     
  12. Mad Kiwi

    Mad Kiwi Friend of Leo's

    Posts:
    2,834
    Joined:
    Nov 17, 2009
    Location:
    Land of the Long White Cloud
    Oh that reminds me, the joyo american in front does wonders.
     
  13. tah1962

    tah1962 Friend of Leo's

    Posts:
    2,650
    Joined:
    Jan 27, 2020
    Location:
    Heart of Illinois
    +1...I also ran a jumper wire on the 18k resistor and It makes the bass control usable now. Prior to running the jumper my settings were, bass 1-2 / treble 8-9. After the mod, bass 3-4 / treble 6-7.
     
  14. Wally

    Wally Telefied Ad Free Member

    Posts:
    35,780
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Note that the 68 SF Custom line of amps are based one the classic AB763 preamps. The AB763 preamp was designed to scoop mids. The stated aims of the modifications was to decrease the amount of midrange scoop to move the amps toward the tweed thing. The 68 SF Custom Prin Rev’s mods are designed to do this to a greater extent than the other 68 SF Custom amps in that the tone stackO values underwent more significant changes. So, if one buys one of these SF Custom Prin Rev amps wanting mid scoop, they have bought the wrong amp. That said, if one buys one of these amps wanting Tweed sounds, they have bought the closest thing Fender has built in the 68 SF Custom line of amps. If one wants to take the 68 SFCPR farther toward tweed sounds, it can be modded to go there. I personally do not understand why that 18K resistor is there, though. Fwiw, in other 68 SF Custom amps, that resistor is the ONLY change.....and I don’t hear much use for that change in those amps. The 68 SF Custom Pin Rev has some of the basic changes that techs have been making to the AB763 preamp for a long time.....but it does not go far enough if one really wants to get there.....or to get to the 6G2 Brown Princeton, either, imho.
    Ime, IF one wants to accomplish a wide variety of sounds with nothing but a guitar and an amp; one needs an amp that has tonal flexibility AND gain flexibility. In the Fender world, there are a very few amps that can do this. The Rivera era amps will do this. the Super Champ being my favorite of those. The Pro Sonic will do it. The Fender 75 with the change of two resistors....one in the mid boost circuit and one in the negative feedback circuit...will do it. The Mesa Boogie MKIV and V amps will do it. Simple one channel amps without flexibility such as a Princeton Reverb/Sf 68 Pin Rev will not yield anything except what it yields without help from outside.
    I have a guitar with 16 different options. I have had young players listen for a few minutes and then say things like...“You don’t need a pedal board with that guitar, do you?” It starts at the guitar and ends up out of the speaker. Lots of variables.....some people like the variables at the guitar. Som elite the variables in the amp. Som elite a lot of pedals on the floor. billy Gibbons uses elaborate EQ systems with every guitar to make every guitar he plays sound like Pearly Gates. Each of those EQs cost more than most of us can to spend for a single amp.
    End of rant....enjoy the search. It is long. It is regarding, too.
     
  15. Axegrinder77

    Axegrinder77 Tele-Holic

    Posts:
    869
    Joined:
    Mar 21, 2019
    Location:
    Springtown
    I like the cpr as a pedal platform. You get a nice thick base tone from which to add. And you get good trem and reverb. The 18k resister jumper mod made this my perfect amp. I need manageable volume, and just mic for gigs. My 1x12 cab angled up gives me enough monitoring. Any louder and I'm getting hearing damage.
     
    Wally likes this.
  16. woodnut

    woodnut TDPRI Member

    Age:
    40
    Posts:
    8
    Joined:
    Jun 3, 2020
    Location:
    UK
    By way of an update, at the weekend I made the following changes :-

    - Installed a MOD 3 spring reverb tank
    - 25K pot installed in place of the 6.8K tonestack mid resistor
    - 500k pot installed for negative feedback adjustment in series with the stock NFB resistor

    There was no R34 resistor on the board and as mentioned the PCB trace was not cut. All other tonestack components seem to correspond to the schematic. I can only guess that when this custom 68 princeton was made, it got forgotten.

    In terms of the mods, I am pleased with these. The mid control is giving me a bit less mid scoop when I want it and a little reduction of negative feedback seems to make the amp feel less tight, a bit more open and responsive. I could possibly use a higher value mid pot as was previously recommended but for now I think I will leave it.

    The reverb tank is much better for me than the stock one that it replaced, the 3 springs I think are contributing to this and are more to my taste. I was constantly fiddling with the reverb control before either having too much or too little reverb whereas now I am able to dial in what I want and then forget it and play. It sounds quite lush.


    One area I am looking at now is the cable that connects the reverb tank to the amplifier. I am unfortunately getting a hum that gets louder when the reverb level is increased. It is a touch louder with the new tank than with the stock reverb tank.

    I was looking to make a cable that is shielded and connected in the best possible way. The stock RCA cable looks very thin and cheap.

    I have read various threads on this and am a bit confused about the best way to wire the cables between the amp and reverb tank. My confusion relates to the mentions of lifting the shields.

    I have some 75ohm coaxial cable that I could use. It has two conductors, a central conductor and a surrounding foil / braid outer that are in contact with each other all the way along the cable.
    If I simply wired this as I normally would a pair of interconnects, I would have the RCA center pins to the inner conductor and the outer foil/braid would be connected to the barrel of the RCA connector. Would two runs of this be the correct and best way to get signal to and from the reverb tank? I think there is something that I am missing here however.

    The new reverb tank chassis is tied to the red (output) outer RCA barrel whereas on the other side of the tank (input) the chassis is not connected to the outer barrel of the RCA. this is the same as the stock tank.

    Many thanks in advance
     
    Wally likes this.
  17. Wally

    Wally Telefied Ad Free Member

    Posts:
    35,780
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Wood it, not having a schematic handy I am wondering what the two resistors in that NFB loop are. I have used not heard of using a pot of that large of a value. I would think that a 10k or 25k pot would be all one would need and would afford a finer, more precise adjustment of the NFB signal. If a loop uses a larger value for the top resistor...say 56k...than a 100k pot might be in order. with that 500k pot in series with the 5.6k resistor, all of the effect of the adjustment is taking place very quickly at the beginning of the sweep of the pot.
     
  18. woodnut

    woodnut TDPRI Member

    Age:
    40
    Posts:
    8
    Joined:
    Jun 3, 2020
    Location:
    UK
    The negative feedback resistor (R10) in my Princeton 68 custom reissue is 5.6K. I have the 500K pot in series. I got that value pot from a couple of threads and also a magazine article (guitarist magazine I think) that I found. I will probably end up swapping it for a switch and fixed resistors once I have spent some more time with it as I find I am setting the pot at pretty much 2-3 positions. I am always preferring a little less negative feedback than stock so far. I guess at full rotation there is very little negative feedback, it gets a little loose and hairy there.
     
  19. Wally

    Wally Telefied Ad Free Member

    Posts:
    35,780
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Woodnut, if you want to explore NFB, Aiken has an excellent article...Aiken Amps, Tech info. I do not understand nor have I ever read anything about using such a large value pot there. The equation will illuminate the subject. If you are curious, give a smaller pot a try....you will see the increase in the precision of the variability. That 500k pot does not give much precision....it very soon approaches the lack of a loop, imho, way down at the beginning of the sweep...as you describe. For instance, in a BF Bassman which uses a 820ohm/47ohm arrangement there, a 5k pot is sufficient....10k at the max. I have been installing adjustable NFB loops for a long time in Fenders and pay close attention to how different resistances work in the circuits.
     
  20. woodnut

    woodnut TDPRI Member

    Age:
    40
    Posts:
    8
    Joined:
    Jun 3, 2020
    Location:
    UK
    Thanks for the tip I will try a smaller pot in there and see how that is. I'm finding that I am not really fiddling with it much hence my idea to move to a switch at some point. I am generally just adding a bit of resistance via that pot (less NFB) to loosen the amp up a bit and then leaving it be. Easy to try a smaller value I guess.

    This is where I got the information from regarding the 500K NFB pot.
    https://guitar.com/guides/diy-workshop/diy-workshop-rift-amp-mod-part-2/
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.