'57 Gibson GA-5 crackling...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by NatStrat79, Dec 15, 2005.

  1. NatStrat79

    NatStrat79 Member

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    I just got a 57 gibson ga-5 amp about a week ago and while it sounded amazing at first it seems to be going downhill..Today when i fired it up i noticed that when i would turn the volume up above about halfway that there would be a drop in the volume of the amp and then it would start crackling and sound kinda farty when id play notes on the lower strings. Id tried changing out all the tubes but that didnt really help. Im planning on recapping it this weekend and crossing my fingers that that helps but im still new at this fixing up old amps thing so i could sure use some advice on some other things to look for while im in there.
     
  2. NatStrat79

    NatStrat79 Member

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    Well i discovered the problem so i thought id post incase someone else has a similar problem in the future. I replaced all the electrolytics including the cathode bypass caps and the crackling was still there. Also at higher volumes the volume would drop down as you played a note as if you were turning the volume down on your guitar a notch or two. Then i also noticed that the volume pot was making a swishy ocean sound when i turned it up and down. Turns out that coupling cap right before the volume control was leaking DC. replaced it with a new mallory 150 and all is well.
     
  3. VintageJon

    VintageJon Member

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    GOOD WORK!

    The wax/parer cap you replaced is no doubt the culprit. When you are doing a cap-job on an old 50's amp replace all the wax/paper also as they leak badly after 50 years.
    Just did a cap job on a McIntosh 100W mono-block and replaced the wax/paper cap along with the electrolytics. Same hold true for any chassis over 30 years old.

    Again- Good Work,
    Jon
     
  4. NatStrat79

    NatStrat79 Member

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    Well i thought i had it fixed...replacing the leaking paper coupling caps fixed alot of the snaps and crackles and the volume dropping went away but when i play at higher volumes there is still a faint crackling on lower bassy notes. Ive cleaned and retensioned all the tube sockets and its still there. I checked all the voltages and some seemed a little high but not unreasonable? Im guessing there high because my line voltage in my house is around 125volts

    V1 plate 183.3 , 165 on schematic
    V2 plate 194 , 165 on schematic
    Power tube plate - 405 , 375 on schematic
    Cathode v - 27.4 , 20 on schematic

    I upped the cathode resistor to 1k measured it to be 930ohms to bring the bias down a little to account for the higher voltages than the schematic. so it should be biased at about 12w...still crackling

    Im beginning to think maybe its the output transformer. I tried to test it using a method i found in a different post. i measured resistance across the primary..green wire to red wire(center tap?)..black wire to red wire.. black to red gave a reading of like 200ohms and green to red was open circuit. I then measured resistance from primary to secondary when i measure green wire to each of the secondaries i got a measurement of a few ohms but green wire to any of the secondaries i get open circuit..Is this bad news?

    Im thinking this is a definite possibility since when i got the amp someone had installed a 1/4inch jack for an extension cab but they had done it poorly and it was cutting in and out..im guessing that in short intervals the output transformer was running with no load.
     
  5. TNO

    TNO Member

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    Try doubling the value of the first filter cap.
     
  6. Swarty

    Swarty Member

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    Have you changed all the coupling caps? Output tube? Replaced all electrolytics? If answer is yes to all, then do you have access to a scope or a replacement OT?
     
  7. NatStrat79

    NatStrat79 Member

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    Ive replaced all coupling caps, all electrolytics, tried different tubes, and i replaced all of the carbon comp resistors that were way out of spec.

    i dont have access to a scope or a replacement OT...is there anything else i can do to check. I think i might look into a mercury magnetics replacement but i want to be sure first..
     

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