Spring reverb squealing

Discussion in 'Burnt Fingers DIY Effects' started by choosebronze, Jul 31, 2019.

  1. choosebronze

    choosebronze TDPRI Member

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    Well, I tried to be creative. That can be dangerous. I pulled apart this old Fender M80 solid state head from the 80's. It had spring reverb, the same tank specs as the Blues Jr (except mounting position). I'd been thinking for a while about building a tube reverb unit, but here I was with a tank build for solid state impedances, and I figured it was worth a shot. The schematics for the M80 and Blues Jr reverb circuits are almost identical. The biggest changes really are a few component values. I created a veroboard layout of the merged circuits.

    I made a couple of mods:

    1. The circuit is meant to run at 15v, so I ran my 9v power through a 7660s-based charge pump from tagboard effects. That's giving me closer to 17v, but the ICs should be able to handle it. I could put a voltage divider in front of everything to try to get closer to 15.

    2. I added a dwell pot. I added a 1M pot right after the first 330p cap in place of the BJr's 910K resistor. Looking back now I don't remember how I decided that would affect dwell - it might have been based on a Billm suggestion of where to add dwell to a BJr?

    Anyway, it works! Audio passes through the board. And it has reverb. The problem is, there's a loud squeal. It sounds like there's a theremin playing along with me. The pitch of the squeal changes as I adjust the pots (fun to play with, but not here). My initial thought would be the 7660s is introducing squeal, I've read they can do that. But I don't have a spare to test. I could build an NE555 doubler instead and try that I guess.

    I think I'm also confused on the RCA cables to and from the tank. The note on the Bjr schematic tank input says it "requires a floating input coil (ie insulated input connector)." I don't get what that means. Is that telling me the rca cable has to be an insulated cable? Like RG174 or something.

    Just wondering if you guys have any other ideas.

     
  2. AAT65

    AAT65 Friend of Leo's Silver Supporter

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    It means that neither the inner nor the outer of the reverb tank input connector is to be connected to ground (you would short out the 47ohm resistor if you did).

    If you’re worried about the 17V supply causing noise (quite likely if it’s a simple charge pump I think) then why not test with a bench PSU if you have one, or 2x 9V batteries if you don’t?
     
    brokenbones likes this.
  3. ICTRock

    ICTRock Tele-Afflicted

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    first thing, you should probably consider shielded rca cables ... yeah you should also have them isolated. be careful about signal wires crossing power supply lines.
     
  4. jimdkc

    jimdkc Friend of Leo's

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    I think I'd try powering it with a couple 9V batteries first. That could quickly confirm or eliminate the power supply circuit as the problem.
     
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  5. choosebronze

    choosebronze TDPRI Member

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    2 9v's! Why didn't I think of that? Thanks.
     
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  6. Frontier9

    Frontier9 Friend of Leo's

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    8EB2C1B = horizontal, open side down mounting plane
    Is this how you have it oriented? I've had pans start to howl if they weren't placed as they were intended (the last letter, B, refers to the mounting plane)
     
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  7. jimdkc

    jimdkc Friend of Leo's

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    Good point! Most reverb tanks have the mounting position specified!
     
  8. J-Flanders

    J-Flanders Tele-Meister

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    It's meant to run at 30volts! -15v to +15v = 30v
    The original design uses a dual power supply.

    You seem to be using a single (split) power supply.
    What changes did you add to the original design to make it run on a single PS? (opamp biasing, DC blocking / AC coupling caps?)
     
  9. choosebronze

    choosebronze TDPRI Member

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    I got caught up in an amp build and haven't had a chance to come back to this reverb. But I used the Tagboard Effects bipolar pump that converts 9v to +17v and -17v. I guess I don't know enough to know how that's different from a "dual" supply or how one would discern either from the original schematic. If the IC pins see +17 and -17 relative to ground, isn't it all the same?
     
  10. jimdkc

    jimdkc Friend of Leo's

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    That's the definition of a dual supply!
     
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