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Can a PRRI really be turned into a Custom 68?

Discussion in 'Glowing Bottle Tube Amp Forum' started by maj34, Sep 5, 2017.

  1. maj34

    maj34 Tele-Meister

    209
    Nov 19, 2013
    Calgary
    I have seen a bunch of forum posts saying you can change (with a few caps/resistors) a Princeton Reverb Reissue into a Custom 68 or vice versa. Here's a website that describes the differences:
    https://guitarampblog.wordpress.com/2017/01/26/fender-65-princeton-reverb-schematic-vs-68/

    Can anyone confirm this? Of course they have different speakers. But aside from that, are there other differences?

    One thing that grabs my attention: the custom 68 is a 12W amp and the PRRI is 15W. Does this mean they have different output transformers as well? I see that the Custom 68 lists a Schumacher transformer, but I can't find any info on the PRRI transformer.
     
    trouserpress likes this.

  2. Wyatt

    Wyatt Tele-Holic

    807
    Nov 3, 2004
    Yes. The PRRI and CPR difference are the 6 components (plus speaker) listed on that link.

    The CDR and CVR have other differences since they have two channels and switched to a tube tremolo.
    Same transformer...same output. They are both ~12 watts RMS. The rest is marketing and how they decided to list it at the time.
     
    cnlbb likes this.

  3. maj34

    maj34 Tele-Meister

    209
    Nov 19, 2013
    Calgary
    Thanks for your reply.

    Marketing. Wow.

    I wonder if there is any sort of standardized test, or generally accepted test, to come up with the output. I suppose not, or we wouldn't be having this conversation.
     

  4. xafinity

    xafinity Friend of Leo's Silver Supporter

    Dec 24, 2015
    my Mom's basement
    Yes just measure voltage at speaker terminals while striking a note and square it and divide by 2 (approx watts) or something like that, just look it up:cool:
     

  5. Wyatt

    Wyatt Tele-Holic

    807
    Nov 3, 2004
    Many ways to do it. None are standardized to my knowledge.

    Like many things it can be dependent on frequency, target THD level, and many other variables. Manufacturers generally estimate, or round up, or round down or whatever.

    Vox AC30? Four cathode-biased EL84's reaching 30 watts RMS? Unlikely.
     
    Last edited: Sep 5, 2017

  6. maj34

    maj34 Tele-Meister

    209
    Nov 19, 2013
    Calgary
    yes, power is the square of voltage over impedance... but as Wyatt says, you need to specify all the test conditions to have a fair comparison.

    Why not? When I worked out hte power output of my AC15 I got both tubes running at around 14 watts per tube. I could cook pancakes inside that thing. But now that I think about it: That's the tube dissipation due to DC bias. I suppose that wattage rating on a tube amp is actually the dissipation due to signal amplification?

    Even with the background to understand these tube amp things, there are so many inconsistent standards and misinformation about guitar tube amps that it is a brutal field to learn.
     

  7. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

    Age:
    65
    Mar 2, 2003
    Lawndale CA
    Actually, yes - but the variations are in the output based on THD % (total harmonic distortion) - meaning who/how they are deciding exactly where the headroom ends and "distortion" creeps in. There's no hard line (most amps have a small amount at every volume level or they sound very "clinical"), hence the "%".

    My guess (without looking to see if they published accurate specs) is that Fender amp marketing goosed the published number for the Custom to help justify the price difference.
     

  8. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    Why can you not make 15W per output pair? The datasheet says 17W at 1% distortion. (top of page two) I do doubt the distortion figure though. I would guess thy were running with fixed bias on the tubes, good for another watt compared to cathode bias.

    http://www.r-type.org/pdfs/el84.pdf

    Mind you on page three they give the wattage out for a sine wave input, 15W for the pair and a more realistic 3.4% distortion. Graph on page 11 says a bit better than that, but it also depends how the pair was biased.
     

  9. Dacious

    Dacious Friend of Leo's

    Mar 16, 2003
    Godzone
    Power has long been a bone of contention. Vox, Marshall and British tended to rate pre-distortion. Hence a JTM45 rated 33 watts.

    Fender Twin Reverb - a late Ultralinear might be 100 watts. An 68 with composite cathode/grid bias probably not in a month of Sunday's. A Vox AC30 in good nick at full volume (distorted) can pump out 36 watts. Same with a Marshall Plexi - 130+ watts.

    The Princeton is limited by the Champ power transformer and dinky output transformer. If you try to pull extra current out of the P/t it'll probably just heat up past a certain level. If you did pull more current, you'd then heat up the O/t. Voltages in the PR and DR aren't that far apart. But one's rated 22 ( probably 20 before distortion) and one's 14 (12). The Deluxe has more iron in both trans plus probably a more efficient phase inverter and a 12" speaker. Hence, louder.

    Fender may claim the 68 is more powerful because I think the Celestion G10 is maybe more efficient than the Jensen C10R in the 65. 3db extra would make it perceptibly louder A/B.
     
    Last edited: Sep 5, 2017

  10. maj34

    maj34 Tele-Meister

    209
    Nov 19, 2013
    Calgary
    Thanks for the info Dacious.

    FYI - I think it's the 65 reissue that Fender claims is 15W, and 68 custom they claim as 12W.

    I can see a lot of the issues with rating guitar amps now. Would be nice if (a long time ago) manufacturers decided to give you a bit of a curve - power vs THD. Perhaps then we could look at the curves for a Vox AC15/Fender Twin/Marshall JCM800 and see which one is loudest while still clean, which one has early breakup, etc. Of course this is way over-complicated and manufactuers have little to no incentive to provide this. It'd be cool though.
     

  11. MilwMark

    MilwMark Poster Extraordinaire Ad Free Member

    Apr 29, 2013
    near Arnold's
    I'm no tech but there's also the practical point that assuming the same circuit, speaker and cabinet, there is no practical (maybe perceivable?) difference between 15w and 12w. The old, you'd need 120w all other things being equal to be twice as loud.
     

  12. maj34

    maj34 Tele-Meister

    209
    Nov 19, 2013
    Calgary
    No - there isn't a big diff between 15w and 12w, but really what we're discussing is how amp manufacturers come up with the number. In the case of hte 68 custom and PRRI they are identical circuits, with a few component changes, and a different speaker - why the number differnece (the answer, if you read above, is: there is no way to justify a difference because there's no standard for how to come up with that number).
     

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