Guitar amp spring reverb unit ?

Discussion in 'General Electronics Chat' started by Man_in_UK, Aug 5, 2009.

  1. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    I have a faulty guitar amp. When the reverb control is turned up, instead of getting reverb I get lots of nasty distortion. I have got a schematic and it shows the spring reverb unit to have an input of 150ohm & output of 2250ohm. From the diagram it would seem that the input & output are only connected via a spring and should not be connected electrically.

    My spring unit measures 150 at the input, 250 at the output & 250 between the input & output.

    Does this sound like my spring unit is at fault ?
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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    I do not know anything about reverb control . But as a general warning. Then you are measuring the resistance of components in circuit. Other components around my influence on your readings.
     
  3. beenthere

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  4. bertus

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    Apr 5, 2008
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    Hello,

    Do you have the schematic of the reverb unit?
    As far as I can recall the spring reverb consists of a kind of speaker connected to a spring.
    At the other end of the spring there is a kind of microphone.
    If the spring is mechanicaly wrong (twisted or bend and touching the case) the sound may become distorted.

    Greetings,
    Bertus
     
  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    I'd say your springs are perfectly okay. If you can unplug the spring unit from the amp (usually just a couple of RCA connectors) see if you still get nasty sounding stuff by cranking up the reverb level. If not, it may be just a bad connection. Reinsert all the RCA connectors and see what happens.

    Eric
     
  6. KL7AJ

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    Nov 4, 2008
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    Actually there are two electromagnetic transducers (pickups) one at each end of the springs. The INPUT transducer is low impedance since it has to produce considerable power to vibrate the springs. The output transducer is very high impedance, so it can output a reasonable voltage from the minuscule vibrations that make it to the far end of the springs. :)

    eric
     
  7. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    OK.........

    All measurements were taken with the unit removed - not connected to the amp circuit.

    The schematic I have shows an almost identical diagram to the one shown in the link.

    Removing the RCA connectors stops the nasty distrotion but obviously stop the reverb too.

    From what I can gather from your suggestions/link its a faulty unit. I THINK having a restance between the two transducers is bad, causing my problem.
    (now I'm wondering if I can cut the springs and rejoin them with an insulator ... might work)
     
  8. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Try connecting just the OUTPUT of the springs to your amp. Rap on the springs and see if you get a clean sounding "boing" through the amp. if this is working, the driver circuit from the amp is probably bad. I doubt there's anything wrong with the spring unit.

    eric
     
  9. Dynaman

    Active Member

    Jan 17, 2008
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    Hi British guy...

    1) Is your reverb unit an Accutronics?

    2) What is the model number of the amp? Sounds like a fender..

    3) Tube or transistor?

    I may already have the schematic, but it would be helpful you could post it.

    Dynaman
     
  10. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    Its not as good as that.....
    Spring unit is made by Hammond. The amp is a Carlsbro transistor that I got new 26 years ago.

    I think all those years in a garrage with out british weather has not done it many favours.

    KL7AJ, good idea I will try that.
    I have put a scope on the input to the spring unit and it looked ok but its a good test all the same.
     
  11. flat5

    Active Member

    Nov 13, 2008
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    If you have removed the spring unit why not open it and inspect it?

    See that the springs are properly attached and that nothing is obstructing their movement (like a dead cockroach or spider).
     
  12. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    I have it on the bench but there is not a lot to look at. The only thing I can find that does not seem right is the resistance between the in & out.

    Its not that I do not have any reverb, you cant tell if its there as the distortion swamps everyting out.
     
  13. Andrew Leigh

    Well-Known Member

    Sep 8, 2008
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    Hi,

    those input and output resistances are typical of a reverb tank. Did you do what a previous posted suggested? Tap the reverb tank heavily, the purpose of this is to establish if the problem is at the input or output. If the springy boingy sound is hear loud and clear that means that the problem is at the input, probably the driver.

    I have no doubt that the tank is OK.

    How it works is that there is a low power signal driver feeds the input transducer. The signal then travels down the spring, as the signal travels so it decays. The purpose of the output transducer is to pick up the decayed signal and to present it to another low power amp which, lifts the signal back to what is was before it was fed into the input transducer.

    Another thing is to check the earthing, there are various earthing options on springs, make sure you have not lost an earth.

    BEWARE, those little tranducer coils can "pop" very quickly so don't connect anything to them other than the intended RCA's. An don't reverse them.

    The reverb circuit runs in parallel to your main signal on the pre-amp side. There are normally two controls, one for depth and one for mix. The depth one adjusts the amount of drive into the spring and the mix one determines how much of the reverb signal is mixed into the clean signal. Isolate the circuit it is as simple as two opamps / transistors and associated components. One drives the signal, the other one lifts it back up again.

    Lastly, I would bet that your reverb tank could have been made by accutronics. Have a look at accutronicsreverb.com you may well see your tank there, if it is then you will get the spec's. Remember they post impedences not resistance.

    Your tank is fine it is elsewhere. Search the web for schematics on your amp, you will find them.

    Cheers
    Andrew
     
    Last edited: Aug 9, 2009
  14. t06afre

    AAC Fanatic!

    May 11, 2009
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    Your guitar amp unit is quite old. Have you checked the your Electrolytic capacitors, and inspected the solder joints. Electrolytic capacitors may dry out over time. So it is a good idea to replace them all
     
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