'68 Custom Deluxe Reverb issue

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Kyle Ashley, Apr 13, 2016.

  1. Kyle Ashley

    Kyle Ashley Supporting Member

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    My 68 CDR has developed a problem. I spoke with Mike at KCA NOS yesterday and got some advice on various tube issue scenarios, but I don't think it's a tube. I swapped out all but the rectifier tube and had no luck.

    Basically the amp is running about half the volume it was. It is lifeless and one dimensional sounding and feels flat and sterile under the fingers. It had such great sparkle, depth and a 3D-like quality before.

    As I thought back over the time I've used it in the last month, I recalled a studio session where we wanted to use tremolo but I didn't have the footswitch to activate it. The house engineer grabbed a couple of footswitches he had lying around, but none would activate the tremolo.

    The next time I turned on the amp, I immediately noticed that something was wrong.

    Does the circuitry of the footswitch jack affect the master output of the amp? I'm wondering if plugging in the wrong footswitches at that studio could have caused some damage or is behind this issue in some way.
     
  2. Blue Strat

    Blue Strat Member

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    Unlikely unless there's some missing information (like he plugged into the wrong jack).

    The rectifier is a remote possibility, definitely worth a shot.

    Try removing all preamp tubes except for V1 and V6. This will allow the normal channel, ONLY, to work. Try various 12AX7s and 12AT7s in those slots and see if you can get any improvement. If not, it's likely power tubes, rectifier or something not tube related.
     
  3. Steppin' Wolfe

    Steppin' Wolfe Member

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    Do you see heater filament activity in all tubes? IF not, that is a problem with either the tube or the filament circuit.
    Does the amp exhibit the same problem in both channels?? IF one channel operates differently from the other, then your problem area has been identified. If the Normal channel is good to go but the Vib channel is not functioning properly, then pull V1 and insert it into V2??? IF nothing changes, put V2 back in its slot and sub that V1 tube into V4. V4 is has one triode that is the reverb return stage and one triode is a gain stage. That gain triode may be bad.
    IF the problem exists in both channels, sub that V1 tube into V6..the phase inverter. IF that doesn't cure the problem, put all of the preamp tubes back where they were.....and then try this....touch the power tubes....you should be able to touch these tubes for a part of second without blistering your fingers. 'Hit it and quit it' type of touch, right? Is one of those tubes cold? IF not, then they are both conducting current. IF one is cold, you have a dead tube for some reason.
    IF none of these tests give you a cure, it is tech time....and might be tech time regardless. If the amp is under warranty, enforce the warranty but be advised that if they know about an improper switch being used they will likely void the warranty.
    there is a low voltage on that switch, but I doubt that it is the source of your problems. I still advise NOT to let others mess with your amp.....plugging odd things in and out.
     
  4. Kyle Ashley

    Kyle Ashley Supporting Member

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    Thanks guys - the amp is at the shop - all the tests and tube swapping got me nowhere. We'll see what they find and I'll update in case it may help someone else down the line.
     

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