Gibson GA-15 Problem. Please Help!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by hayman2020, Jan 6, 2008.


  1. hayman2020

    hayman2020 Member

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    Hi Everyone,

    I'm hoping someone out there can give me advice on my Gibson GA-15 amp. The amp is british-made and has Trace Elliot "velocette" printed circuit board. It uses two 12AX7's and 2x EL84's (10in speaker). The amp is in excellent condition and the insides look pristine.

    The problem is this: the amp has lost alot of volume and oomph. Alot. It also distorts a little earlier than before the problem developed. The problem was initially intermittent. The amp would work fine and then suddenly the volume would drop alot and the distortion would become "fizzier." The sound of the amp lost alot of it's meatiness. It sounded like someone installed a master volume and turned it down to two. Even the background noise level at full volume is scaled way down. It really now sounds like a 1-2 watt amp. Initially, the volume would occasionally unexpectedly return after a loud "krrrrk" noise. (The amp when functioning properly is really loud for 15 watts, so it's not very pleasant to have it suddenly playing at top volume.) Well,... the volume hasn't returned in quite a while.

    I tried to change out both pre-amp and power tubes. I also sprayed out the tube sockets and jacks with Deoxit G5 and both dials with F5. I've heard about the OT's on some of these amps being bad, but I thought that they just blew out and stopped working altogether.

    I like the amp, but I'm not sure how much money I want to put into it. Thats why I'm asking if anyone has any guesses as to what the problem is. (I'm just not ready to take it in for repairs just yet.) It seems that most of the amp it working fine and so there can't be that many things that could cause the problem. THis condition has to have happened to someone here before. Please give me a hand. Thanks in advance for any input, guys.

    Rob
     
  2. Swarty

    Swarty Member

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    Output transformers on these were problematic. For awhile they were replacing them free.
     
  3. hayman2020

    hayman2020 Member

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    Thanks for the post, Swarty.

    I've heard of the OT problems on these amps, but I'm not sure what the symptoms of a bad output trannie are. Doesn't the amp just fail to work at all? Can a bad output transformer reduce the output power to where it sounds like a 2 watt amp? Thanks again for the input.

    Rob
     
  4. John Phillips

    John Phillips Member

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    Yes, a bad OT could do that.

    It could also be a tube socket problem - the ones they used were crappy plastic and shrink with heat, sometimes making a poor contact to the tube (which would cause this if one power tube stops working - or one side of the phase inverter, though the power tube sockets are most at risk) and sometimes actually cracking the glass. You can fix it permanently by replacing the sockets with ceramic ones.

    Or it could be a broken joint on the rather poorly-supported PCB, again probably in the power section rather than the preamp.

    While you're in there, replace the power tube cathode resistor (120 ohms) with a 150-ohm one. The stock value idles the tubes at over 18W (on a 12W tube) usually, and is part of what cooks them and the sockets.
     
  5. hayman2020

    hayman2020 Member

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    Thanks for the reply, John!

    I'm pretty sure that the tube sockets on my amp are ceramic. At least they look ceramic. I sprayed them out with Deoxit D5, followed by G5. I did this because, before, when I jiggled one of the power tubes, the amp lost output. After I sprayed, this stopped happening.

    The circuit board is mounted upside-down on standoffs, and it seems that I can't flip it over without disconnecting some of the wires to the transformers, since there is no slack in the wires. If I do flip the circuit board, would I be able to see a bad joint if it's there? Is replacing a power transformer technically difficult? Are the wire colors universal? It seems like the wires are not soldered, but are connected with screw-down connectors, and then globbed with epoxy or something. If I can turn a tiny screw-driver, can I do this alone? Is there a definitive test for a bad power transformer? Thanks again for all your help.

    Rob
     
  6. John Phillips

    John Phillips Member

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    I think you'll find they are cream/white colored plastic. If they show any sign of going like the ones in this pic

    [​IMG]

    (note the discoloration and melted appearance) you must replace them.

    It's still a bad sign...

    Yes, but you're better to try to identify it first by poking components from the top side and see if anything makes it cut out. Be careful, since you'll need the amp fully powered up to do it.

    Yes. The goop is hot-melt glue, not epoxy, and comes off quite easily.

    Unless one side is reading open circuit or obviously shorted, it's difficult to get a definitive result by measurement - even one shorted turn will stop it working properly and probably won't show up on a meter. Really the only absolutely definitive way is to measure the power generated by the tubes and the power output of the amp, and if the output is obviously much less than the input (transformers are typically around 90% efficient) you've got a problem. It may even be difficult to identify that if it's intermittent though.
     
  7. hayman2020

    hayman2020 Member

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    Thanks again, John! I really appreciate all your help.

    I'm going to have another look at the tube sockets and poke around some more.
     

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