Need help with reverb tank

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Luthierwnc, Feb 3, 2008.


  1. Luthierwnc

    Luthierwnc Member

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    Hi Guys;

    I'm stumped on this one. I am working on an Ampeg Gemini-based reverb and can't get any signal through the tank. With a 1k sine going in I get a correct wave form all the way through the big coupling cap into the tank. Absolutely nothing is coming back in the other jack.

    I have a correct Accutronics tank. When I jumped the RCA jacks the signal was a little distorted coming through the recovery phase but was otherwise fine. I got 212R between the transducer pins on both sides of the springs. With the tank plugged I get 25K resistance between the pins. The tank is new (bought it a year ago but never installed it) and I don't have another high-impedance tank to sub. The volts are in the ballpark for the 6CG7 and 12AX7. I put the tank transducers under a magnifying glass and it looks fine. No obvious breaks or shorts.

    If you have any ideas how to troubleshoot a tank or some other tests I might try, please let me know. With some continuity, I hope the tank isn't dead as disco but right now that is the most obvious possibility.

    Thanks, Skip
     
  2. rockon1

    rockon1 Member

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  3. Luthierwnc

    Luthierwnc Member

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    Type 4. FB in and out. long delay, no lock, open face down
     
  4. rockon1

    rockon1 Member

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    Somebody here should be able to point you in the right direction. It measured correctly at 200 ohms on boths sides so I cant imagine why its not getting thru the tank? Bob
     
  5. mark norwine

    mark norwine Member

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    Here's your problem.

    Inside the amp, Ampeg very wisely connected the shield on only one of the pan cables.....this to avoid ground loops. The ground connection of the unwired lead was accomplished because both jacks on the old pan were grounded.

    By contrast, examine your new F pan. See that one of the RCA connectors is isolated from the pan's box. Ergo....no connection to the shieled in the amp....i.e.: incomplete circuit.

    Inside your new pan, run a jumper wire from the case-grounded RCA connector to the ground ring on the other RCA connector. Reassemble the amp. Your reverb will work.
     
  6. rockon1

    rockon1 Member

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    Sweet! Now theres an answer!
     
  7. Luthierwnc

    Luthierwnc Member

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    Thanks Mark. I'll give that a try tonight (last night I was watching a little football ;-)

    I recapped an Ampeg V4 about 8, 9 years ago and remember how much wire they used to keep the grounds on the star. With Fenders it is sort of ground it where you found it.

    sh
     
  8. Luthierwnc

    Luthierwnc Member

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    I got a look in the tank today and neither of the jacks are grounded -- either to the tank or each other. They are completely isolated by a phenolic spacer. Should one/both be attached to the tank frame? If so, should I lift the ground off one of the chassis jacks to avoid a loop?

    Danka, Skip
     
  9. mark norwine

    mark norwine Member

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    Well, to satisfy Ampeg's internal circuit [i.e. shield not connected inside the amp on one of the leads] you'll need to connect the grounds of the RCA jacks to one another. For sure.

    As for grounding the actual tank, I see no harm in doing so, and you may gain some hum protection, too. But the tank itself has nothing to do with your original problem.....that's caused by the RCA jacks not being tied together.
     
  10. Luthierwnc

    Luthierwnc Member

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    That was it. I need to shield some wires but it twangs. Thanks, Skip
     
  11. mark norwine

    mark norwine Member

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    Happy to help.

    About 15 years ago, I returned not one, but 2 "F type" pans to my supplier because they were "defective". I learned this one the hard way!
     

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