'61 Fender Reverb unit problems

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Argonaut4, Nov 17, 2005.

  1. Argonaut4

    Argonaut4 Member

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    Aug 31, 2005
    I plugged into my reverb unit today and the sound coming through my amp was extremely faint. I plugged directly into my amp (Supro) and the volume was normal. I plugged the reverb unit back in the chain and turned the mix all the way down and the volume was normal. With the mix turned up, extremely faint.

    The tubes are good as new (pulled and tested them). Also, a few months ago, I completely recapped it (electrolytics) and replaced the tank. Up 'til today, this unit has been sounding phenominal.

    Any ideas?

    Thanks,

    J
     
  2. VintageJon

    VintageJon Member

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    Location:
    Austin TX
    "new tubes" don't mean they are good!

    Start with V1 and replace with old tubes you pulled, assumng they worked.

    Have seen any number of "new" failures. Sovtek,EH, EI, GT are
    prime suspects...

    -Jon
     
  3. Argonaut4

    Argonaut4 Member

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    Hi Jon,

    Yep, I'm positive the tubes are good. I tried the old ones with the same results and, for good measure, I tested them in my Hickok. I'm using an RCA 6K6, Mullard AT7, and I think it's an RCA 5751 (can't quite remember that one). In other words, good quality tubes.

    Everything lights up correctly, just a faint reverb sound. I'm thinking it might be the small trannie that connects to the tank.:mad:

    Anybody know what the impedance of the tank should be? Or what impedance that trannie should read? It's the 6G15 Reverb.

    Thanks,

    J
     
  4. TheAmpNerd

    TheAmpNerd Member

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    Tejas
    Argo,

    Now that you checked the tubes...

    Time to check the electronics of the tank
    and the connections!

    Pull and swab the reverb tank connections
    with deoxit or similar, input and output jacks.

    Assuming no change, then
    with the unit operational
    and a signal running through it:

    Work your way from output plate
    to input grid along the signal path.

    Once you find the problem area,
    then check voltages, values, joints,
    wiring etc. This will ID the faulty
    component, wire, joint, etc.

    Good luck and let us know what you find.
     
  5. VintageJon

    VintageJon Member

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    Austin TX
    Well, they DO sound like good tubes. (I'm a Hickok-man, myself).

    Gotta be in the Mix section.

    AmpNerd is dead-on. Time to haul out the voltmeter. Voltage-analysis is a very good place to start. If you have plate voltage then look at cathode voltage. This will tell you if the tube is conducting, (you probably knew this already...)

    Best,
    Jon
     
  6. Enzo

    Enzo Member

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    Oct 3, 2005
    Location:
    Lansing, Michigan, USA
    If i read you right, the dry signal comes through but no reverb effect.

    Rock the unit, if spring noise comes out, then we have a drive problem. if no sound comes out, then the return circuit has the problem.

    First suspect is the pan itself. The impedance won't be wrong, it will either be right, or it will be open. Disconnect the cables to avoid false paths,and measure for continuity through each end of the pan. One end should measure almost shorted, and the other end 100-200 ohms or so.

    If the pan is OK, test the system. With it set up to run, pull the return cable from the OUTPUT jack on the pan, and touch the tip of the plug with a finger. You should get a loud hum. If not, the return circuit has trouble. if hum, the return is working.

    The drive end can be pulled from the pan and connected to a small speaker. Hear anything? If so, the drive is working, if not the drive circuit has trouble. You could also connect it to a test amp or even scope it or AC volt meter it.

    The drive transformer impedance wouldn't have changed, your problem is not wrong impedance. If you want to test the tranny, disconnect it from the pan and measure continuity through the windings. Power off for this. Open means bad. I would just pull the drive cable and measure continuity across that RCA plug. If it is there, OK. As to the primary of the tranny, if there is B+ on the plate of the drive tube, it had to have come through an intact tranny primary.

    Assuming the pan and the tranny are not open, whichever circuit has the problem - drive or return - basic troubleshooting finds the trouble. You can trace the circuit, but often as not voltage readings will work. For all the 12AX7 type tubes look for plate volts on pins 1 and 6. if absent, it should be easy to find out why. Then look at pins 3 and 8 for cathode voltages. If zero volts, the tube is not conducting. The drive tube should have appropriate volts on each pin.

    You could have an open pot or bad coupling cap or something, so for those you can signal trace. Apply a signal to the input. An audio generator is fine, but for this symptom all we care about is go or no go, so anything wil do. I have a line out from the shop stereo for such. Now follow along with your scope. No scope? Set your meter to AC volts and read the signal level with it.
     
  7. Argonaut4

    Argonaut4 Member

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    Thanks all for the extremely helpful suggestions! Enzo, those are some great suggestions for pinpointing the problem. I haven't had much time this weekend to mess with it, but I have ruled out the following:

    The tubes are good.

    The tubes are conducting properly.

    The pan is good.

    The drive circuit is working properly (I initially thought this was the problem).

    I'll continue from there. Thanks again, all!:)
     
  8. jchabalk

    jchabalk Member

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    Apr 5, 2009
    bump - i've got what sounds like the same problem, just wondering if anyone had any suggestions
     
  9. kevmin

    kevmin Supporting Member

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    Mine had the same symptoms and it ended up being the return cable from the reverb tank.
     

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