Reverb tank issue?

Discussion in 'Amp Tech Center' started by tele_savales, Dec 5, 2019.

  1. tele_savales

    tele_savales Tele-Meister

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    If an acutonics tank measures 177 ohms at the output but only .6 ohms at the input, would that indicate a problem with the tank? The cables check fine and the voltages on my drive and return tubes look kosher. I'm not getting any reverb but when I plug the tank into the back of the amp re-built SF twin) I hear a small jump in volume and brightness thru the speaker.
     
  2. Wally

    Wally Telefied Ad Free Member

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    Iirc...and for a Fender tube Reverb tank...that input resistance should read somewhere close to 2 ohms, so that .6ohms is Lo....and might be what your meter would read from probe to probe??? As with speakers, resistance readings are not true impedance readings. Your other number Indicates a good inductor on that end.
    https://www.mojotone.com/kb-reverb-tanks/Reverb-Tank-Specifications
     
  3. tele_savales

    tele_savales Tele-Meister

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    Thanks, yeah I guess I have to decide whether to have a look at the transducers by peeling the tape back or jus get another tank. The jack wires are sturdy but I touched up the greyish solder anyway. The amp had been thrown, dropped or chucked down a flight of stair or out of a van, the pilot light was cracked and the entire front of the chassis was massively bent in. I had to yank the brass grounding bus with a pair of vice grips, even after bending the front panel as straight as I could.
     
  4. tubelectron

    tubelectron Tele-Holic

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    Your values are not surprising. A 4AB3C1B Accutronics/Belton is usually 1R at drive coil and 200R at recovery coil.

    -tbln
     
  5. Nickfl

    Nickfl Tele-Afflicted

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    Are you sure your DMM is accurate at low resistances? I don't completely trust mine in the vicinity of 1 ohm or less and I usually break out my ESR meter if I am trying to accurately measure a really small resistance, like when selecting 1% 1R resistors for measuring cathode current or something like that.
     
  6. Wally

    Wally Telefied Ad Free Member

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    I just measured a number of Fender tube Reverb tanks.....two short tanks and two long 4AB3C1B tanks. Three of them measured 1.2-1.3 ohms. One of my ?Super ?Champ tanks measured 2.2 ohms. This was with a Fluke 77 meter, which reads .3 ohms probe to probe. I have never measured one that was below 1 ohm. That is why I said that .6 ohms was a bit low...and considering meter error, it looks bad. I could be wrong, but the only way to know I suppose is to run that tank on another amp...or run a known good tank on this wrecked amp.
     
  7. tele_savales

    tele_savales Tele-Meister

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    According to the manual, 0.1 +/- 1.5% up to 400ohms. But I really don't know. It's brand new at any rate.
     
  8. Wally

    Wally Telefied Ad Free Member

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    that accuracy doesn’t leave much room for misinterpretation. .6 x .015 = .009. And.....what does the meter read from probe to probe. I have never seen one read ‘0’.
     
  9. tele_savales

    tele_savales Tele-Meister

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    I'll check it as soon as get home.
     
  10. tele_savales

    tele_savales Tele-Meister

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    probe to probe reads .1
     
  11. Peegoo

    Peegoo Tele-Holic

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    Many Hammond and Accutronics pans use push-in connectors that over time (vibration, rough handling, etc.) cut all the way through the conductor in the wire.

    These connectors are the same concept behind those crappy blue wire splice connectors in trailer light kits: when you close the connector, a little blade on each side of the wire cuts through the insulation and pinches the copper conductor. As you may be aware, copper 'work hardens' (it gets brittle with flexure) and this pinch point makes the wire break. Look here:
    upload_2019-12-5_21-51-40.jpeg
    See where the green wire and black wire attach to the transducer's connection block? That's the problem area. Check for continuity on both wires through that connection block, from the RCA jack on the side of the pan to the solder connection on the transducer side of the connection block. Move the wire around a little bit in the connector and see if you have a poor or intermittent connection.
     
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  12. tele_savales

    tele_savales Tele-Meister

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    I'll take a closer look at that, thanks!
     
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  13. tele_savales

    tele_savales Tele-Meister

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    I found the problem! But now I have another one! I decided there was nothing wrong with the tank, and at about 1 in the morning looked at the schematic and realized my reverb return tube plate wasn't hooked up at pin 6. Also, I neglected a jumper between pins 3 and 8 on the same tube.

    Now the reverb works and sounds pretty lush. However, the amp is now so unbelievably loud I can't get it past 1 on the volume pot. And it's super noisy, whereas before, it was perfectly quiet with excellent, genuine Fender tone... I detached the type 3 MV because that seemed to be generating a lot of noise, and the noise increased when turning the pot. Removing it helped somewhat. I am getting a lot of rustling and some pops.

    I will add a .002 bypass cap to the reverb grid leak resistor- it was wired like that when I inherited the chassis and I desoldered it.

    I also took my circular saw to the particle board baffle and have some nice birch ply to put in. i'll post some pics of that part of the project as it unfolds.
     
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