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'65 Pro fixed, thanks! Now the reverb...

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  • '65 Pro fixed, thanks! Now the reverb...

    Thanks and a tip of the hat to MWJB; my Pro Reverb is now working. However, the reverb is not working, and is making some noise when I turn up the reverb knob. The return from the tank is working (I hear the coils when I tap the tank), but I probably need to test the input. What should I measure to make sure the tank is okay?

    The reverb signal has a mod. The reverb footswitch jack has been replaced by a potentiometer A1MΩ. 2 pics are below. Wiring is as follows:
    • A dark green cable from the transformer (?) in between 12ax7's N°s 3 & 4 goes to the "reverb input " jack
    • A green cable goes from pin 2 of 12ax7 N° 4 to the "reverb output" jack
    • A red shielded cable goes from a bridge between pins 2 & 7 of 12ax7 N° 3 to the center arm of the mod pot
    • The shield from that red cable goes to the right arm of the mod pot
    • A shielded white cable in a grey housing goes from the left arm of the mod pot to a cap on the circuit board (see pic N° 2)
    • The shield of that cable goes from the right arm of the mod pot to a ground connection on the CB


    What I'd like help with is:
    1. to test the tank to make sure that is not the problem
    2. if the tank is okay, to decide wether or not to disable the mod while I work on the reverb issue
    3. to understand the mod


    Any help would be greatly appreciated!

    Cheers,

    Peter



  • #2
    the dwell pot simply replaces a 1meg resistor that sat on the board between the mod white wire & ground (shield). That 1Meg resistor was the load for the 12AT7 grid, like a volume pot set fully up. The pot just allows you to vary the signal strength going to the tank.

    The easiest way to test the tank is to replace it with another.

    If your 12AT7 in V3 is good & V4 is good, the most common reverb failures are the tank (transducers at the end of the springs die), cables/plugs to & from the tank and the reverb transformer (between V3 & V4).

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    • #3
      Hi there. I put a known good tank in and still no reverb. I did a continuity test on the cables going to the tank; they're okay. I assume it's time to get the multimeter out?

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      • #4
        Do you have an 8ohm speaker/cab, some wire and a spare RCA plug/1/4" to RCA adaptor? You can plug a speaker into the Reverb tank "input" jack (fed by the reverb transformer dark green wire) and see whether you get sound from the speaker.

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        • #5
          Yup, that works. Interesting trick ;-)

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          • #6
            OK so you have signal from the transformer, through the cables (that work), through the known good tank, through the other cable that works...so that leaves us with V4, the 110K plate resistor, its .003 coupling cap & the reverb pot on the front panel.

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            • #7
              Another problem fixed! Thanks. I got the reverb working, but it was noisy. So I swapped out tubes 3 & 4 and it went away. 3 was the culprit and is now in position 6. I'll be re-tubing the amp at the end of this process.

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