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Thread: Fender 65 Princeton Reverb RI-reverb tank feedback

  1. #1
    Supporting Member nevetslab's Avatar
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    Fender 65 Princeton Reverb RI-reverb tank feedback

    I have one of our Fender Princeton Reverb amps (65 reissue), with reverb tank feedback. It's NOT the stock Accutronics 4AB3C1B tank, but instead a Ruby RRVL2AB1C1BV4 8 ohm input (0.9 ohm DCR), 150 ohm output (46 ohm DCR) 2-spring long tank, short decay, input insulated, output gnd’d. While I like the sound of this tank, it takes very little to send this into gross howling feedback.

    I ordered a replacement stock tank, but while waiting, I've tried a number of things to see if I could defeat it. Loaded the output of the tank, first with 100k, then with 20k (220k input grid bias resistor on the return circuit), with no change. Reversed the polarity of the Tank Input wires, then the Tank Output wires, neither producing any change. Lacking any fiberglass insulation sheets, I tried some 2" grey packing foam, cut to inside cabinet dimensions, pushed in atop the tank/below the speaker. No change. Tried some 1.5" white packing foam, same approach, no change.

    I've no idea who installed this Ruby tank. I don't think that's stock from Fender. Hopefully the 4AB3C1B tank, when it shows up, will restore order.

    Any other ideas? Even with the tank out of the cabinet, held away as far as the cables will allow, turning up the Reverb will still make the system feed back.

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    Old Timer Leo_Gnardo's Avatar
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    FWIW Fender has been sourcing octal sockets from Ruby for quite a while now. Maybe 10 years, even longer? So it may not be unusual for other Ruby parts to show up in Fender factory builds.

    I'd wait until you have another tank arrive. Maybe the one in that amp is a particularly badly behaved unit.

    What I do with "bad actors" like this, aside from dumpsterizing them, is "mummifying" the tank with foam weatherstrip tape. Home Despot sells foam tape that's about 1.5" wide (4.5cm for the rest of the world) intended to keep chilly breezes from blowing into camper backs set atop pickup truck beds. About $10 a roll, enough to cover 3 regular size reverb tanks, top bottom sides front & back. Then a tank "sock" over the whole mess. It's always worked for me, and the tape is good for lots of other applications. If that don't fix it your tank is haunted.

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  3. #3
    Supporting Member The Dude's Avatar
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    Does touching the transducers in the tank stop the oscillation? I've had to insert a little rubber cement in the middle of transducers to stop oscillation. If loose they will vibrate.

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    I had a princeton(or was it a deluxe?) reissue of the drip edge silverface series in for service and found that its reverb tank had a 3 or 4" piece of sheet metal riveted to the open side of the tank covering one end. I don't remember if it was covering the input transducer or the output. Has anyone seen one of these?
    Nevertheless when I get amps in with the new production tanks that howl I first try lining the open side with aluminum foil.
    And it sometimes works.

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by pontiacpete View Post
    I had a princeton(or was it a deluxe?) reissue of the drip edge silverface series in for service and found that its reverb tank had a 3 or 4" piece of sheet metal riveted to the open side of the tank covering one end. I don't remember if it was covering the input transducer or the output. Has anyone seen one of these?
    Nevertheless when I get amps in with the new production tanks that howl I first try lining the open side with aluminum foil.
    And it sometimes works.
    The Ruby tank that's presently installed has that riveted shield in place. I removed it so I could get at the two coil wires for flipping the polarity. As stated, changing the input/output polarity made no difference. Underneath the tank inside the vinyl pouch is the fiber board cut to the same size as the tank's flange perimeter.

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by The Dude View Post
    Does touching the transducers in the tank stop the oscillation? I've had to insert a little rubber cement in the middle of transducers to stop oscillation. If loose they will vibrate.
    That's something I haven't tried yet. If I get a chance later today, I'll see if it does. Never thought of this technique.

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  7. #7
    Supporting Member nevetslab's Avatar
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    The Accutronics 4AB3C1B Reverb tank arrived, so I moved an Eden WT-800 off the bench to see if this would be a quick slam-dunk. Would that it were so.

    The new tank howls just as badly as the Ruby tank, so, I think it's NOT the tank itself. So, I pulled up the schematic for the 65 RI Princeton Reverb, to see the tubes in use. V2 is the Reverb drive tube, 12AT7, and, that's what is installed. Just in case, I pulled it and installed a 12AU7. Got the same results...howling. So, put the 12AT7 back in. What's in the driver stage on this amp? 12AT7? No, 12AX7 was what I found plugged in, and, that's what is called out on the schematic, so the tubes are the right ones. Power tubes 6V6GT, with both running at 20mA plate current, according to my bias probes, so that's in the ballpark.

    I disconnected the RCA cables connected to the now-mounted/bagged reverb tank, and grabbed the Ruby tank I had removed, and plugged it in with a longer dual RCA cable. Stretched it away from the amp as far as it would reach (3 ft). Yup. still howling, regardless of orientation.

    I can put my finger on the long springs where the two shorter springs meet in the middle, and that will stop the howling. Though I don't think this was what The Dude was referring to: Does touching the transducers in the tank stop the oscillation? I've had to insert a little rubber cement in the middle of transducers to stop oscillation. If loose they will vibrate.

    Dude, are you referring to placing a little rubber cement at the middle connections of the two long springs? I don't have any rubber cement, but, do have some RTV that I'll try. First try, just the tiniest bit applied to both spring joints in the middle of the long springs. Still howling. Added a bit more. Now, while that seemed to stop it, the tank now sounds dark, lacking highs. Removed most of the two small blobs, still leaving a bit at each spring. Got more highs out of it, and still not howling. I'm trying this on the Ruby tank, outside the cabinet with a different cable attached so I could mess with it.

    Now pulling the Accutronics tank out and putting this modified Ruby tank back into the pouch and mounting it again. THAT seems to be working. Still a bit too dark to my liking, but, it's no longer feeding back, and I can drive the amp hard, dial in enough reverb to my liking without the loud screaching it had when it came in. Thanks for the suggestion, if that WAS what you meant.

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    Last edited by nevetslab; 10-17-2019 at 09:59 PM.
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    Old Timer Leo_Gnardo's Avatar
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    Maybe I missed if you checked this, is some sort of unexpected signal being sent out the reverb drive line? FWIW I've had reverb drive transformers apparently with internal insulation breaking down, maybe arcing, sending random pops or crackly noises into the tank. I'm thinking maybe you have something like that going on, but with a much more obnoxious noise coming out instead.

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  9. #9
    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by Leo_Gnardo View Post
    Maybe I missed if you checked this, is some sort of unexpected signal being sent out the reverb drive line? FWIW I've had reverb drive transformers apparently with internal insulation breaking down, maybe arcing, sending random pops or crackly noises into the tank. I'm thinking maybe you have something like that going on, but with a much more obnoxious noise coming out instead.
    I wasn't hearing anything that suggested random pops or crackling noises, plus got the same results with the new tank. I've removed the new tank, and, if you read my edited post from a few minutes ago, I think The Dude's suggestion, or my interpretation of it has restored order.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by nevetslab View Post
    I wasn't hearing anything that suggested random pops or crackling noises, plus got the same results with the new tank. I've removed the new tank, and, if you read my edited post from a few minutes ago, I think The Dude's suggestion, or my interpretation of it has restored order.
    I thought there might be some howling noise coming out of the tank drive jack, but upon reading your edited post #7 seems not. I'll have to keep The Dude's suggestion on file too.

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  11. #11
    Supporting Member The Dude's Avatar
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    I was talking about between the transducer coil and core. I've drawn an arrow on this picture for clarity. Often times, it's really loose and will vibrate causing oscillation. I think you did something else, but hey, if it works......

    Click image for larger version. 

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    Supporting Member nevetslab's Avatar
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    Glue applied to middle of long springs where they join together

    Quote Originally Posted by The Dude View Post
    I was talking about between the transducer coil and core. I've drawn an arrow on this picture for clarity. Often times, it's really loose and will vibrate causing oscillation. I think you did something else, but hey, if it works......

    Click image for larger version. 

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    I kind of thought that was the area you were referring to. It was tight on both ends on the Ruby tank, while a tiny bit of play on the Accutronics tank.

    What I had done here, after touching the springs that were vibrating with the howling, was to apply a tiny bit of RTV at the junction of the middle of the two shorter springs joined to make up each long spring, as shown below:

    Click image for larger version. 

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    I would have tried rubber cement here, but didn't have any on hand. I think the RTV wasn't the best choice, though it did succeed in halting the howling feedback, though it changed the character of the tank. I will have to look at a couple other Princeton Reverbs in our inventory, as I rarely find this symptom.

    I'll definitely keep your method in mind to try in the future. Thanks!

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  13. #13
    Don't forget the joker g1's Avatar
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    I think they used to secure those joints with something harder, like epoxy or nail polish. Same as at the ends where the springs hook into the transducer cores.

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    I think they used to secure those joints with something harder
    Yes, I'd use a drop of high viscosity Super Glue (cyanoacrylate) which gets even harder than epoxy. Softer stuff tends to damp high frequency reflections.

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