Fender Brownface Concert Amp issues

Discussion in 'Amp Tech Center' started by rockman627, Aug 13, 2019.

  1. rockman627

    rockman627 TDPRI Member

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    I refer you to http://ampwares.com/schematics/concert_6g12a.pdf.
    A friend of mine retired from playing and gave me his brownface Fender Concert amp.It wasn't working so I ended up rebuilding the circuit board. I managed to not screw anything up; however, the normal channel was way louder than the vibrato channel. I routed the normal channel to the vibrato circuit and vice versa and used the amp for a couple of years. Then I started getting weak or no sound intermittently. So I put it aside and used something else.
    Then I read on Rob Robinette's site that old tube sockets could give you that problem, so I replaced them all and that seems to be working. But I always wanted to find out why the vibrato channel was lower and address that before I put the chassis back in. I think I finally found it.
    V1 pin 6 goes straight to a .05 capacitor while V2 pin 6 goes to a 100k resistor, and then a .05 cap.
    My initial plan was to switch the 100k resistor with the 10k next to it, but I may be messing with some function I don't know about, so I figured I better check with the experts.If that works what about putting a resistor between V1 pin 6 and the .05(actually .047)cap? The normal channel is very loud and starting to break up at 10 o'clock and I would like to tame that a little.
    Thanks for your consideration on this.
     
  2. D'tar

    D'tar Friend of Leo's

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    That layout is a mess and can cross you up pdq.

    Don't do this ^^^

    A complete voltage chart and a metric sh*(&^t ton of pictures will be required. Not knowing your experience here, this may need to see a tech.
     
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  3. peteb

    peteb Friend of Leo's

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    I agree that you found an area causing signal loss that could be changed.


    I don’t see all the caps you mention, I see others and can’t predict the effect of caps bypassing resistors but the 100k followed by a 10k in series to AC ground, B, is a voltage divider and if there were only resistors here 91% of the signal would go to ground. See how they take the signal out after the 100k and before the 10k.


    10/ 10+100= 9% of signal left



    You could make the 10k larger, but probably not too much larger. Try a pot? Dial it in?


    You could make the 100k smaller. You could do this by paralleling the existing 100k with another 100k or any other resistor or a pot.



    It could mess with the balance of the trem circuit.
     
    Last edited: Aug 13, 2019
  4. D'tar

    D'tar Friend of Leo's

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    .Have you tried a new tube in v5. I bet that trem could kill a tube over time.

    Edit: try swapping v1<>v2 also?



    routing the normal channel through vib channel made things operate as desired?

    I count an unused triode;)
     
    Last edited: Aug 13, 2019
  5. rockman627

    rockman627 TDPRI Member

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    the vib channel output was very weak. I re-directed the output of the loud normal channel from the phase inverter to the vibrato circuit. I re-directed the output of the weak vibrato channel from the vibrato circuit straight to the phase inverter. This gave me the stronger sound and the luscious tremolo. If I don't figure out why the vibrato channel is so much weaker, I'll just keep it this way. But I would like to figure it out.
     
  6. BobbyZ

    BobbyZ Doctor of Teleocity

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    Post some pictures so we can better wrap our collective brains around this.
     
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  7. Snfoilhat

    Snfoilhat Tele-Afflicted

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    You may have made a mistake. A component may be messed up or drifted out of spec. But leaving that possibility aside the Normal channel is louder than the Vibrato channel by design. There are threads on TDPRI where after people marvel at how cool and rare the harmonic tremolo is in Fender offerings, the person who owns the amp will say they really only ever use the Normal because they like its gain structure better. The large brown amps are all like that. The reason is even in your post -- the Normal channel has a lot of gain, more than an amp designed and marketed in 1961 needed. The Vibrato channel has lots of headroom.

    The aggressive voltage divider knocks the signal back so that the extra gain stage of the vibrato channel has plenty of headroom and the total signal can be brought back up to the neighborhood of other Fender amps.

    If the Vibrato channel is not hot enough, then you can just change the ratio of that split plate load, leaving the sum at 110k and leaving the coupling cap at the intersection of the two resistors. 90k+20k, or 80k+30k, or 70k+40k.

    Similarly the normal can be split into 10k+90k, or 20k+80k, etc., and the coupling cap moved to the intersection of the (now) two resistors.

    Preserving the total plate load resistance means the tube will continue to operate as it was designed. Good luck
     
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  8. D'tar

    D'tar Friend of Leo's

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    Excellent^^^^ Snfoilhat,

    If all is wired correct and component values correct. A signal injected into either channel should be practically identical measured at the plate of the second gain stage with vol,tmb set the same on both channels.

    I would confirm this before changing the divider.

    Why can't I find this highlighted cap on the layout? Am I just blind... That cap will dump some small amount of signal so to compare channels you may want to lift one leg.


    upload_2019-8-14_11-1-30.png
     
  9. rockman627

    rockman627 TDPRI Member

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    OK this is what happened:
    In the vibrato channel, I removed the .003 cap (actually .0022), removed the 10k resistor, moved the 100k to where the 10 K was. This makes the normal preamp and the vibrato preamp literally identical. I did get an increase in volume on the vibrato channel, but the normal was still a bit louder, as opposed to way louder. Then it occurred to me to put a smaller gain tube in V1 so I put a 5751 in there. It tamed the normal channel nicely. The volume of the normal channel at 3 equals the volume of the vibrato channel at 4 1/2. I suppose the remaining difference may be due to the input resistors and/or the tone controls drifting off value. I'm going to keep it this way because I am trying to keep what I can original, although with a new circuit board and new tube sockets it's starting to turn into an amp kit. It does however, have a sweet crunch. Can't wait to take it out to some jams.

    BTW the .003 cap does not show up on the layout. I built the circuit board according to the layout and when I was done the vibrato channel had no sound. It took me a while to see the schematic had an extra component. I checked with the old circuit board and figured out where to put the .0022 cap. And here are some pictures:
    20190817_151148.jpg 20190817_150717.jpg
     
  10. ocduff

    ocduff Tele-Meister

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    The Vibrato channel on larger Brown amps - or specifically the 6G series (and even 5G) I love and had experience with, the Vibrato channel is always weaker than the Normal channel.

    I believe it’s just the drag of the Vibrato on the Vibrato channel. On the 6G there is half a 12ax7. I wonder why Leo didn’t opt to use this as a make up gain stage.

    BUT - Vibrato channel is nice and mellow for humbucker and Normal channel is nice and warm for single coils.

    It depends on amp and condition of a lot of things as far as how hot the normal channel is and how weak the Vibrato channel is in comparison.

    My experience in messing with it is just to leave it alone. Amps are weights and balances and often mods ruin some other positive attribute while fixing a bad attribute.

    IE your amp is normal.
     
  11. Luthier Vandros

    Luthier Vandros Tele-Holic Silver Supporter

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    More pics! I'm so intrigued by the Brown Concerts.
     
  12. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    OK, OK!

    #53331

    One speaker was replaced before I bought it - with the same general type Oxford. I've had tremendous luck with Oxfords in Fenders, and this amp has what I feel is THE prototypical Fender clean tone for its output level. Just stunning - especially paired with the matching Reverb unit! Guts 9-6-2012.jpg 1962 Concert - front view.jpg Speakers - 3 orig 1 repl corr type.jpg 1963 Fender Reverb.JPG
     
  13. ocduff

    ocduff Tele-Meister

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    Egads that’s sexy. You almost never see Concerts with the original speakers. Love those “cappuccino” Oxfords. Nice creamy tone.

    And yes the Normal Channel with sound gainier than Vibrato channel, and fatter. I’ve heard Normal channels in Browns and Blondes with vib/trem that were too gainy to use. And I’ve heard Vibrato channels that were positively thin and anemic.

    They both should sound healthy and the Vibrato a bit skinnier and not quite as gainy and the Normal channel should sound about like the gain of a stock 5F6-A setup.

    I think on Blackfaces there was an extra gain stage as well to makeup for reverb and tremolo losses.
     
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  14. Luthier Vandros

    Luthier Vandros Tele-Holic Silver Supporter

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    That's GORGEOUS!
     
  15. powerwagonjohn

    powerwagonjohn Tele-Meister

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    I have a 62 Pro and a 62 Vibrolux. The brown amps have a sound I really enjoy, I think they are very special amps. Not that I don't like my blackface and Silverface amps. The browns are more MY clean punchy sound with just a bit or hair on it.

    Thanks John
     
  16. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Why?
     
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