Attn Twin Reverb experts - questions about a 68 (pics)

Discussion in 'Amps and Cabs' started by rusted, Dec 1, 2005.

  1. rusted

    rusted Member

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    Questions pertain to this 68 (warning, big pics).

    I know it's not the AB763 circuit, but it is a non-master volume.

    First, if any of you experts can offer any comments about the circuit or components based on those pics I would certainly appreciate it. It doesn't appear molested to me, but I know a bit less about vintage amp circuits than vintage guitars (for example Blackfacing alterations that may have been made).

    Secondly, it was loaded with those extremely heavy Electro-Voice EVM 12L speakers, and the weight had broken one of the baffle mounts off the inside of the cab (pictures in link). Those things were just held on by two big staples. What would be a proper repair? Drill it for two screws?

    I'll probably start another thread about what speakers I might put in this, but if you have an opinion, would Eminence Red Coat Red Fangs be alright wattage-wise? I happen to have two in a Bassman cab (and two lesser wattage Webers to go in that cab). I think this Twin is 100W. Those speakers are rated 30W rms/60W peak. I'm not rocking any stadiums, and in fact main use would probably be Rhodes piano, then Tele and lap.

    Appreciate input and advice. Thanks.
     
  2. Reeek

    Reeek Member

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    I have a 1969 and LOVE IT TO DEATH. I have original JBL's in mine.

    As for the pics, they all look pretty much untouched except (and I am not an expert) for those three caps on the terminal board in pic number two. My 69 had all chocolate drops before electronic refurbishing but then, I could be way off base and those caps could also be stock. It would make sense since the filter caps look original and by that, I mean anyone who would go inside and replace some poly caps would most likely do the filter caps for general purpose but it's all speculation on my part.

    One thing is for sure, send that amp to a good Fender tech and spend about $100 to $250 ($250 give or take for a blackface job if you so desire) on it and you have yourself a fantastic amplifier.

    As for thebaffles mount, that's what I would do. I'd doa careful drill job for some screws making sure they don't inpede the back panel screws and I would probably glue it before I screwed it down.

    Here is the Fender Field Guide page for SF Twins:

    http://www.ampwares.com/ffg/twin_reverb_sf.html

    Here is the Fender Field Guide main page:

    http://www.ampwares.com/ffg/index.html
     
  3. Shea

    Shea Member

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    Kind of hard to say how much of that is original inside. Looks to me like it might have been worked on by an excellent tech who favors original-looking parts doesn't leave a lot of tell-tale signs of his work behind.

    I have a '68 that had that exact same circuit, but there weren't any of those axial blue caps in it. It only had baby poop caps and maybe one radial blue cap.

    So, the blue caps may have been added, or you may have gotten one of the last amps that had blue caps installed originally. Either way, keep them because they sound great.

    Some of the plate resistors and tone stack slope resistors may have been replaced. Although the schematic calls for 1/2 watt resistors in those positions, I'm almost positive mine had 1 watt resistors intead. And you can see flecks in the wax around the eyelets where someone was probably doing some de-soldering and re-soldering.

    The orange drop caps were obviously added. I'd keep those. You might ask, "Aren't orange drops supposed to be harsh sounding?" Well, what really matters is whether they're polypropylene orange drops, because those caps pass through high end more clearly than polyester caps do, which is why some people believe they're "harsh" compared to polyester caps such as Mallory 150s, mustard caps, or those blue caps. But having just 3 orange drops in your amp isn't going to make the whole thing sound harsh, so leave 'em in there, I say.

    Overall, it looks beautiful inside and I wouldn't let the possibility that some of this stuff was changed deter me from buying it if I wanted this amp. Whoever worked on it did nice work.

    In my '68 Twin, I got rid of that cathode biasing setup, changed the hum/bias balance control to a bias voltage control (a la blackface circuit), got rid of the suppression caps on the power tubes, and changed the pi to blackface specs (it's halfway there already, being a transitional circuit). But you might try playing it before you do any of that stuff. If you really love it, you might just decide to leave it as is.

    But definitely change those electrolytic caps, at least the filter caps.

    Hold onto those those Sylvania 6L6s with your life. They rule!

    Shea
     
  4. Lord Valve

    Lord Valve Active Member

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    The PI coupling caps have been replaced with orange drops.
    There are two reasons to replace these - one is for failure
    (uncommon but not impossible) and one is because some
    wanker believed that "matched" PI couplers will do something
    magical. (In fact, they will...your money will magically appear
    in the wanker's pocket.) This replacement won't hurt anything
    (other than your feelings) so leave it alone.

    All of that cathode bias crap needs to be stripped out of the
    output stage and the bias circuit needs to be converted to the
    blackface type. The PI stage will need blackfacing, too. The
    amp will almost certainly require a cap job. Once this is done,
    you will have a *smokin'* blackface Twin, even though it won't
    look like one. The lead dress, especially the filament string,
    needs to be cleaned up. Smooshed-down filament wiring
    causes hum; you'd be surprised what you can accomplish
    with just a spudger in the way of hum reduction. Those twisted
    green wires need to be as far up in the air as they can go, and
    dressed nice and straight.

    When replacing speaker baffle supports, I like to pre-drill the
    block for a couple of screws, run them in and then back out, and
    then shoot some hot glue underneath the block and tighten
    the screws while the glue is still hot. They'll never come loose
    again.

    Lose the EVMs, all they do is cause shoulder pain. A pair of
    Weber Chicago 12" (the ceramic version) will put your tone
    into the near-legendary area.

    Routine maintenance like socket re-tensioning (if required) and
    cleaning the jacks, controls, and switches will make more of
    a difference in the amp's sound than you might think. You have
    a good set of power tubes, and if they are reasonably well
    matched, I'd leave them in. I didn't see your preamp tubes,
    so I'll reserve judgement on those. I'd use a set of the same
    preamp tubes I put into Derek Trucks' amps if your amp was
    on my bench. Lastly, I'd remove any grounds that are derived
    by wires attached to ring terminals underneath transformer
    mounting nuts and solder those wires directly to the chassis.

    You have an amp that can be absolutely *killer* if you can get
    it to someone who knows how to tweak it. As is, it's probably
    pretty average-sounding.
     
  5. rusted

    rusted Member

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    Thanks for the responses.

    On an amp of this size do techs take in just the chassis for work, by shipment or even in person?

    I'd be game for shipping the chassis to a good tech, but if anyone has recommendations in the LA area please let me know. Problem in SoCal, I would guess, may be cost and high volume of work.
     
  6. Shea

    Shea Member

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    540
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    Some other ideas:

    3 conductor power cord - that's almost essential.

    Replace the ground switch with one that has a center off position.

    If you ever plan on using the convenience outlet, you can replace it with a 3-terminal one available from Allied Electronics. It's kind of expensive for one lousy outlet, but I did that with my '68 Twin and I actually use the thing sometimes.

    Shea
     
  7. trisonic

    trisonic Member

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    You got some great answers from the other guys.
    I don't know about Socal (well I did but it's some ten years out of date).
    When I'm having comprehensive work done on an old amp like yours I tend to leave the whole thing with a tech (that I trust). Just so he can test as a "whole package". As you've got to replace the speakers that may be a moot point.

    Best, Pete.
     
  8. Wakarusa

    Wakarusa Member

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    The rosin splash and different body color on the two plate load resistors is pretty obvious rework, but there's (usually) a really easy way to identify rework in old Fenders - even when the tech is very careful.

    All of the old Fenders built with eyelets on fishpaper were bottom-soldered. A bottom-soldered joint tends to have a raised, rounded lump of solder. Further, the solder usually won't completely cover the eyelet (there will be a small ring of "bare metal"). Rework done from the top of the board will usually lack both of these attributes (no lump, and the wetting action of the rosin/flux makes the solder cover the eyelet completely) -- and it's really rare for a tech working on one of these to pull the whole board to work from the bottom.

    Some folks will look for oxidation of the joint (dull grey instead of shiny), but a dull joint can be just a cold (bad) joint or really old rework.
     
  9. RussB

    RussB low rent hobbyist Silver Supporting Member

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    Cool Amp!

    Very similar to my '69. The baffle was replaced because the JBL's were too much for it over time. Previous owner made a real nice piece.

    My amp was "attacked and hacked" at some point in time. I converted mine to blackface spec's...and gave it the big servicing. I have a set of Celestion Vintage 30's in it. I don't miss the JBL's at all!


    Here's what the chassis looks like after everything was said and done...

    [​IMG]
     
  10. doctorx

    doctorx Member

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    Lord Valve and the others give the right advise about converting to the AB763 Blackface circuit, and replacing the electrolytics. You have the round blue caps in the preamp circuit, keep those they are the good ones.

    I have a Bassman 100 whose cabinet was really beat up, and the baffle mounts were loose. I sanded the old glue off, straightened the staples and reglued them using a good wood glue, clamping them tight while the glue dried. You maybe might want to swipe some wood filler into the holes before you do it. A couple of screws wouldn't hurt, but be careful you use the right size.

    regarding the evms, if you play clean they are ok and they take a lot of power, but they don't have much character. Ted usually gives good recommendations, I would listen to him.

    regis
     
  11. RussB

    RussB low rent hobbyist Silver Supporting Member

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    About speakers...

    Red Fangs are a bad choice. their power handling capacity is too low for use in a Twin.

    Those EV's will sell real well if you need to unload them
     
  12. thintele

    thintele Member

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    I highly recommend Skip Simmons ...he is one of the best with old Fenders. He is up in the Bay Area. If you are going to ship I would just ship the chassis, but if you going to drive I would take him the whole amp. I'm sure he would have a couple of recommendations as far as speakers as well ...if you decide to change them out .
     

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