'68 twin reverb-reverb/tube questions!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mlynn02, Aug 30, 2003.

  1. mlynn02

    mlynn02 Supporting Member

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    I've got a '68 twin reverb that I love--it sounds awesome. But today I fired it up for the first time in a few weeks and the reverb doesn't work. The rest of the amp seems to be working fine and flipping the switch on the footpedal just makes a popping noise but doesn't turn on the reverb.

    Now I've recently had new reverb tank put in and at that time I had the wires that make the connections into the back of the amp replaced. So is this probably just a tube problem? I tried switching around some of the tubes in there that were the same type (7025 for another 7025 a couple slots over) but that didn't help.

    I've got no experience working on amps so I can't figure out what's probably a simple problem. And if it is a simple tube replacement, are there any magical tube layouts that you all have found to get the most from this amp? I've never really been disappointed by the sound, but if there is a way to get it sounding warmer and smoother that'd be great too. While I'm buying tubes, why not get this thing cooking, right? Thanks for your help!
    matt
     
  2. jda

    jda Member

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    Do all the tubes glow when powered? I know that's not definitive...

    Is it possible that the wires to the tank are reversed? I have a few amps whose reverb won't work if the cables are flipped.

    Folks seem to like RCA and GE tubes in Twins...
     
  3. John Phillips

    John Phillips Member

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    It's much more likely to be the reverb tank or the connections than a tube. Just because the tank and cables are new means nothing.

    If you shake the amp with the reverb turned up, does it make a crashing noise? If so, you know the return circuit is working, and the problem is in the drive side. If no, the problem is in the return.

    If you get a pop with the footswitch, it's almost certainly not the reverb return tube (the 12AX7/7025 on the power tube side of the reverb transformer), but it still could be the connections or the reverb tank transducer on the return side.

    Like jda says, reversing the connections will give a very weak reverb signal, but I'm guessing it's not that if it was working before and you haven't changed anything.
     
  4. mlynn02

    mlynn02 Supporting Member

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    ok, thanks for the ideas. I turned it on with the reverb on 4 or so and shook the amp (not as easy as it sounds). It made the crashing noise as expected. I also fiddled with the connections (i didn't switch them b/c it was working before like this) and it still doesn't work. So does that mean that it's not a tube? do I have to drag it in to somebody who can open it up and look at it? Thanks again!
     
  5. John Phillips

    John Phillips Member

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    OK, the return is working. Have a look at the 12AT7 (next to the reverb transformer on the side away from the power tubes). That's the reverb driver tube. Chances are it's either this or a bad connection in the send circuit.
     

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