1962 Fender Brownface Pro - loss of presence after service

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by paulbrainard, May 18, 2011.

  1. paulbrainard

    paulbrainard Member

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    May 18, 2011
    Hi all, wondering if anyone can shed some light on this predicament. I have a 62 Pro that has been my main gigging amp for 15 years (Pedal Steel.) With a JBL D-130F, this has been a magical amp, full, bright, warm, clean. . But I had an elusive short that dogged me forever, causing crackling and pops at inopportune times. Over the years I have basically had the thing rebuilt chasing this down, and finally I think I got it licked by replacing a tube socket. At the same time, the tech who did the work recommended replacing some caps that had strayed from spec, putting in a fresh set of preamp tubes, and replacing some more caps in the tremolo section to try to get rid of a thump it had developed at higher intensity settings

    After I got it back the amp sounds great in some regards (quieter for sure) but there has been a drastic tonal change. It lost a lot of presence/high end, seems to be on both channels. Dialing the Treble knob up brings it back but it's a thinner brightness than before, not as "open", chimey, glassy, Fendery as before. The Normal channel is muted, and the Vibrato channel is choked & nasally. In addition, the Presence knob has lost a lot of its effect, it does add in some crispy top end but to my ear a much narrower band of it.

    So, we've started going backwards, restoring some of the original parts, putting the old tubes back in. . . mostly this has just brought back the noise from before, nothing has restored the previous tonal character of the amp. At this point, the only thing left to roll back is the tremolo circuit, there are about 5 caps in there that he replaced with poly film types, and one big gumdrop on the intensity knob (which was changed to reign in the range of the trem sweep which he thought was causing the thump when it bottomed out. . .)

    My question, before I take it back one more time and make the poor guy undo another batch of his work - is it possible that these components in the trem section would have an overall affect on the tone of the amp? Or is there something else that we should be looking at that would do this? The tube socket?? Of course the power tubes, speaker, output tranny. . . but none of those were messed with (unless there was some secondary impact from the other changes. . .) I know that it's not always possible to go backwards with these things, and it may have just sounded a certain way with the parts that were in it. . . but I want to make sure we don't go on a wild goose chase when maybe one of the steps that we took had an unforseen effect that could be undone. Also, with all the other work that has been done on this amp (not that many components in there are original, really) it has always retained its basic character. Something this time made a pretty drastic difference.

    Thanks!!!

    BTW the tech is being very accommodating about helping me sort this out, and he's done great work for me on other amps. I think we're both going to learn something from this!
     
  2. phsyconoodler

    phsyconoodler Member

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    Quote:"commended replacing some caps that had strayed from spec,"

    That's where the issue lies.The trem caps have zero effect on tone.
    Whatever he did is where the problem is,for sure.If he did some dumb stuff like increase the values of filter caps or just plain used the wrong cap values altogether.
    When I recap an old amp there is never a 'loss of tone' because I don't alter the original values,layout or anything that could possibly ruin the tone.
    Some techs think they can 'improve' the amp by doing stuff that's just plain wrong.It's not malicious by any means,just mis-informed.
     
  3. Structo

    Structo Member

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    Also in those old amps a lot of component values have drifted, especially with the carbon composition resistors.
    They usually drift up in value.

    Capacitor values can vary widely and even new electrolytic (filter caps) can have a 20% tolerance.

    When working on a vintage amp, if the owner is super happy with the way it sounds, then it is probably best to measure things carefully and replace components with those values.

    For instance, say the V1a plate load resistor is marked as 100K ohms.
    But when you put a meter on it it measures 120K.

    If you replace that with a 100K that measures 100K, it may not sound the same, especially if you replace a handful of components that have all drifted.

    I think many times the tone we love about some of these old amps can be attributed to component drift.

    Because certainly there are dogs among the old amps that may have never sounded good.

    If the amp tech is not experienced with these old amps, he may rip out all the vintage coupling caps (Blue Molded) and replace them with polypropylene orange drop caps.
    Well that certainly can change the tone and is usually a mistake unless the caps are leaking DC current badly.
    Even then, they should be replaced with a Mylar or polyester cap to stay true to the amp.

    Carbon comp resistors tend to be noisy after a lot of years but many claim they impart something to the tone that is desirable.
    So it may be best to replace bad ones with another carbon comp resistor.

    It can really be a crap shoot. The best thing is try to use a minimal approach when servicing these old amps and just try to fix the problem at hand rather than rebuild the whole amp to make it "better".
     
  4. paulbrainard

    paulbrainard Member

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    He was actually really careful with values, even tracked down some hard-to-find ones like a .003 cap and a NOS .005 one (which measured .006 but was replacing one that read .007 - we put that back in thinking maybe it was the secret agent we needed! But alas, it just brought back some noise. . .) as well as a correct vintage tube socket. And the trem caps he used are the small poly film barrels, not big orange blobs. . . He's into correct parts, while I was there he got a shipment from UPS, a bunch of unused vintage caps that someone had found in a basement (Gibson Honeybees, etc.) and the first thing he did was open the ziploc bag they were in, hold it over his face and take a deep breath like some people would with a bag of fresh green, well you know. . .

    I started wondering about the trem caps because I know some brownface amps use bias-modulating trem and thought that any changes in that section might affect the overall character of the power stage. But I think this amp (6G5-A) uses the 2.5-tube "harmonic" trem instead so maybe that theory doesn't really hold up. But don't things all kind of interlock and affect each other anyway to some degree?

    My fear is that if nothing we roll back restores the character to what it was (and I honestly don't think I'm crazy. ..), then we may have stressed either the output tranny or speaker in testing (we did crank it up pretty good with an SG at one point, which I have rarely if ever done before. . .) I can test it out with another speaker easily enough, but actually that sounds like it could be a transformer huh?

    Thanks for the feedback. . .
     
  5. Ronsonic

    Ronsonic Member

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    Transformers do not get stressed and change their sound. They just don't do that. I doubt it's the trem caps.

    He should have cleaned things while in there. Dirt sounds bad, in the process of changing things, moving things dirt gets disturbed and interferes in ways it didn't before. Like moving the furniture you find crud you didn't know you had. Sometimes when you clean, some crud gets displaced to somewhere worse. Those are high voltage high impedance circuits on that board, I would double check the cleaning.

    How many hours do you have on the amp since the overhaul?

    If less than 100 I'll suggest you may need to burn it in more.

    I know this is controversial and some people will accuse voodoo, mojo and witchcraft. Break in is a real phenomenon. Or at the least is so often perceived that I'll accept it as an explanation. If you can't stand listening to it in the meanwhile, plug in an FM tuner (set for low output level) and give it between-station noise, at living room volume and walk away for a day.

    A few ideas. Hope it helps.
     
  6. smolder

    smolder Gold Supporting Member

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    I am often guilty of overlooking the most basic things. Could the subtlety of tone you are describing be attributed to something as simple as a reordering of the preamp tubes... putting them back in in different (wrong) positions?
     
  7. paulbrainard

    paulbrainard Member

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    Well I've tried doing that and also put the entire old set back in (I think in the same order) - some subtle changes but nothing overall like what I'm noticing.

    However, I just tried cross-connecting the Pro and my SF Deluxe Rev (these two amps used to have a pretty similar tone at low-mid volume) and the JBL is sounding muddy when driven by the DR, while the Pro sounds pretty good through the 12" Deluxe cab, so I think maybe we did smash the voice coil a little when we rocked out. It's an old speaker - stock, I think (I believe this was an original option on these) and as far as I can tell has never been reconed, so it may well have taken a hit from the onslaught of a 70's SG for which it was never designed.

    I've got a Standel JBL-clone somewhere in storage that I think is very similar, I'll go find it & throw it in there to see what that does before I torture my tech any more. I think I heard tell of someone in town here (Portland OR) with NOS parts to recone these things. . .
     
  8. colinesquire

    colinesquire Member

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    That certainly could be it! I blew a pair of '69 jbls with a bandmaster reverb. Thing is it's pretty easy to tell if you've blown a speaker. I was hearing a ratty & physical clank sound. Good luck, sounds like a pain but old fenders are the best.
     

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