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Thread: Reverb Oscillation

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    Member PfeifferElectronics's Avatar
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    Reverb Oscillation

    I have a new build of a blackface pro reverb. If I turn the reverb up past 4 or 5 it begins to oscillate. Ideas on what would be causing this?

    Thanks

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    Old Timer olddawg's Avatar
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    Quote Originally Posted by PfeifferElectronics View Post
    I have a new build of a blackface pro reverb. If I turn the reverb up past 4 or 5 it begins to oscillate. Ideas on what would be causing this?

    Thanks
    How are you mounting the tank? Try using an RCA cord and barrel connectors to locate the pan outside of the cab temporarily. The tank should be loosely mounted on thick rubber grommets or on a piece of 1" foam. Otherwise it will become microphonic from the the vibrating cabinet.

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    Senior Member TimmyP1955's Avatar
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    It's important that the tank and the amp have the correct grounding scheme on the RCAs.

    On the tank for an oldie Fender, the tank's input RCA should be floating, and the output RCA should be grounded to the tank (per the specs on the tank - the amp schematic does not detail this). Looking at an amp schematic, it appears that the send RCA is grounded to the chassis, and the return RCA is floating (it seems to me that this should not work - curious). However the schematic also shows that the shields of both cables are grounded. A 65RI schematic shows both amp RCAs to be grounded, which makes sense.

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    Supporting Member The Dude's Avatar
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    Just got in a brand new Deluxe Reverb RI with the same problem. I've discovered that removing the tank from the chassis corrects the problem, relating primarily to the tank's proximity to the speaker. I believe the speaker magnet is causing the coils in the tank to vibrate, as I can stop it by holding my fingers on the coils. Further inspection shows that the coils are extremely loose and "floppy"- far more so than I remember in previous tanks I've had out. I've inserted some glue in between the coils and the core they're mounted on to stop them from vibrating. I'll report back on how it works after the glue dries.

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    Supporting Member The Dude's Avatar
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    Amp fixed. The "glue-job" worked.

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    The metal pan itself needs to be damped- they get to singin' all on their own...long strips of old inner-tube rubber super glued on helps dull the ringing of the tank.

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    Cheers,

    Alexander
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    www.retrodyne-austin.com

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    Supporting Member The Dude's Avatar
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    The tank had foam strips on the bottom and was enclosed in one of the typical Fender "baggies". It wasn't the pan that was singing in my case. It was the little transformers. As I said, I could stop the oscillation by touching the coils. They were extremely loose on the core. Hard to describe, but I've not seen this on Accutronics tanks before. Maybe a bad batch? Anyway, your pan tip is still a good one!

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    Last edited by The Dude; 01-16-2015 at 04:42 AM.
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    Bent Member Chuck H's Avatar
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    I've done about everything (other than Dude's coil snuggie) to stop vibration from getting to a reverb pan. Rubber strips, location adjustments, loose bagging, memory foam wrap, you name it!

    Persistence is the key. Read and try until it works. My most difficult one was an independent reverb amp. A twelve watt tube amp in an 18"x22"x9" cabinet dedicated to reverb. It's enough reverb to give a full wet sound to a clipping Marshall Getting the three spring, long pan to stay stable up to full volume was a real grind.

    My latest is a 60W twin twelve combo. I tried all my experienced moves and it still let's go a little (moans quietly at about 500Hz). I may need to try Dude's "touch the transducer" test.

    A good tip I got along the way (but haven't used yet) is to sub in a three spring short tank. Supposedly sounds just great but a lot less prone to acoustic feedback. I've heard a few short three springers and they do sound good (not just good enough).

    I think Belton makes a digital box that plugs right in where a pan would go. I'm game if it sounds good.

    Lot's of options and stuff to try. The bad news is that reverb tanks/pans aren't getting any better and they're not as good as they were.

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    There's a possibility it could be a bad cathode bypass cap on V4.
    Unplug the tank and see if it still oscillates.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Alexander View Post
    The metal pan itself needs to be damped- they get to singin' all on their own...long strips of old inner-tube rubber super glued on helps dull the ringing of the tank.
    I've done similar, using foam tape meant for mounting camper tops on pickup trucks. It's about 1.5 in wide and 1/8 thick. Under $10 a roll at Home Despot, enough to mummify 3 tanks. Stick it on front back sides, every metal surface. Cover the open side of the tank with cardboard like Fender, then slide the whole meshugah into a tank "sock". If THAT doesn't do it, it's well and truly doomed.

    There are tanks that just never sound good no matter what you do. And that was the case even 25 years ago when Accutronics was king.

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    Last edited by Leo_Gnardo; 01-16-2015 at 02:55 PM.

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    Quote Originally Posted by The Dude View Post
    Amp fixed. The "glue-job" worked.
    I check for 'floppy coils' on all repairs that have an Accutronics tank.
    Reglue as needed.

    If all of the above does not cure the ringing, sometimes it helps to put a piece of squishy foam directly underneath the springs.

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    Supporting Member Randall's Avatar
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    I wish I could visualize the glue job, but I'm not understanding it exactly.

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    It's weird, because it WAS working fine.....

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    Supporting Member Jazz P Bass's Avatar
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    Transducer image.

    Click image for larger version. 

Name:	transducer_2.gif 
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ID:	32447

    You do not want the transducer 'wiggling' on the laminated steel.
    On the attached pic, the glue bead can be run along the plastic plate & the laminate. (right above where it says "coil")
    You have to look at each transducer to decide where to bond it, as they are setup differently.

    Good reading: http://www.oberlin.edu/staff/thinder...rb/reverb.html

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    Last edited by Jazz P Bass; 01-16-2015 at 03:05 PM.

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    The OP said the circuit was oscillating, I took it to mean whistle not tank feedback, but who knows.

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    Quote Originally Posted by Chuck H View Post
    I've done about everything (other than Dude's coil snuggie) to stop vibration from getting to a reverb pan. Rubber strips, location adjustments, loose bagging, memory foam wrap, you name it!
    One more thing you can try for tank feedback, the 4 springs that hold the inner pan to the outer chassis are adjustable. The support springs can be moved to different holes in the outer chassis. Sometimes you can make gains this way.

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    Senior Member TimmyP1955's Avatar
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    Quote Originally Posted by pdf64 View Post
    There's a possibility it could be a bad cathode bypass cap on V4.
    Unplug the tank and see if it still oscillates.
    Yep, I've seen this before. Removing the cap made it much worse. New cap and all was well.

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    Old Timer Tom Phillips's Avatar
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    Have you tried a different tank and verified that the problem is independent of the tank?

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    Member PfeifferElectronics's Avatar
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    Wanted to give everyone an update... First of all, thank you so much for all the help. Pulling the tank out of the cab eliminates the problem with the oscillation/feedback, but the reverb is still "too hot". there is too much reverb. Also I noticed that the reverb driver tube is hotter than the rest of the pre-amp tubes. I also noticed that the tank is for the stand alone reverb unit where it is mounted differently. The tank is a 4AB3C1C. I think the original spec for a pro reverb is a 4AB3C1B. I'm thinking I need to order a different tank. Thoughts?

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    Yes, and it sounds much better with a different tank.

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    Supporting Member Jazz P Bass's Avatar
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    The last letter indicates the plane in which the tank is oriented when mounting.

    DIGIT #7 - MOUNTING PLANE
    A = Horizontal Open Side Up
    B = Horizontal Open Side Down
    C = Vertical Connectors Up
    D = Vertical Connectors Down
    E = On End Input Up
    F = On End Output Up

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    Last edited by Jazz P Bass; 01-17-2015 at 09:23 PM.

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    That's normal, it's a minipower amp, there's 432V plate to cathode at 1.9mA per triode ~1.7 W total, compared to 0.55 W total for V6 http://bmamps.com/Schematics/fender/...a165_schem.pdf

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    Bent Member Chuck H's Avatar
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    Already mentioned earlier. I just want to keep it in play. The recovery triode does a lot of amplifying. There's still a possibility that the recovery stage is microphonic until a new tube is tried there.

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by PfeifferElectronics View Post
    ...The tank is a 4AB3C1C. I think the original spec for a pro reverb is a 4AB3C1B. I'm thinking I need to order a different tank. Thoughts?
    As JPB said in post #20 that last character is just the mounting configuration.
    Also note post #15 where g-one talked about the four suspension springs that hold the inner reverb spring tray assembly to the outside case.
    You can reconfigure a 4AB3C1C to be a 4AB3C1B by moving the suspension springs to different holes in the outside case. The idea is to suspend the inner tray roughly in the center of the horizontal and vertical mounting plane to prevent undue reverb crashing.
    However, I don't think that is the cause of your stated problem.

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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by PfeifferElectronics View Post
    Yes, and it sounds much better with a different tank.
    Of same or different type? What was the number on the tank that sounded better? What brand?

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    As mentioned, even back when, different tanks of the same spec / part # can sound unbelievably different.

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    the tank that I am using has no numbers on it. It is accutronics. the one that had a problem was a MOD tank. Anyway, different tank, all is good. I never realized that reverb tanks have personality, that is each one is unique. Thanks for the help!

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    Bent Member Chuck H's Avatar
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    Indeed! Each tank is unique in it's specific frequency range for each spring, as well as sensitivities, interaction between the springs, resonance with a particular amp, etc... If one has enough tanks around it's a very worthy effort to try them all with any given amp to find the one that performs best with THAT particular amp. I've even had some limited success tweaking a tank to work with a particular amp by making minor spring length adjustments. It's possible to tune the springs so they don't resonate out of control with a given amp. I don't recommend trying this since I did eventually ruin that tank My point is that the resonant sensitivities of the tank need to cooperate with the amp it's installed in. The very best solution is to have several tanks to try out. I've never had that luxury myself.

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    An old thread I know, but it seemed a good place to add my findings on reverb related oscillations on the 65 Priceton reverb, just in case anyone came searching for answers,

    I found two different kinds oscillation. The first happens when you crank the reverb, the treble and dial in the volume, it bursts into full power instability around 24KHz. This was fixed by adding a 470pF cap in parallel with the existing 220pF on the RCA socket for the signal back from the reverb.

    The second sounded very much like acoustic feedback from the speaker to the tank, but it wasn't. I could run into a dummy load and still get the problem. The volume and reverb needed to be well up to get it to happen. I found that if I lowered the load impedance I could stop it. That told me that the feedback was coming from the output stage. I tried variations of dress of the leads to the output transformer with zero effect. I started to look the the speaker jacks. I found that if I removed the negative feedback I could stop it in in tracks, surprising since that would increase the loop gain. Suspecting the negative feedback wire was coupling capacitively to something, I tried moving the feedback resistor off the PCB to right next to the jack so attenuating and coupling from the wire. This had no effect so it must be the current though R7, the 47 ohm feedback divider resistor. I lifted the ground side of that and hunted around for a different ground that didn't add hum. The ideal place turned out to be the -ve end C26. Problem solved. I have no doubt that changing the tank could significantly affect and even cure this issue, but only because it would have moved the resonance to a place where the loop gain was lower.

    How this helps someone else with similar issues.


    Piccie of mods:


    PS: I upload the wrong image at first. Updated.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	65_PR-Osc_mods_anno.jpg 
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ID:	53326  

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by nickb View Post
    An old thread I know, but it seemed a good place to add my findings on reverb related oscillations on the 65 Priceton reverb, just in case anyone came searching for answers,

    I found two different kinds oscillation. The first happens when you crank the reverb, the treble and dial in the volume, it bursts into full power instability around 24KHz. This was fixed by adding a 470pF cap in parallel with the existing 220pF on the RCA socket for the signal back from the reverb.

    The second sounded very much like acoustic feedback from the speaker to the tank, but it wasn't. I could run into a dummy load and still get the problem. The volume and reverb needed to be well up to get it to happen. I found that if I lowered the load impedance I could stop it. That told me that the feedback was coming from the output stage. I tried variations of dress of the leads to the output transformer with zero effect. I started to look the the speaker jacks. I found that if I removed the negative feedback I could stop it in in tracks, surprising since that would increase the loop gain. Suspecting the negative feedback wire was coupling capacitively to something, I tried moving the feedback resistor off the PCB to right next to the jack so attenuating and coupling from the wire. This had no effect so it must be the current though R7, the 47 ohm feedback divider resistor. I lifted the ground side of that and hunted around for a different ground that didn't add hum. The ideal place turned out to be the -ve end C26. Problem solved. I have no doubt that changing the tank could significantly affect and even cure this issue, but only because it would have moved the resonance to a place where the loop gain was lower.

    How this helps someone else with similar issues.


    Piccie of mods:


    PS: I upload the wrong image at first. Updated.
    nice work, I saved it in my 65 prin/rev reissue folder.
    Interesting that the optimum connection for noise was not -ve node of C29, which is the filter supply for the stage input for the feedback. Just wondering if fender used a higher impedance voltage divider for the feedback, limiting the current through that point, would've improved the noise issue?
    Regardless, nice fix, and appreciate you sharing it.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by SoulFetish View Post
    nice work, I saved it in my 65 prin/rev reissue folder.
    Interesting that the optimum connection for noise was not -ve node of C29, which is the filter supply for the stage input for the feedback. Just wondering if fender used a higher impedance voltage divider for the feedback, limiting the current through that point, would've improved the noise issue?
    Regardless, nice fix, and appreciate you sharing it.
    It is interesting. I think the problem is an unintended side effect of R60 in the ground between the power and preamp. The feedback current has to flow through this to get back to the output transformer.

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