Gibson GA-5 Skylark too dark

Discussion in 'Amp Tech Center' started by AlexPerry, Jul 27, 2021.

  1. AlexPerry

    AlexPerry TDPRI Member

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    Hi,
    I have a 1960 Gibson GA-5 Skylark that is very dark sounding. Pretty much unusable with any neck pickup without some serious treble boost from a pedal.

    But when I turn it up m, the overdrive, while still very dark, has a fizzy top to it.

    Would like to fix both, but my main question is if anyone has any tips to what resistor/capacitor value to change to make it brighter. Or is it an idea to add a bright cap?

    Schematic:
    http://www.0rigami.com/gg/GA-5.pdf

    Thanks
     
  2. Jon Snell

    Jon Snell Tele-Holic

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    Perhaps the speaker cone has gone 'soft', (come away from the speech coil), causing low HF response. Try another speaker.
    The fizz could be the 6V6 suffering. Check voltages are in order first before changing things at random.
     
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  3. AlexPerry

    AlexPerry TDPRI Member

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    I have a new speaker in it. I have tried two different 6v6’s and the voltages seem to be fine
     
  4. Jon Snell

    Jon Snell Tele-Holic

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    Try this;
    Screenshot 2021-07-27 at 07.52.44.png
     
  5. AlexPerry

    AlexPerry TDPRI Member

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    Not too good at reading schematics, but does that mean a bright cap?
     
  6. timfred

    timfred Tele-Meister

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    Before changing anything inside the amp, take a 1/4” jack and connect a 1M resistor from tip to sleeve. Then plug this into the unused input jack.

    This will make the input circuit more Fender-like.

    But even without that, this should not be a “dark” amp. Something is wrong somewhere. I’d start by measuring the values of all the resistors and making sure there’s no DC voltage after the coupling caps.
     
    Last edited: Jul 27, 2021
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  7. Lynxtrap

    Lynxtrap Tele-Afflicted

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    That is basically a tweed Champ, perhaps a little hotter and even a little brighter. So I agree that something seems to be wrong.
    Did it always sound like that?
     
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  8. AlexPerry

    AlexPerry TDPRI Member

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    I haven’t had it for that long, but yes.
    My friend also bought one recently and it is dark sounding too (and it had just been serviced). Kind of like when you roll the tone of your guitar down.

    Now, dark sounding is of course an objective thing, but it is really muddy when I use a neck pickup. And the clean sound are for my taste way to dark.
    I am comparing it to my 64 crestline version of the Skykark, which is very bright (which I like for these kind of amps)
     
  9. Lynxtrap

    Lynxtrap Tele-Afflicted

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    OK.

    The 1M experiment mentioned by @timfred is worth trying first, to set the input impedance for the guitar signal. If it improves the sound, you need so solder in a 1M resistor between input and ground.

    If you want to mod the amp, the bright cap across the volume pot as suggested by @Jon Snell is an easy one.

    After that, pretty much every capacitor (excluding filter caps) is up for grabs = lower values. C1, the bypass cap for the V1 cathode, would be an obvious place to start. You could probably go as low as 1 MFD on that one.

    I would be tempted to disconnect the negative feedback and bypass the V2 cathode with something like 5-10 MFD.
     
  10. AlexPerry

    AlexPerry TDPRI Member

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    Thanks for the tips! Where exactly is the negative feedback resistor? I see it on the schematic but can’t really understand where that is in the actual amp. Is it shown on the picture below?

    ps: the old 3-in-1 filter cap is disconnected and replaced with new ones by previous owner, but left in, I have since removed it
    E210DD4C-A07F-4A18-B9AF-9A7BE97D5C3C.jpeg
     
  11. Lynxtrap

    Lynxtrap Tele-Afflicted

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    Start with a voltage meter before you do anything and make sure there is no voltage present anywhere in the circuit!

    It's a little hard to tell the pinout from that picture. NFB is probably a wire from pin 3 or 8 (the cathodes). One of those pins is the the one with a 2.2K resistor and a cap across it going to ground at the input jack.
    Look at the other one, whether it's 3 or 8, and trace the wiring to that one. That pin should also have a 2.2K resistor connected to it, but no cap across it. Unless the amp has already been modded.

    But I would recommend you look at possible mods in the order that I suggested in the previous post.

    If you are not sure what you are doing, ask further and post more pictures and the folks here will be able to help you.

    Also, I think I see some safety issues there, like exposed parts of plate supplies and the live end of cathode components seemingly hanging above the chassis supported only by the wires.
     
    Last edited: Jul 27, 2021
  12. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    GA-5s aren't generally considered dark amps. Especially not "very dark." And you have fizz, too; as folks above are saying, something is likely wrong. I'd overhaul before I started modding.

    Is that paper multicap original? Unless it's been recently re-stuffed with modern filter caps (noting the waxy end), that could be your problem for sure. I'd automatically have that replaced, test all resistors for drift, and if that doesn't help, consider replacing *all* the old caps.

    For folks who haven't unpacked the link above:

    GA-5 copy 2.jpg
     
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  13. AlexPerry

    AlexPerry TDPRI Member

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    the old 3-in-1 filter cap is disconnected and replaced with new ones by previous owner, but left in, I have since removed it
     
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  14. muscmp

    muscmp Tele-Afflicted

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    i have a 64 ga5t with two 6aq5s and it is quite bright so yea you have some digging to do.

    good luck.

    play music!
     
  15. Jon Snell

    Jon Snell Tele-Holic

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    Hint: If the old capacitor is sealed with bees wax, most were, keep it handy to remove or insert awkward screws that are not magnetic by placing a lump of wax on the tip of the screwdriver.
     
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  16. AlexPerry

    AlexPerry TDPRI Member

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    My 64 is also very bright, but they are totally different amps, though they share the same name
     
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