PRRI Ruby Reverberation Tank Problems

Discussion in 'Amp Tech Center' started by DennisM, Oct 6, 2019.

  1. DennisM

    DennisM Tele-Meister

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    The tank for this amp should be 8 ohms in, 2250 ohms out. My reverb circuit has a bad hum and takes in to sqealing feedback at 7 on the volume. I took out the ruby tank and measured 1.5 ohms in and only 209 ohms out. I ordered a new MOD tank, just wondering if I measured the ohms right and if you think the Ruby tank is the cause of the problem? I changed v2 and v3, no help, changed cables no help. Hope this sorta made sense.
     
    Last edited: Oct 7, 2019
  2. max_twang

    max_twang Tele-Afflicted

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    i know this sounds obvious, but make sure you have it hoked up right -- input to input, output to output. Don't ask why I know it's important to check... :)
     
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  3. Wally

    Wally Telefied Ad Free Member

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    Those readings you took are correct for that tank and the application. The 8ohms/2250ohms numbers are impedance and are not directly reflected in resistance meters. Your tank is good as far as the meter readings are concerned.
    One thing I do for all Reverb tanks...even new ones....is to secure the inductors in their mounts. If they are loose, the tank yields sloppy, harsh sounds. Is your tank in a bag with a piece of cardboard fitted to the bottom?
     
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  4. schmee

    schmee Poster Extraordinaire

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    It could be one of the reverb tubes. I also had a squealing issue on a vintage Fender that turned out to be dirty RCA connectors on the cables at the amp end.
     
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  5. DennisM

    DennisM Tele-Meister

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    Thanks Wally. Yes, in the bag with cardboard, out of the bag, upside down, vertical, sideways, out of the cabinet, on top of the amp. The inductors seem tight in their mounts. I can play it up to about 3, after that the hum starts getting worse and starts squealing at about 6 or 7. I don't play it at 6 or 7, but the hum at 3 and 4 is quite annoying. I've checked grounds in the chassis. If the new MOD tank does it too it'll just have to be I guess.
     
  6. DennisM

    DennisM Tele-Meister

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    Thanks schmee. I've changed out all the tubes with no improvement. I've made sure the V2 is an AT7...tried several. I've tried different cables and cleaned the output/input jacks with alcohol. Hoping it's not the reverb driver tranny.
     
  7. charlie chitlin

    charlie chitlin Doctor of Teleocity Silver Supporter

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    Hmmm...floppy inductors.
    I have that.
    A dab of silicone?
     
  8. schmee

    schmee Poster Extraordinaire

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    Yeah, that little tranny is pretty cheap. But it seems they rarely go bad. Do you have a tank to try? Reverb on 7 can be pretty noisy though anyway...
     
  9. Wally

    Wally Telefied Ad Free Member

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    That would do it. I use a super glue.
     
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  10. DennisM

    DennisM Tele-Meister

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    The MOD tank I ordered from AES should be here around Wednesday. We'll see. It just sounds harsh to me, even on 3 (hums there too), like it's got too much juice. The transducers are tight and snug. Sure hope the new tank fixes it.
     
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  11. DennisM

    DennisM Tele-Meister

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    Seems I remember one of the amp builder gurus used toothpicks :)
     
  12. tubelectron

    tubelectron Tele-Holic

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    @DennisM,

    1 - the DCR you measure are normal. Circa 1-2R and 200-250R is correct for a PR, it matches the 8-10R and 2250R impedances.

    2 - the correct Accutronics designation would be 4AB2C1B (medium delay) or 4AB3C1B (long delay).

    3 - if you got hum, there's 2 possible causes - supposing the contacts to the RCA reverb connectors is OK.

    A - your reverb tank is not in the best sense - I mean position, not connectors, OK ? - try to swap it : one of the transducers is more sensitive to the power transformer hum field than the other, so a swap can attenuate neatly the hum issue.

    B - your reverb tank lacks shielding. The carton underneath the unit can be covered with aluminium foil GND'ed : it will help. I usually do this plus wrap all the unit in an aluminium foil GND'ed to completely shield it : it should be enough to make the unit hum quiet.

    4 - if you got howling, squealing when you increase your reverb level setting, that's a Larsen effect vs. the loudspeaker and the amp vibrations. Take a pair of wool sleeves coming from an old pull-over and slip the reverb unit inside them, plus use an old blue-jean pant sleeve to make a reverb bag loose enough. Screw the whole at the bottom of the cab loosely, and no more Larsen effect.

    Here is a set of picture showing the process I used for that :

    [​IMG]

    Hope this helps ! ;)

    -tbln
     
  13. DennisM

    DennisM Tele-Meister

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    Thank you for taking the time to explain this method. I will give them a try. Rather than the sweater sleeves and pant leg, wouldn't the reverb bag do the same thing? How do you ground the foil?
     
  14. Wally

    Wally Telefied Ad Free Member

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    Imho, one need place the foil only on a cover for the open side of the tank. Take a piece of cardboard and cover it with foil and make sure the foil contacts the metal of the tank. That creates a full shield. And...yes, a tank bag will do the job and leaves the tank more accessible, which is important, imho.
    Fwiw, a Reverb tank can function fine without a bag or even without shield on the open side, ime. Does the hum go away when you unplug the tank? Does the hum go away if you move the tank out of the cab. Does the hum go away when you ‘spin’ the tank 180 degrees....end to end...which would place the connection toward the outside of the cab?
     
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  15. Wally

    Wally Telefied Ad Free Member

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    That would seem like a hack job to my eye....and is less than permanent. If that is all I had and needed to get the tank back in operation for a gig, I would do it....and correct it at the first opportunity.
     
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  16. DennisM

    DennisM Tele-Meister

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    Yes. The hum goes away when the tank is unplugged or when I use the foot switch to disengage it. Still hums out of the cab. Still hums when spun 180*. I can get the hum to stop somewhat by putting the tank on top of the amp and tilting it randomly...sometimes I hit the right spot and it calms down a bit, but still feeds back terribly on 6 or 7. I've tried it upside down with the open end up too. No change. I'll give the shielding thing a try with the foil over the cardboard. I have the original bag. Thanks Wally!
     
  17. Wally

    Wally Telefied Ad Free Member

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    And what about the inductors? Are they loose? Or did you stick some toothpicks in there? (;^)
     
  18. charlie chitlin

    charlie chitlin Doctor of Teleocity Silver Supporter

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    Toothpicks!!!
    Please!!!
    3/32" double-terminated white birch dowels!
     
  19. tubelectron

    tubelectron Tele-Holic

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    Sure the reverb bag can replace the pant leg, but adding the sweater sleeves is a great plus for acoustic insulation against feedback.

    The foil is GND'ed because it is pinched by the alu foil covered carton while assembling the reverb tank to that carton.

    Another possible cause of reverb hum : a lack of filtering in the reverb driver amp circuit. This is common on SF and BF Fenders. A supplementary RC HV filter eradicates that source of hum (usually 1.5-3.3K 1W + 10-22µV 500V).

    As you describe it, it's the power transformer hum radiation which spreads on one of the reverb transcducers, typically. :confused:

    But it's strange though that turning the tank 180° left-right in the cab gives no improvement in hum, by moving away from the Xfo the hum-sensitive transducer : I often noticed a neat improvement doing this... o_O

    -tbln
     
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  20. Wally

    Wally Telefied Ad Free Member

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    Yes, there could be a bad bypass cap in the Reverb circuit.
     
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