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Thread: Gibson Ranger GA20-T Rebuild

  1. #71
    Senior Member guitician's Avatar
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    That's good to hear Tom. The speaker is a personal preference thing that is best suited to what the player wants out of the amp. Yes, the P12R was probably the orig. model and is a real good performer, but since the owner can put anything that will fit in it, they should find their ideal speaker if it's to be their "sound".
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  2. #72
    Supporting Member TomCarlos's Avatar
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    Gents.... A follow up to this thread with a question about tube sockets.

    My friend has been using this amp during the last year. He had someone else swap out the rectifier and output tubes. The amp is plenty loud - EXCEPT that is drops in volume for short spurts then goes back up to full power. That is the problem - it is a quick drop in volume then back to full power. So it is hard to get voltage measurements or to see this on the scope. I'll try to use a sine wave to see if this will happen again and if I can see it on the scope.

    I noticed this problem with BOTH channels.

    My initial testing included -

    (1) Swap out the Rectifier tube - No difference
    (2) Swap out the 6V6 Power tubes - No difference.
    The tubes mentioned above are tight in their sockets and no visible corrosion.


    So... knowing that this happens going through both channels, that leads me to the Phase Inverter and beyond.

    I noticed that all the 9 pin preamp tubes are very loose, including the Phase Inverter. So I want to swap out these sockets. I tried to retension the sockets but I think they are beyond help. I see that Antique Radio Supply sells shield base tube sockets - some have the Center Shield, others do not. I don't know if there is an advantage to having the center shield. Anyway, this is something that needs to be done and at least I will be able to rule out a loose tube connection.

    After that, if the problem continues, I'll dive into the right side of the circuit.

  3. #73
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    How abrupt is the drop & recovery? If it's heater related it takes a second or two. Watching the heaters in a dark area may identify an issue if you can get it to exhibit the fault while you are watching.
    If it's a more abrupt issue, have you tried prodding around with chopstick or similar?
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  4. #74
    Supporting Member TomCarlos's Avatar
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    Hi Greg....

    Once the amp warms up, my bud will begin playing at a medium volume level. Maybe around the 4 or 5 minute mark, the volume drops suddenly. Then, maybe 10 or 15 seconds later, it shoots back up to a normal level.

    This amp has a new power transformer and all caps have been replaced. I think we'll start by replacing the tube sockets (9 pins) because it needs it anyway.

    Yes, I poked around and couldn't nail down anything that would cause the volume drop. I'll try to get a scope on this thing. I wish I had a generator with pulsing!!

    The only remaining original component is the Output Transformer. But I am thinking if it were bad, it would be bad all the time... not just intermittently.

    Tom

  5. #75
    Bent Member Chuck H's Avatar
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    Poking around should probably also include thumping on the tubes, wiggling the pots at the knobs and giving the amp the ol' Enzo whack.

    Also, has the guy been using any other guitars, amps or cables with the amp in question being the only common piece of gear in association with the problem? If not then the problem can't be isolated to the amp yet.
    "I'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob." überfuzz

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  6. #76
    Supporting Member TomCarlos's Avatar
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    Yes Chuck....

    We did all the above- different guitars, cables, whackaroo on the case, etc. Since it is a problem with both channels, that is why I thought it would be from the Phase Inverter and beyond. But I will check the heaters because if there is a drop there, then that would be affecting all tubes.

  7. #77
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    I had a sort of similar issue with a project amp half a decade back....it would drop in volume for awhile, then recover and go back to full volume, and then drop again, and then back to full again....it ended up being an oscillation from a trem tube interfering with a gain stage that was too close. Not saying that is the same problem in your amp though, but perhaps there is an oscillation going on somewhere? The one in this amp I described I could not see the oscillation on the scope and I have a dual trace Tek 100 MHz scope. My brother's amp doesn't have the problem your friend's has so the layout isn't a problem with these amps.

    I think you are right in starting with phase inverter and beyond, and also heaters....anything that is common to both channels essentially.

    Greg

  8. #78
    Supporting Member TomCarlos's Avatar
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    Thanks...

    I removed the tubes from Channel 2 (gain and trem tubes) and tested using Channel 1 only. Then I removed the tube from Channel 1 and tested using Channel 2- same resilts. So it helps me nail down the area to test. I found a video that talks about using a #20 gauge twisted pair for the heaters. This amp probably uses #22 and the wires are just laying there on the chassis, non-twisted. So for cleanup purposes, that is something else I will address.

    I'm taking a short break and gave the amp back to my bud. I'll get back on this one in a couple weeks. I want to solve this problem!!

    Tom

  9. #79
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    Replacing the large filter caps on my Gibson Lancer GA35-RVT... the ones in there are brown paper (CDF 6673 CAP = RED wire #1 20MFD 450v, Red Wire #2 20MFD 450V, blk wire common/neg) and have 3 wires. What do I replacement with as most caps have 2 leads?

  10. #80
    Supporting Member Jazz P Bass's Avatar
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    If you can find a dual 20uf/ 450 can cap, that will do.

    What is inside is two separate capacitors.
    Red+ & Black- for each cap.
    The Black leads just happen to be tied together.

  11. #81
    Supporting Member TomCarlos's Avatar
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    I used separate caps... You can get something like an F&T 22uf, 500V for less than $5.

  12. #82
    Lifetime Member Enzo's Avatar
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    I'd just use individual caps, but AES does sell made uop dual ones for just such a purpose. Electrically it doesn't matter.
    Education is what you're left with after you have forgotten what you have learned.

  13. #83
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    GA-20t amp question...

    Hope I can get a response to an older posting here re: Gibson 1958 GA-20t...

    I'm working on one of these and am experiencing a lot of interaction from the depth control on channel 1 volume. I know the tremolo is used on channel 2 only, but is it normal for this to occur. It really cuts the volume almost entirely off. Just curious.

  14. #84
    Supporting Member TomCarlos's Avatar
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    I want to make sure I understand the problem.... You are plugged into Channel 1, playing along, but you then turn the Depth control (other channel) and you loose volume on Channel 1? Very odd indeed. Have you tried removing the 5879 and 6SQ7 tubes to see if you have the same problem?

    By the way... are we talking about original parts in this amp? No odd-ball modifications?

    And what about playing through Channel 2? If you adjust the Depth Control, do you loose volume on that channel?

  15. #85
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    Yes, just as you said, but channel 2 has no issues itself. Just channel one volume is the issue.

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